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Lessons Learned

Project Environment Setup

  • Install necessary packages on the host:

    sudo apt update

    sudo apt install build-essential lzop u-boot-tools net-tools bison flex libssl-dev libncurses5-dev libncursesw5-dev unzip chrpath xz-utils minicom

  • Setup the workspace:

    • custom_drivers/
    • downloads/
    • patches/
    • source/

  • Download boot images and root filesystem

    • Boot images - pre-built-images.zip
      • am335x-boneblack.dtb - Device tree binary of BBB
      • MLO - Primary boot loader (Memory LOader)
      • u-boot - U-boot bootloader image
      • uEnv.txt - U-boot commands and environment settings
      • uImage - Kernel image
    • Debian root filesystem - bone-debian-9.9-iot-armhf-2019-08-03-4gb.img.xz (https://beagleboard.org/)

  • Prepare $\micro$SD card

    • Partition 1 - BOOT / FAT16 / Stores boot images (e.g., MLO, U-boot, kernel image) / 512MB
    • Partition 2 - ROOTFS / EXT4 / Stores Debian root filesystem / Rest of the $\micro$SD card

  • Download cross-compiler and toolchain

  • Add the toochain binary path to the PATH variable (.bashrc in home directory)

    1. Go do the home directory

    2. Open .bashrc file using an editor

    3. Add the following command to the .bashrc file

      export PATH=$PATH:<path_to_toolchain_binaries>

      e.g., export PATH=$PATH:/home/klee/linux-device-drivers/workspace/downloads/gcc-linaro-7.5.0-2019.12-x86_64_arm-linux-gnueabihf/bin

      Or simply do

      echo "export PATH=$PATH:<path_to_toolchain_binaries>" > ~/.bashrc

    4. In the terminal, type arm and hit tab to see if the system recognizes the binaries.

Connecting the Host with the Target (USB-to-Serial TTL Cable)

  • Pin mapping

    USB-to-Serial TTL Cable Pins BBB J1 Header Pin Outs
    Pin 1 - GND (Black) Pin1 - GND
    Pin 4 - RXD (Oragne) Pin 5 - TXD
    Pin 5 - TXD (Yellow) Pin 4 - RXD

    Make sure to cross-connect TXDs and RXDs!

Boot Sequence of BeagleBone Black Board

  • The boot sequence is determined by the SYSBOOT[4:0] bit field of the control register. This bit field value changes depending on whether the boot button is pressed on power-up or not.
  • AM335x boot sequence
    • [Default] Boot button NOT pressed on power-up:
      1. MMC1 (eMMC)
      2. MMC0 ($\micro$SD)
      3. UART0
      4. USB0
    • Boot button pressed on power-up:
      1. SPI0
      2. MMC0 ($\micro$SD)
      3. USB0
      4. UART0

Usage of Power Button, Reset Button and Boot Button

  • Power button

    By pressing and holding this button for 10 to 20 seconds, you can power down the board. Once you power down the board, gently pressing this button one time will power up the board again. Instead of connecting and disconnecting the power source to your board, you can use this button to power down and power up.

  • Reset button

    Pressing this button resets the board. Note that the boot sequence is not affected by the reset action.

  • Boot button

    This button can be used to change the boot sequence during the power up of the board.

$\micro$SD Preparation

  1. 8+ GB $\micro$SD card can be used

  2. Connect the $\micro$SD card to PC using card reader

    • Use dmesg or lsblk command to check if your system recognizes the device (e.g., sda)

  3. Launch the GParted application

  4. Create 2 partitions (FAT16 and EXT4)

    • Partition 1 - BOOT / FAT16 / Stores boot images (e.g., MLO, U-boot, kernel image) / 512MB
    • Partition 2 - ROOTFS / EXT4 / Stores Debian root filesystem / Rest of the $\micro$SD card

    [!] Note: Make sure to click "Apply" button (green check) after creating partitions.

  5. Configure the flags of BOOT partition; lba, boot

    micro-sd-card-partitions

    Once you close GParted app, you'll see the newly created partitions appear on your system.

  6. Copy boot images on FAT16 partition (BOOT partition)

    sudo cp -a workspace/downloads/pre-built-images/SD-boot/* /media/klee/BOOT/

    am335x-boneblack.dtb - Device tree binary of BBB

    MLO - Primary boot loader (Memory LOader)

    u-boot - U-boot bootloader image

    uEnv.txt - U-boot commands and environment settings

    uImage - Kernel image

    sync

    To flush left-over contents in the buffer to the media

  7. Copy Debian root filesystem on EXT4 partition (ROOTFS partition)

    • Decompress the downloaded Debian image:

      unxz bone-debian-9.9-iot-armhf-2019-08-03-4gb.img.xz

    • Right-click the .img file $\to$ Open With Disk Image Mounter. This will mount the image to the filesystem.

      Check the mount status by running:

      lsblk
      mounted-rootfs

    • Copy the contents of mounted rootfs/ into /media/klee/ROOTFS.

      sudo cp -a /media/klee/rootfs/* /media/klee/ROOTFS

      sync

      To flush left-over contents in the buffer to the media

  8. Unmount and remove the $\micro$SD card from PC

  9. Power down the board, insert the $\micro$SD card into BBB $\micro$SD card slot

  10. Boot from $\micro$SD card (MMC0 interface)

Booting from $\micro$SD Card Interface (MMC0)

  1. Make sure that BBB board is NOT powered up

  2. Connect BBB board and host using serial debug cable

    • Check if connection has been successfully established by running dmesg.

      sub-to-ttl-cable-dmesg

      This means that the host will serial communicate with the target board over the device file ttyUSB0.

    • Run minicom:

      (sudo) minicom

      Run minicom -s to configure the environment such as selecting the serial device (e.g., /dev/ttyUSB0).

      ctrl + a, x to exit.

  3. Insert the SD card to BBB board

  4. Power up the board using mini USB cable

  5. Press and hold the boot button (S2)

  6. Press and hold the power button (S3) until the blue LED turns off and turns back on. (If the blue LED doesn't turn back on, gently press the power button.)

  7. Release the S2 button after 2 to 5 seconds.

Check if the board is successfully booting from $\micro$SD card.

Pro Tip!

  • It would be great if we could force the board to boot from the $\micro$SD card (MMC0 interface) by default without having to manipulate any buttons.

    $\to$ This can be done by making eMMC (MMC1 interface) boot fail in the default boot sequence so that the board attempts to boot from the $\micro$SD card (MMC0 interface)

  • When BBB is pre-installed with older Debian eMMC image

    As a root: sudo -s

    1. Create a temporary mount point:

      mkdir /media/tmp1

    2. Mount the partition 1 of the Debian image to the mount point created in the previous step:

      mount /dev/mmcblk1p1 /media/tmp1/

      BOOT partition (partition 1)

    3. cd to the mount point and alter the name of MLO file to something else (e.g., MLO.bak):

      cd /media/tmp1/

      mv MLO MLO.bak

      MLO can be restored easily by changing the name back.

      This will prevent the board from finding the MLO file during its boot process, and in turn cause boot from eMMC failure.

    4. Reboot and see if the board boot from $\micro$SD card successfully.

  • When BBB is pre-installed with newer Debian eMMC image (You may not see /dev/mmcblk1p2 partition and MLO file. You'll see mmcblk1p1 only!)

    newer-emmc-flasher-debian-image-partitions

    As a root: sudo -s, we'll take a snapshot of MBR and zero out it to cause boot failure.

    1. Take a snapshot of MBR:

      dd if=/dev/mmcblk1 of=emmcboot.img bs=1M count=1

      if/of - Input/output file

      bs - Block size

    2. Zero-out MBR:

      dd if=/dev/zero of=/dev/mmcblk1 bs=1M count=1

    3. Reboot and see if the board boot from $\micro$SD card successfully.

    [!] Note: To restore the MBR:

    dd if=emmcboot.img of=/dev/mmcblk1 bs=1M count=1

Linux Kernel Source Update

  1. Clone the kernel source from BBB official GitHub repository (https://github.com/beagleboard/linux) $\to$ This will clone the master branch of the repository. You will need to checkout a specific branch (i.e., kernel source version).

    • 4.14 is used for this project

      In the workspace/source/ directory:

      git clone https://github.com/beagleboard/linux.git linux_bbb_4.14

    • cd to linux_bbb_4.14/ and run git checkout 4.14.

  2. Compile and generate the kernel image from the downloaded kernel image directory (workspace/source/linux_bbb_4.14/)

    • Step 1:

      make ARCH=arm distclean

      Removes all the temporary folder, object files, images generated during the previous build.

      Also, deletes the .config file if created previously.

    • Step 2:

      make ARCH=arm bb.org_defconfig

      Creates a .config file by using default config file given by the vendor (Default config file can be found in workspace/source/linux_bbb_4.14/arch/arm/configs/)

    • Step 3 (Optional):

      make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- menuconfig

      Run this command only if you want to change some kernel settings before compilation.

    • Step 4: Compile kernel source

      make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- uImage dtbs LOADADDR=0x80008000 -j4

      Creates a kernel image uImage.

      Compiles all device tree source files, and generates dtbs.

      #error New address family defined, please update secclass_map. can be resolved by:

      1. scripts/selinux/genheaders/genheaders.c $\to$ comment out #include <sys/socket/h>
      2. scrpts/selinux/mdp/mdp.c $\to$ comment out #include <sys/socket.h>
      3. security/selinux/include/classmap.h $\to$ add #include <linux/socket.h>

      fatal error: mpc.h: No such file or directory error may arise, which is caused by the lack of multiple precision complex floating-point library development package libmpc-dev. Resolve this error by running:

      sudo apt install libmpc-dev

    • Step 5: Build kernel modules

      make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- modules -j4

      Builds and generates in-tree loadable(M) kernel module (.ko).

    • Step 6:

      sudo make ARCH=arm modules_install

      Installs all the generated .ko files in the default path of the computer (/lib/modules/<kernel_ver>).

      Now, you should be able to see /lib/modules/4.14.108+/ directory.

  3. Update the $\micro$SD with the new kernel image, dtb and kernel modules

    • To update the kernel image and dtb in the $\micro$SD:

      • Copy workspace/source/linnux_bbb_5.10/arch/arm/boot/uImage to /mdeia/klee/BOOT/ ($\micro$SD card).

      • Copy workspace/source/linnux_bbb_5.10/arch/arm/boot/dts/am335x-boneblack.dtb to /mdeia/klee/BOOT/ ($\micro$SD card).

        Don't forget update dtb! It will hang the boot process at 'Starting kernel ...'.

        Select the dtb file whose name matches the one that is present in the BOOT partition of the $\micro$SD card.

    • To update the kernel modules in the $\micro$SD:

      • Copy newly installed kernel modules /lib/modules/4.14.108+/ to /media/klee/ROOTFS/lib/modules/ ($\micro$SD card).

      Run sync to flush left-over contents in the buffer to the media

  4. Boot the board from the updated $\micro$SD card.

  5. Check the kernel version after login:

    uname -r

    It should display the updated kernel version. (4.14.108+ in my case)

Enabling Internet Over USB

  • Internet over USB
    • BBB board can communicate with the Internet over the USB cable by sharing the host PC's internet connection.
    • A separate Ethernet cable is not necessary for the BBB board's internet connection.
    • The required drivers are enabled by default in the kernel and loaded when Linux boots on the BBB board.
    • But, you need to enable the internet sharing feature on your host PC to use this service.

Target Settings

  1. First run ifconfig and see if your system recognizes usb0 interface.

    If usb0 interface does not show up, reboot the board and check again.

    If it still does not show up, execute the following commands and check again:

    sudo modprobe g_ether
sudo ifconfig usb0 192.168.7.2 up
ifconfig

    At this point you'll be able to ping 192.168.7.1 (to host), but not ping www.google.com (to the Internet).

  2. Add name server address in /etc/resolv.conf:

    nameserver 8.8.8.8
nameserver 8.8.4.4

  3. Add name server address in /etc/network/interfaces

    iface usb0 inet static
	address 192.168.7.2
	netmask 255.255.255.252
	network 192.168.7.0
	gateway 192.168.7.1
	dns-nameservers 8.8.8.8		<-- add this
	dns-nameservers 8.8.4.4		<-- add this

  4. Add default gateway address by running the following command:

    sudo route add default gw 192.168.7.1

    We are using the host PC as the default gateway.

    Whenever rebooting the board, you need to run this command to get Internet connection. For simple SSH connection to the host PC, running this command is not required.

Host Settings

  1. Run the following commands:

    sudo iptables --table nat --append POSTROUTING --out-interface <network_interface_name> -j MASQUERADE

    <network_interface_name> - Your primary connection to the network could be wireless or wired. You must use the name as listed by the command ifconfig. (In my case wlp61s0)

    sudo iptables --append FORWARD --in-interface <network_interface_name> -j ACCEPT

    sudo -s
echo 1 > /proc/sys/net/ipv4/ip_forward

    Simply running sudo echo 1 > /proc/sys/net/ipv4/ip_forward won't work!

    Whenever rebooting the board, you need to run these commands. So, may be a good idea to create a short script and execute it on every reboot. For example:

    #!/bin/bash
##To run this script do
##1. chmod +x usbnet.sh 
##2. ./usbnet.sh 
iptables --table nat --append POSTROUTING --out-interface <network_interface_name> -j MASQUERADE
iptables --append FORWARD --in-interface <network_interface_name> -j ACCEPT
echo 1 > /proc/sys/net/ipv4/ip_forward

    Make sure th replace <network_interface_name> with a real name.

Linux Kernel Module

  • Objectives:
    • Write a simple hello world kernel module
    • Compile the kernel module using kbuild
    • Transfer a kernel module to BBB board, load and unload

Linux Kernel Module (LKM)

  • Linux supports dynamic insertion and removal of code from the kernel while the system is up and running. The code we add and remove at run-time is called a kernel module. (In other words, a LKM is like a plug-in to the running Linux kernel.)
  • Once the LKM is loaded into the LInux kernel, you can start using new features and functionalities exposed by the kernel module without even restarting the device.
  • LKM dynamically extends the functionality of the kernel by introducing new features to the kernel such as security, device drivers, file system drivers, system calls etc. (modular approach)
  • Support for LKM allows your embedded Linux systems to have only minimal base kernel image (less run-time storage) and optional device drivers and other features are supplied on demand via module insertion.
  • Example: when a hot-pluggable new device (e.g., USB drive) is inserted, the device driver (i.e., LKM) gets loaded automatically to the kernel.

Static & Dynamic LKMs

  • When building the kernel, modules can be directly linked into the kernel (i.e., static), or built as independent modules that can be loaded/unloaded into/from the kernel at run-time (i.e., dynamic).

    • Static (y)

      When you build a Linux kernel, you can make your module statically linked to the kernel image (module becomes part of the final Linux kernel image). This method increases the size of the final Linux kernel image. Since the module is "built-int" into the Linux kernel image, you cannot "unload" the module. It occupies the memory permanently during run-time.

      Increases the size of the Linux kernel image.

    • Dynamic (m)

      When you build a Linux kernel, these modules are NOT built into the final kernel image, and rather they are compiled and linked separately to produce .ko files. You can dynamically load and unload these modules from the kernel using user space programs such as insmod, modprobe, and rmmod.

User Space vs. Kernel Space

  • User space (Restricted mode) - User-level programs
  • Kernel space (Privileged mode) - Kernel-level code (e.g., Linux kernel, subsystems and LKMs)

user-space-kernel-space

Linux Kernel Module (LKM) Structure

  • Since LKMs are the code running in kernel space, there are certain rules to follow when writing one.
  • Sections of an LKM:
    • Header
    • Code
    • Registration
    • Module description

Header

  • This section lists the header files to include.

    /*****************************************************************************************
 * INCLUDE (Inclusion of the necessary header files)
 ****************************************************************************************/
#include <linux/module.h>

    Every kernel module must include the header file linux/module.h. It provides various macros for writing kernel modules.

  • All the kernel header files can be found in the kernel source tree LINUX_SRC/include/linux.

  • Kernel header vs. User-space header

    • Since kernel modules are to be executed in the kernel space, kernel headers must be used. Do NOT include any user-space library headers like C standard library header files (e.g., stdio.h).
    • No user-space library is linked to the kernel module.
    • Most of the relevant kernel headers can be found in linux_source_bse/include/linux/.

Code

  • This section implements what the kernel module should do.

  • Code section contains 2 entry points:

    • Module initialization function (or entry point)

      • Prototype: int fun_name(void);

      • Must return a value:

        • 0 on success

        • Non-zero on module initialization failure where the module will not get loaded in the kernel.

      • This is an entry point to your module.

        • In the case of static modules, this function will get called during the boot-time.

        • In the case of dynamic modules, this function will get called during the module insertion time.

      • There must be one module initialization entry point in every kernel module.

      • What it typically does:

        • Initialization of devices
        • Initialization data structures private to devices
        • Requesting memory dynamically for various kernel data structures and services
        • Request for allocation of major-minor numbers
        • Device file creation

      • The module initialization function is module-specific and should never be called from other modules of the kernel. It should not provide any services or functionalities which may be requested by other modules. Hence, it makes sense to make this function private using static though it is optional to do so.

    • Module clean-up function (or entry point)

      • Prototype: int fun_name(void);
      • This is an entry point when the module gets removed
      • Since a static module cannot be removed at run-time, clean-up function will get called only in the case of a dynamic module when it gets removed by using the user space command such as rmmod.
      • If you write a module and you are sure that it will always be statically linked to the kernel, then it is not necessary to implement this function.
      • Even if your static module has a clean-up function, the kernel build system will remove it during the build process if there is an __exit marker.
      • What it typically does:
        • In general, it is the reverse operation of the module initialization function; undoing init function.
        • Free the memory requested by the init function
        • De-init the device or leave the device in the "proper state"

  • Example:

    /*****************************************************************************************
 * CODE (Implementation of what the module does)
 ****************************************************************************************/

/* Module initialization entry point */
static int __init helloworld_init(void)
{
    /* Kernel's printf */
    pr_info("Hello world!\n");
        /* 'pr_info()' is a wrapper function for 'printk()'. */

    return 0;
        /* Module load will be successful only when the module init entry
           point function returns 0. If it returns non-zero value for any reason,
           loading the module will be unsuccessful. */
}

/* Module clean-up entry point */
static void __exit helloworld_exit(void)
{
    pr_info("Good bye world!\n");
}

  • Function section attributes:

    • __init

      #define __init			__section(.init.text)
#define __initdata		__section(.init.data)
#define __initconst		__section(.init.rodata)

      __section(.init.xxx) - Compiler directives, which direct the compiler to keep data or code in the output section .init.

      • __init and __exit makes sense only for static modules (built-in modules)
      • __init is a macro which will be translated into a compiler directive, which instructs the compiler to put the code in .init section of the final ELF of the Linux kernel image.
      • .init section will be freed from memory by the kernel during boot-time once all the initialization functions get executed.
      • Since the built-in driver cannot be unloaded, its init function will not be called again until the next reboot. This means that it's not necessary to keep references to the init function after its execution. So, using the __init macro is a technique, when used with a function, to make the kernel free the code memory of that function after its execution.
      • Similarly, you can use __initdata with variables that will be dropped after the initialization. __initdata works similar to __init but it works for init "variables" rather than functions.

    • __exit

      #define __exit			__section(.exit.text)

      __section(.exit.xxx) - Compiler directives, which direct the compiler to keep data or code in the output section .exit.

      • For the built-in modules, clean-up function is not required. So, when the __exit macro is used with a clean-up function, the kernel build system will exclude the function during the build process.

static-modules

Registration

  • Module entry point registration example:

    /*****************************************************************************************
 * REGISTRATION (Registration of entry points with kernel)
 ****************************************************************************************/
module_init(helloworld_init);
module_init(helloworld_exit);

    These are the macros used to register the module's init and clean-up functions with the kernel.

    Here, module_init/module_exit are NOT functions, but are macros defined in linux/module.h.

    • module_init() macro will add its argument to the init entry point database of the kernel.
    • module_exit() macro will add its argument to exit entry point database of the kernel.

Module Description

  • Module description example:

    /*****************************************************************************************
 * MODULE DESCRIPTION (Descriptive information about the module)
 ****************************************************************************************/
MODULE_LICENSE("GPL");  /* This module adheres to the GPL licensing */
MODULE_AUTHOR("Kyungjae Lee");
MODULE_DESCRIPTION("A simple kernel module to pring Hello World");
MODULE_INFO(board, "BeagleBone Black REV A5");

    MODULE_LICENSE is a macro used by the kernel module to announce its license type. If you load a module whose license parameter is a non-GPL(General Public License), then the kernel triggers warning of being tainted. This is the way of the kernel letting the users and developers know it's a non-free license based module.

    The developer community may ignore the but reports you submitted after loading the proprietary-licensed module.

    The declared module license is also used to decide whether a given module can have access to the small number of "GPL-only" symbols in the kernel.

    Go to include/linux/module.h to find out what are the allowed parameters which can be used with this macro to load the module without tainting the kernel.

    You can see the module information by running the following command on the .ko file:

    arm-linux-gnueabihf-objdump -d -j .modinfo helloworld.ko

Introduction to Kernel Module Build

  • A kernel module can be built in 2 ways:

    • Statically linked to the kernel image

    • Dynamically loadble $\leftarrow$ our focus!

      • In-tree module (Internal to the Linux kernel tree)

        These modules are the ones approved by the kernel developers and maintainers that are already part of the Linux kernel source tree.

      • Out-of-tree module (External to the Linux kernel tree)

        A module written by a general user, which is not approved by the kernel authorities and may be buggy, to be built and linked to the running kernel, is called an out-of-tree module.

        This method taints the kernel. Kernel issues a warning saying that out-of-tree module has been loaded. You can safely ignore the warning!

  • Building a kernel module (out-of-tree)

    • Modules are built using "kbuild" which is the build system used by the Linux kernel.

    • Modules must use "kbuild" to stay compatible with changes in the build infrastructure and to pick up the right flags to GCC. Also, the "kbuild" will automatically choose the right flags for you.

    • To build external modules, you MUST have a prebuilt kernel source available that contains the configuration and header files used in the build. This is because the modules are linked to the object files found in the kernel source tree.

      You cannot compile your module on one Linux kernel version and load it into the system running on a different kernel version. The module load may not be successful, and even if it is, you'll still encounter run-time issues with the symbols.

      [!] Rule of thumb: Have a prebuilt Linux kernel source tree on your machine and build your module on it.

      Two ways to obtain a prebuilt kernel version:

      • Download kernel from your distributor and build it by yourself
      • Install the Linux-headers- of the target Linux kernel

    • This ensures that as the developer changes the kernel configuration, his custom driver is automatically rebuilt with the correct kernel configuration.

    • Reference: https://www.kernel.org/doc/Documentation/kbuild/modules.txt

Building an Out-of-Tree Module

  • Command to build an external module:

    make -C <path_to_linux_kernel_source_tree> M=<path_to_your_module> [target]

    1. make -C <path_to_linux_kernel_source_tree> switch will trigger the top-level Makefile of the Linux kernel source tree.

      It will enter <path_to_linux_kernel_source_tree> and run the top-level Makefile. At this time, kbuild rules (e.g., compiler switches, dependency list, version string) will be utilized to build the kernel modules.

    2. M=<path_to_your_module> will direct the top-level Makefile to trigger the local Makefile in your working directory where the external modules to be compiled are stored.

    In Makefile syntax, it can be re-written as:

    make -C $KDIR M=$PWD [Targets]

    • -C $KDIR - The directory where the kernel source is located. make will change it to the specified directory when executing and will change it back when finished.

    • M=$PWD - Informs kbuild that an external module is being built. The value given to M is the absolute path to the directory where the external module (kbuild file) is located.

    • [Targets]

      • modules - The default target for external modules. It has the same functionality as if no target was specified.

      • modules_install - Install the external module(s). The default location is /lib/modules/<kernel_release>/extra/, but a prefix may be added with INSTALL_MOD_PATH.

      • clean - Remove all generated files in the module directory only

      • help - List the available targets for external modules

  • Creating a local Makefile - In the local Makefile you should define a kbuild variable as below:

    obj-<X> := <module_name>.o

    obj-<X> is the kbuild variable and X takes one of the following values:

    • X = n - Do not compile the module
    • X = y - Compile the module and link with kernel image
    • X = m - Compile as dynamically loadable kernel module

    Example:

    # Makefile
obj-m := main.o

    The kbuild system will build main.o from main.c, and after linking the kernel module main.ko will be produced.

Exercise 1 (Host - Host)

  • Check the prebuilt kernel version by running uname -r and build your own kernel module.

    In the 00_hello_world/ directory, run the following command to build the LKM against the host PC's Linux kernel version:

    make -C /lib/modules/5.19.0-41-generic/build/ M=$PWD modules

    Then, insert the LKM into the running kernel:

    sudo insmod main.ko

    Run dmesg to check if the LKM has successfully printed Hello world!.

    Remove the LKM from the running kernel:

    sudo rmmod main.ko

    Run dmesg again to check if the LKM has successfully printed Good bye world!.

  • Note

    • At first, sudo insmod main.ko did not work on my PC. This turned out to be due to the "Secure Boot" option that was enabled on my PC. Disabling this option in the BIOS resolved this issue.
    • Although both Hello world! and Good bye world! showed up, the warning messages did not appear at insertion. Need to check why!

Exercise 2 (Host - Target : Cross-Compile)

  • In the file /etc/sudoers append export PATH=$PATH:<path_to_toolchain_binaries> to the secure_path. For example,

    Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin:export PATH=$PATH:/home/klee/linux-device-drivers/workspace/downloads/gcc-linaro-7.5.0-2019.12-x86_64_arm-linux-gnueabihf/bin"
Defaults        use_pty

  • In the 00_hello_world/ directory where the LKM is located, run the following command to build the LKM against the target's Linux kernel version:

    sudo make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -C /home/klee/repos/linux-device-drivers/workspace/source/linux_bbb_4.14/ M=$PWD modules

    You are cross-compiling this time!

    You can run file main.ko, modinfo main.ko, or arm-linux-gnueabihf-objdump -h main.ko to check if the build has been successful.

  • Transfer the built module to the target

    scp main.ko [email protected]:/home/debian/drivers

    You'll be asked to enter the PW of the target, and then the file will be transferred.

  • Insert the LKM into the running kernel:

    sudo insmod main.ko

    It will print the message right on the creen.

    Another way to see the message is to run dmesg | tail.

  • Remove the LKM from the running kernel:

    sudo rmmod main.ko

Update the Makefile

  • Update the local Makefile to automate the out-of-tree module build process for both the host and the target:

    obj-m := main.o
ARCH=arm
CROSS_COMPILE=arm-linux-gnueabihf-
KERN_DIR=/home/klee/repos/linux-device-drivers/workspace/source/linux_bbb_4.14/
HOST_KERN_DIR=/lib/modules/$(shell uname -r)/build/

all:
    make ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) -C $(KERN_DIR) M=$(PWD) modules

clean:
    make ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) -C $(KERN_DIR) M=$(PWD) clean

help:
    make ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) -C $(KERN_DIR) M=$(PWD) help

host:
    make -C $(HOST_KERN_DIR) M=$(PWD) modules

Building an In-tree Module

  • Add the LKM inside the Linux kernel source tree and let the Linux build system build it.
  • To list your kernel module selection in kernel menuconfig, create and use a Kconfig file.

Steps to Add In-Tree Module to Kernel Menu Configuration

  1. Create a directory in linux_bbb_4.14/drivers/char/my_c_dev/

  2. Copy main.c

  3. Create Kconfig file and add the following entries:

    menu "Kyungjae's custom modules"
	config CUSTOM_HELLOWORLD
		tristate "helloworld module support"
		default n
endmenu

    CUSTOM_HELLOWORLD is an identifier by which the kernel identifies your custom module.

    User custom module can be selected or unselected. If selected, it can be either static module or dynamic module.

    Each kernel module has 3 states; y, m, n. You can specify it by using the tristate and default keyword.

    • n - Unselected by default
    • y - Select by default

  4. Add the local Kconfig entry to upper-level Kconfig:

    Go one level up (i.e., linux_bbb_4.14/drivers/char/), open Kconfig and add the following line at the end:

    source "drivers/char/my_c_dev/Kconfig"

  5. Create a local Makefile in linux_bbb_4.14/drivers/char/my_c_dev/.

  6. Add obj-<config_item> += <module>.o to the local Makefile

    config_item - The custom module identifier (e.g., CUSTOM_HELLOWORLD)

    obj-$(CONFIG_CUSTOM_HELLOWORLD) += main.o

    Since the state of this module will be selected via menu, we cannot specify it at the time of writing the Makefile.

    $(CONFIG_CUSTOM_HELLOWORLD) will be replaced by the selected state.

  7. Add the local level Makefile to higher level Makefile:

    Go one level up (i.e., linux_bbb_4.14/drivers/char/), open the Makefile and add the following line at the end:

    obj-y += my_c_dev/

    This is how you direct the higher-level Makefile to run another Makefile.

    -y since you want my_c_dev directory always selected.

  8. Run the Kernel Configuration:

    make ARCH=arm menuconfig

    Select M for helloworld module support and exit saving your new configuration!

    kconfig-custom-module-1 kconfig-custom-module-2 kconfig-custom-module-3

  9. Open the updated linux_bbb_4.14/.config file and search for CONFIG_CUSTOM_HELLOWORLD.

    If it is there, it means that your custom module is now part of the kernel source tree.

    dot-config

  10. Build the kernel modules!

    In the Linux kernel source directory linux_bbb_4.14/ run:

    make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- modules -j4

    You'll be able to see main.ko generated.

  11. Check the module info and see if the built module is marked as "intree":

    Go to linux_bbb_4.14/drivers/char/my_c_dev and run:

    modinfo main.ko

    Then, you'll see the intree: field is marked Y.

[!] Reference: https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt

Debugging with printk

  • print is one of the best debugging tools we have in user-level applications.

  • When you work in kernel space, you will not have any access to the C standard library functions like printf or scanf.

  • As their counterpart, the kernel has its own printf-like API called printk, where the letter k signifies "Kernel space printing".

  • Examples;

    printf("User space.\n");
printk("Kernel space.\n");						/* Kernel space */
printf("data1 = %d, data2 = %d\n, d1, d2");
printk("data1 = %d, data2 = %d\n, d1, d2");		/* Kernel space */

  • When using printk, the message will go into the kernel ring buffer (a.k.a. Kernel log) and we can print and control the kernel ring buffer using the command dmesg.

    To check the latest 5 kernel messages:

    dmesg | tail -5

    To check the first 20 kernel messages:

    dmesg | head -20

  • printk format specifiers

    Variable Type		printk Format Specifier
==================	=======================
int					%d or %x
unsigned int		%u or %x
long				%ld or %lx
unsigned long		%lu or %lx
long long			%lld or %llx
unsigned long long	%llu or %llx
size_t				%zu or %zx
ssize_t				%zd or %zx
s32					%d or %x
u32					%u or %x
s64					%lld or %llx
u64					%llu or %llx

    printk does not support floating-point formats (%e, %f, %g).

    [!] Reference: https://www.kernel.org/doc/Documentation/printk-formats.txt

Kernel Log Levels

  • Based on the kernel log level you can control the priority of the printk messages.

  • There are 8 log levels

    • The lower the level number, the higher the priority
    • The default printk log level or priority is usually set to 4. (i.e., KERN_WARNING)
    /* include/linux/kern_levels.h */

#define KERN_SOH		"\001"			/* ASCII Start Of Header */
#define KERN_SOH_ASCII	'\001'

#define KERN_EMERG		KERN_SOH "0"	/* system is unstable */
#define KERN_ALERT		KERN_SOH "1"	/* action must be taken immediately */
#define KERN_CRIT		KERN_SOH "2"	/* critical conditions */
#define KERN_ERR		KERN_SOH "3"	/* error conditions */
#define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
#define KERN_NOTICE		KERN_SOH "5"	/* normal but significant condition */
#define KERN_INFO		KERN_SOH "6"	/* informational */
#define KERN_DEBUG		KERN_SOH "7"	/* debug-level messages */

#define KERN_DEFAULT	""				/* the default kernel loglevel */

  • The log level will be used by the kernel to understand the priority of the message. Based on the priority the kernel will decide whether the message should be presented to the user immediately by printing directly on to the console.

  • All kernel messages will have their own log level.

  • You may have to specify the log level while using printk. If not specified, the kernel will add the default log level set by the kernel config item CONFIG_MESSAGE_LOGLEVEL_DEFAULT whose value is 4.

    So, the following statement

    prink("Kernel space\n");

    is equivalent to

    prink(KERN_WARNING "Kernel space\n");
//    ------------  --------------
//      Log level      String arg

    Notice that there's NO comma (,) between the log level and the string argument!

  • Default message log level can be configured via the kernel menuconfig (make ARCH=arm menuconfig)

    kconfig-default-message-log-level

Significance of the Kernel Log Level

  • There is another log level we need to consider; the current console log level.

  • The kernel message log level will be compared with the current console log level. If the kernel message log level is lower (i.e., higher priority) than the current console log level, the message will be directly printed on the current console.

  • By default, the console log level will have the value of config item CONFIG_CONSOLE_LOGLEVEL_DEFAULT whose default value is set to 7. This value can be changed via the kernel menuconfig or running commands.

    To check the current console log level status, run

    cat /proc/sys/kernel/printk
7       4       1       7

    The results shows the current, default, minimum and boot-time-default log levels.

    pr_info (i.e., KERN_INFO level) used in the hello world LKM is of log level 6. This is why the message is getting printed when the LKM gets inserted into the kernel.

  • At run-time, you can change the current console log level (for example to 6) by running the following command:

    sudo -s
echo 6 > /proc/sys/kernel/prink

    Now, if you check the current console log level:

    cat /proc/sys/kernel/printk
6       4       1       7

    The current console log level has changed from 7 to 6.

    Since the current console log level is now the same as pr_info (i.e., KERN_INFO level), the message will not get printed when the hello world LKM gets inserted into the kernel.

  • prink wrappers (Defined in include/linux/printk.h)

    Name Log Level Alias Function
    KERN_EMERG "0" pr_emerg
    KERN_ALERT "1" pr_alert
    KERN_CRIT "2" pr_crit
    KERN_ERR "3" pr_err
    KERN_WARNING "4" pr_warning
    KERN_NOTICE "5" pr_notice
    KERN_INFO "6" pr_info
    KERN_DEBUG "7" pr_debug (works only if DEBUG is defined)
    KERN_DEFAULT "" -

    pr_err - Useful for reporting errors

    pr_info - Useful for printing general information

    pr_warning - Useful for reporting warning

    pr_alert, pr_crit - Useful for reporting critical situations

    For example:

    printk(KERN_INFO "Kernel version 4.14\n");

    Can be re-written as

    pr_info("Kernel version 4.14\n");

Device Driver

Overview

  • Linux device drivers are software components that enable the Linux operating system to communicate with and control specific hardware devices. Device drivers also expose interfaces to the user applications so that those applications can interact with the hardware device.

  • They act as a bridge between the hardware and the operating system, providing a standardized interface for applications and the kernel to interact with the device.

  • Device drivers are typically written as kernel modules, which are loadable pieces of code that extend the functionality of the Linux kernel.

  • They abstract away the low-level details of the hardware, allowing applications to access the device's functionality without needing to know the specific implementation details.

  • Linux device drivers are essential for supporting a wide range of hardware devices, such as network adapters, sound cards, graphic cards, storage devices, and input devices.

  • Writing a Linux device driver requires knowledge of the Linux kernel internals, device-specific protocols, and programming in C or another supported language.

  • Types of Linux device drivers

    • Character device drivers (Char device - RTC, keyboard, sensors, etc.)

    • Block device drivers (Storage devices - SDMMC, EEPROM, Flash, hard disk)

    • Network device drivers (Network devices - Ethernet, wifi, bluetooth)

  • Device files

    • Devices are handled as a file in a UNIX/Linux systems.

    • A device file is a special file or a node which gets populated in /dev directory during the kernel boot-time or device/driver hot plug events.

    • By using a device file, a user application can communicate with a driver.

    • Device files are managed as part of VFS subsystem of the kernel.

    • Device driver should trigger the creation of device file.

      device-files

  • Major number vs. minor number

    major-number-minor-number
    • Major number - Identifies the device driver or device driver class that the device belongs to.
    • Minor number - Distinguishes individual devices (i.e., device instances) within a driver class or specifies different functionalities of the same device driver.

    Device driver will use the "minor number" to distinguish on which device file the read/write operations have been issued.

Character Driver (char driver)

  • A Linux character device is a special type of device file that allows sequential character-based input or output to be processed, such as terminals or serial ports.

  • A Linux character driver is a software component that enables communication and control of character-oriented devices by providing an interface for applications to read from and write to them character by character.

  • Character driver accesses data from the device sequentially (i.e., byte-by-byte like a stream of characters) not as a chunk.

  • Sophisticated buffering strategies are usually not involved in char drivers, because when you write 1 byte, it directly goes to the device without any intermediate buffering, delayed write-back or dirty buffer management.

  • Examples: Sensors, RTC, keyboard, serial port, parallel port, etc.

Block Driver

  • A Linux block device is a type of device that allows data to be read from or written to in fixed-sized blocks, typically used for storage devices like hard drives or solid-state drives.
  • A Linux block driver is a software component that facilitates communication and control of block devices, handling the read and write operations of fixed-sized blocks of data between the operating system and storage devices.
  • Block drivers are more complicated than char drivers because they should implement advanced buffering strategies to read from and write to the block devices, which involves disk caches.
  • Examples: Mass storage devices such as hard disks, SDMMC, NAND Flash, USB camera, etc.

Kernel Header File Details

Kernel functions and data structures Kernel header file
alloc_chrdev_region()
unregister_chardev_region()		 include/linux/fs.h
cdev_init()
cdev_add()
cdev_del()		 include/linux/cdev.h
device_create()
class_create()
device_destroy()
class_destroy()		 include/linux/device.h
copy_to_user()
copty_from_user()		 include/linux/uaccess.h
VFS structure definitions include/linux/fs.h

Device Number Representation

  • The device number is a combination of major and minor numbers.

  • In Linux kernel, dev_t (typedef of u32) type is used to represent the device number.

  • Out of 32 bits, 12 bits are to store major number, and the remaining 20 bits are to store minor number.

  • You can use the following macros to extract major and minor parts of dev_t type variable device_number:

    int minor_no = MINOR(device_number);
int major_no = MAJOR(device_number);

    MINOR(), MAJOR() macros are defined in include/linux/kdev_t.h.

  • Also, major and minor numbers can be turned into device_number:

    MKDEV(int major, int minor);

Kernel APIs and Utilities Used in Driver Code

  • Creation

    /* Create device number */
alloc_chrdev_region();

/* Make a char device registration with the VFS */
cdev_init();
cdev_add();

/* Create device files */
cdev_init();
cdev_add();

    When a module is loaded, these creation services must be executed and the driver must be ready to accept system calls from the user space program. So, it would make sense for this part to be taken care of by the module initialization function.

  • Deletion

    /* Delete device number */
unregister_chrdev_region();

/* Delete the registration */
cdev_del();

/* Delete device files */
class_destroy();
device_destory();

    Taken care of by the module cleanup function.

APIs in Detail

  • Dynamically register a range of char device numbers

    int alloc_chardev_region(dev_t *dev,		/* output param for first assigned number */
                         unsigned baseminor,/* first of the requested range of minor number */
                         unsigned count,	/* number of minor numbers required */
                         const char *name);	/* name of the associated device or driver */

    baseminor is typically 0.

    name is NOT the device file name. It is a pointer to a string that represents the name or identifier of the character device region being allocated. This name is typically used for identification or debugging purposes and is not directly related to the functionality of the device.

    Usage:

    /* Device number creation */
dev_t device_number;
alloc_chardev_region(&device_number, 0, 7, "eeprom");

    Again! eeprom here is not the name of a device file. It is just an identifier that indicates the range of device numbers.

    alloc-chardev-region

  • Initialize a cdev structure

    /* fs/char_dev.c */

/**
 * cdev_init() - initialize a cdev structure
 * @cdev: the structure to initialize
 * @fops: the file_operations for this device
 *
 * Initializes @cdev, remembering @fops, making it ready to add to the
 * system with cdev_add().
 */
void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
    memset(cdev, 0, sizeof *cdev);
    INIT_LIST_HEAD(&cdev->list);
    kobject_init(&cdev->kobj, &ktype_cdev_default);
    cdev->ops = fops;
}

    cdev structure is defined in include/linux/cdev.h:

    struct cdev {
    struct kobject kobj;
    struct module *owner;
    const struct file_operations *ops;
    struct list_head list;
    dev_t dev;
    unsigned int count;
} __randomize_layout;

    ownder - A pointer to the module that owns this structure; it should usually be initialized to THIS_MODULE. This field is used to prevent the module from being unloaded while the structure is in use. (THIS_MODULE is essentially a pointer and is defined in linux/export.h as #define THIS_MODULE (&__this_module).)

    ops - A pointer to file_operations structure of the driver.

    Usage:

    /* Initialize file ops structure with driver's system call implementation methods */
struct file_operations eeprom_fops;
struct cdev eeprom_cdev;
cedv_init(&eeprom_cdev, &eeprom_fops);

    If you are dealing with 10 devices, then you may have to create 10 cdev structures.

  • Add a char device to the kernel VFS

    /* fs/char_dev.c */

/**
 * cdev_add() - add a char device to the system
 * @p: the cdev structure for the device
 * @dev: the first device number for which this device is responsible
 * @count: the number of consecutive minor numbers corresponding to this
 *         device
 *
 * cdev_add() adds the device represented by @p to the system, making it
 * live immediately.  A negative error code is returned on failure.
 */
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
    int error;

    p->dev = dev;
    p->count = count;

    error = kobj_map(cdev_map, dev, count, NULL,
             exact_match, exact_lock, p);
    if (error)
        return error;

    kobject_get(p->kobj.parent);

    return 0;
}

  • Dynamic device file creation in Linux

    • In Linux, you can create a device file dynamically (on demand), i.e., you don't need to manually create device files under /dev directory to access your hardware.

    • User-level program such as udevd can populate /dev directory with device files dynamically. (udevd is a user level daemon which runs in the background and scans for uevents generated by the kernel. By analyzing the uevents, udevd populates /dev directory with device files.)

    • udev program listens to the uevents generated by hot plug events or kernel modules. When udev receives the uevents, it scans the subdirectories of /sys/class looking for the dev files to create device files.

    • For each such dev file, which represents a combination of major and minor number for a device, the udev program creates a corresponding device in /dev directory.

    • udev

      • Relies on device information exported to user space through sysfs.
      • uevents are generated when device driver uses kernel APIs to trigger the dynamic creation of device files or when a hot pluggable device such as a USB peripheral is plugged into the system.

    • All a device driver needs to do, for udev to work properly with it, is to ensure that any major and minor numbers assigned to a device controlled by the driver are exported to user space through sysfs.

    • The driver exports all the information regarding the device such as device file name, major, minor number to sysfs by calling the function device_create.

    • udev looks for a file called dev in the /sys/class/ tree of sysfs, to determine what the major and minor number is assigned to a specific device.

    • class_create and device_create

      • class_create

        /**
 * class_create() - Creates a directory in sysfs: /sys/class/<your_class_name>
 */
struct class *class_create(struct module *owner, const char *name);
        struct class *eeprom_class;
eeprom_class = class_create(THIS_MODULE, "eeprom_class");

      • device_create

        /* drivers/base/core.c */

/**
 * device_create() - Creates a subdirectory under /sys/class/<your_class_name> with your
 * device name. Also, populates sysfs entry with dev file which consists of the major 
 * and minor numbers separated by a ':' character.
 * @class: Pointer to the struct class that this device should be registered to
 * @parent: Pointer to the parent struct device of this new device
 * @devt: dev_t for the char device to be added
 * @drvdata: Data to be added to the device for callbacks
 * @fmt: String for the device's name
 */
struct device *device_create(struct class *class, struct device *parent,
                            dev_t devt, void *drvdata, const char *fmt, ...);
      class-create-device-create

  • Remove a device, destroy a struct class structure, remove cdev registration from the kernel VFS, and unregister a range of device numbers

    • device_destroy()

      /**
 * device_destroy() - Removes a device that was created with 'device_create()'
 * @class: Pointer to the struct class that this device was registered with
 * @devt: device_number of the device that was previously registered
 */
void device_destroy(struct class *class, dev_t devt);

    • class_destroy()

      /**
 * class_destroy() - Destroys a struct class structure
 * @cls: Pointer to the struct class that is to be destroyed
 */
void class_destroy(struct class *cls);

    • cdev_del()

      /**
 * cdev_del() - Removes cdev registration from the kernel VFS
 * @p: Pointer to the cdev structure to be removed
 */
void cdev_del(struct cdev *p);

    • unregister_chrdev_region()

      /**
 * unregister_chrdev_region() - Unregisters a range of device numbers
 * @from: The first in the range of numbers to unregister
 * @count: Number of device numbers to unregister
 */
void unregister_chrdev_region(dev_t from, unsigned count);

inode Object

  • Unix makes a clear distinction between the contents of a file and the information about a file.
  • An inode is a VFS data structure (struct inode) that holds general information about a file.
  • Whereas VFS file data structure (struct file) tracks interaction on a file opened by the user process.
  • An inode contains all the information needed by the filesystem to handle a file.
  • Each file has its own inode object, which the filesystem uses to identify the file.
  • Each inode object is associated with an inode number, which uniquely identifies the file within the file system.
  • The inode object is created and stored in memory when a new file (regular or device) gets created.

file Object

  • Whenever a file is opened, a file object is created in the kernel space. There will be one file object for every open regular/device file.
  • A file object stores information about the interaction between an open file and a process.
  • This information exists ONLY in kernel memory while the file is open.
  • The contents of a file object is NOT written back to disk unlike the case of an inode.

Character Driver File Operations Methods

  • VFS file_operations structure

    /* include/linux/fs.h */

struct file_operations {
    struct module *owner;
    loff_t (*llseek) (struct file *, loff_t, int);
    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
    ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
    ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
    int (*iterate) (struct file *, struct dir_context *);
    int (*iterate_shared) (struct file *, struct dir_context *);
    unsigned int (*poll) (struct file *, struct poll_table_struct *);
    long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
    long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
    int (*mmap) (struct file *, struct vm_area_struct *);
    ...
} __randomize_layout;

    Collection of various function pointers to the possible file operation methods for a regular file or a device file.

Summary

  • When device file gets created

    1. Create device file using udev (init_special_inode() gets called; can be found in fs/inode.c)

      void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
    inode->i_mode = mode;
    if (S_ISCHR(mode)) {
        inode->i_fop = &def_chr_fops;	/* dummy file operation; fs/char_dev.c */
        inode->i_rdev = rdev;	/* initialize i_rdev with newly created device's device # */
    } else if (S_ISBLK(mode)) {
        inode->i_fop = &def_blk_fops;
        inode->i_rdev = rdev;
    } else if (S_ISFIFO(mode))
        inode->i_fop = &pipefifo_fops;
    else if (S_ISSOCK(mode))
        ;   /* leave it no_open_fops */
    else 
        printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for"
                  " inode %s:%lu\n", mode, inode->i_sb->s_id,
                  inode->i_ino);
}

      rdev - Device number

      mode - Device type (e.g., char device, block device, etc.)

    2. inode object gets created in memory and inode->i_rdev field is initialized with the device number

    3. inode->i_fop field is set to dummy default file operations (i.e., def_chr_fops)

      /*
 * Dummy default file-operations: the only thing this does
 * is contain the open that then fills in the correct operations
 * depending on the special file...
 */
const struct file_operations def_chr_fops = { 
    .open = chrdev_open,
    .llseek = noop_llseek,
};

  • When user process executes open system call:

    1. User invokes open() system call on the device file
    2. file object gets created (VFS opens a file by creating a new file object and linking it to the corresponding inode object.)
    3. inode's i_fop gets copied to file object's f_op (dummy default file operations of char device file def_chr_fops)
    4. Open function of dummy default file operations gets called (chrdev_open)
    5. inode->i_cdev field is initialized to cdev that you added during cdev_add (lookup happens using inode->i_rdev field)
    6. inode->cdev->fops (this is a actual file operations of the driver) gets copied to file->f_op
    7. file->f_op->open method gets called (actual open method of the driver)

  • open() system call behind the scenes 1:

    open-system-call-behind-the-scenes-1

  • open() system call behind the scenes 2:

    open-system-call-behind-the-scenes-2

Exercise: Pseudo Character Driver

  • Write a character driver to deal with a pseudo character device
  • The pseudo-device is a memory buffer of some size
  • The driver you write must support reading, writing and seeking to this driver
  • Test the driver functionality by running user-level command such as echo, dd, cat and by writing user level programs

pseudo-char-driver

Connection Establishment between Device File Access and the Driver

  1. Create device number
    • Request the kernel to dynamically allocate the device numbers(s)
  2. Make a char device registration with the Virtual File System (VFS). (CDEV_ADD)
  3. Create device files
  4. Implement the driver's file operation methods for open, close, read, write, llseek, etc.

File Operation Methods

  • Open

    /**
 * pcd_open() - Pseudo char driver open method
 * @inode: Pointer to an inode associated with the filename
 * @file: Pointer to a file object
 *
 * Opens a file, 
 * returns 0 on success, negative error code otherwise
 */
int pcd_open(struct inode *inode, struct file *filp)
{
    return 0;
}
    • Initialize the device or make device respond to subsequent system calls such as read() and write().
    • Detect device initialization errors
    • Check open permission (O_RDONLY, O_WRONLY, O_RDWR)
    • Identify the opened device using the minor number
    • Prepare device private data structure if required
    • Update f_pos if required
    • open method is optional. If not provided, open will always succeed and driver is not notified.

  • Close (e.g., close(fd) system call from the user space)

    /**
 * pcd_release() - Pseudo char driver method to handle close() system call
 * @inode: Pointer to an inode associated with the filename
 * @filp: Pointer to a file object
 *
 * VFS Releases the file object. Called when the last reference to an open file is closed, i.e.,
 * when the f_count field of the file object becomes 0.
 * Returns 0 on success, negative error code otherwise
 */
int pcd_release(struct inode *inode, struct file *filp)
{
    return 0;
}

    • Does the reverse operations of open method. Simply put, release method should put the device in its default state, i.e., the state before the open method was called.

      e.g., If open method brings the device out of low power mode, then release method may send the device back to the low power mode.

    • Free any data structures allocated by the open method.

    • Returns 0 on success, negative error code otherwise (e.g., the device does not respond when you try to de-initialize the device).

  • Read (e.g., read(fd, buff, 20) system call from the user space)

    /**
 * pcd_read() - Pseudo char driver method to handle read() system call
 * @filp: Pointer to a file object
 * @buff: Pointer of user buffer
 * @count: Read count given by user
 * @f_pos: Pointer of current file position from which the read has to begin
 *
 * Reads a device file @count byte of data from @f_pos, returns the data back to @buff (user),
 * and updates @f_pos. Returns the number of bytes successfully read, 0 if there is no bytes to 
 * read (EOF), appropriate error code (negative value) otherwise.
 */
ssize_t pcd_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos)
{
    return 0;
}

    __user

    • Optional macro that alerts the programmer that this is a user level pointer so cannot be trusted for direct dereferencing.
    • A macro used with user level pointers which tells the developer not to trust or assume it as a valid pointer to avoid kernel faults.
    • Never try to dereference user given pointers directly in kernel level programming. Instead, use dedicated kernel functions such as copy_to_user and copy_from_user.
    • GCC doesn't care whether you use __user macro with user level pointer or not. This is checked by sparse, a semantic checker tool of Linux kernel to find possible coding faults.
    • Read count bytes from a device starting at position f_pos.
    • Update the f_pos by adding the number bytes successfully read.
    • A return value less than count does not mean that an error has occurred.

  • Write (e.g., write(fd, buff, 20))

    /**
 * pcd_write() - Pseudo char driver method to handle write() system call
 * @filp: Pointer to a file object
 * @buff: Pointer of user buffer
 * @count: Write count given by user
 * @f_pos: Pointer of current file position from which the write has to begin
 *
 * Writes a device file @count byte of data from @f_pos, returns the data back to 
 * @buff (user) and updates @f_pos. Returns the number of bytes successfully written,
 * appropriate error code (negative value) otherwise.
 */
ssize_t pcd_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos);
    • Write count bytes into the device starting at position f_pos.
    • Update the f_pos by adding the number of bytes successfully written

  • llseek (e.g., llseek(fd, buff, 20))

    Used to alter the f_pos.

    /**
 * pcd_lseek() - Pseudo char driver method to handle llseek() system call
 * @filp: Pointer to a file object
 * @off: Offset value
 * @whence: Origin 
 * 		- SEEK_SET: The file offset is set to @off bytes
 *		- SEEK_CUR: The file offset is set to its current location plus @off bytes
 * 		- SEEK_END: The file offset is set to the size of the file plus @off bytes
 *
 * Updates the file poitner by using @off and @whence information. Returns the newly updated 
 * file position on success, error otherwise.
 */
loff_t pcd_lseek(struct file *filp, loff_t off, int whence);

Check

  • Do make host and see if you are getting pcd.ko from pcd.c.

  • Insert the LKM (sudo insmod pcd.ko), run dmesg and see if the messages are getting printed.

  • Check /sys/class/ if you see pcd_class/ which should be created by the class_create() kernel function.

    • pcd_class/ directory should contain pcd (the same name as your LKM) directory
    • pcd_class/pcd/ directory should contain dev file whose contents is the device number <major:minor>
    • pcd_class/pcd/ directory should also contain uevent whose contents is major number, minor number and devname.

    udev creates the device file under /dev directory according to these details which are created and populated by the device_create() kernel function.

  • Check /dev/ if you see the device file pcd.

  • Remove the LKM (sudo rmmod pcd.ko), run dmesg and see if the messages are getting printed.