EBC Device Drivers

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thumb‎ Embedded Linux Class by Mark A. Yoder


Chapter 8 of the text [1] gives a nice example of a minimal device driver. The purpose of this lab is to implement that driver.

Minimal Device Driver Example


Since you are going to make several changes, it's a good idea to use git to help keep track of them. First check the current status

host$ cd bb-kernel/KERNEL
host$ git status
# On branch v3.8.13-bone28
nothing to commit (working directory clean)

Good, so far no changes have been made. Now create a new branch.

host$ git checkout -b hello1
host$ git status
# On branch hello1
nothing to commit (working directory clean)

This creates a new branch called hello1 and checks it out.

Follow the 5 steps given in Section 8.1.4 on page 205. Hint: details are given in the pages that follow 205. exercises/modules has a copy of hello1.c. Once finished you will have a file called hello1.c in .../drivers/char/examples and have the kernel configure file and Makefile updated for the new driver. See Section 4.4 on page 89 for help with modifying the config files.

Note: There is a typo in Listing 8-2.

Assuming you have done the steps from the book, check git's status

host$ cd bb-kernel/KERNEL/drivers/char/
host$ git status
# On branch hello1
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#	modified:   Kconfig
#	modified:   Makefile
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#	examples/
no changes added to commit (use "git add" and/or "git commit -a")

Now add the files you just edited/created.

host$ git add Kconfig Makefile examples/Makefile examples/hello1.c
host$ git status
# On branch hello1
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#	modified:   Kconfig
#	modified:   Makefile
#	new file:   examples/Makefile
#	new file:   examples/hello1.c
host$ git commit -m "Files for hello1 kernel module"
[hello1 99346d5] Files for hello1 kernel module
 4 files changed, 33 insertions(+)
 create mode 100644 drivers/char/examples/Makefile
 create mode 100644 drivers/char/examples/hello1.c

If you have created the crossCompileEnv.sh file and sourced it, all you have to do to make the modules is cd to the top of the kernel directory and then:

host$ cd Your path/bb-kernel
host$ tools/rebuild.sh

Mine took a while the first time as it compiles all the modules. The second it only took 34 seconds.

Moving to Beagle

Here are two choices for moving your module to the Beagle. The first is the complete way, the second is the fast way.

Option 1, copy everything

If you want to install all the modules that are created

host$ make INSTALL_MOD_PATH=~/BeagleBoard modules_install

This will create lib directory in ~/BeagleBoard with everything that goes in /lib on the Beagle. Then

host$ rsync --progress -avhe ssh ~/BeagleBoard/lib root@beagle:/

Could take a while to transfer.

Option 2, copy just what you need

Here's a faster way, just copy the new file you created

host$ scp …/drivers/char/examples/hello1.ko root@bone:.

On the Bone

bone$ cd /lib/modules/`uname -r`/kernel/drivers/char/
bone$ mkdir examples
bone$ cd examples
bone$ mv ~/hello1.ko  .

Now build a new dependencies file

bone$ depmod -a

I suggest putting the scp command in the Makefile since you may use it several times while developing your code.

Inserting your module

See if your module is there

bone$ modinfo hello1
filename:       /lib/modules/3.8.13-bone28/kernel/drivers/char/hello1.ko
license:        GPL
description:    Hello World Example
author:         Chris Hallinan
srcversion:     602AF803EAAB89A5CB5DAD7
intree:         Y
vermagic:       3.8.13+ SMP mod_unload modversions ARMv7 p2v8 

That looks good, now modprobe the module and check the log file.

bone$ modprobe hello1
bone$ dmesg | tail -4
[    9.106206] snd-usb-audio 1-1:1.0: usb_probe_interface
[    9.106244] snd-usb-audio 1-1:1.0: usb_probe_interface - got id
[    9.813239] usbcore: registered new interface driver snd-usb-audio
[  109.308551] Hello Example Init

You should see your Init message. And then...

bone$ rmmod hello1
bone$ dmesg | tail -4
[    9.106244] snd-usb-audio 1-1:1.0: usb_probe_interface - got id
[    9.813239] usbcore: registered new interface driver snd-usb-audio
[  109.308551] Hello Example Init
[  241.037368] Hello Example Exit

should show your Exit message.

Documenting your work with a patch file

If you set up a branch on git as shown above, capturing all your changes in a patch file is easy. First be sure you have checked in everything.

host$ cd bb-kernel/KERNEL/drivers/char
host$ git status
# On branch hello1
nothing to commit (working directory clean)

Good, everything is committed. Now just

host$ git format-patch v3.8.13-bone28 --stdout > hello1.patch

Where v3.8.13-bone28 is the branch you were on before creating the hello1 branch. This creates a file, hello1.patch, that captures all the changes you made in installing your hello1.c module. how-to-create-and-apply-a-patch-with-git shows how someone can take your patch file and apply it to their kernel, therefore getting your hello1.c kernel module.

Improvements to the Code in the Book

The code in Listing 8-10 is incomplete. If you compile it as is and load the module, it will work, but if you then remove the module and try to reinsert it it may fail with an error. The reason for this is that the module is registered with the kernel on load with a command like this:


But it is never unregistered when the module is unloaded. To properly unload the module, add this line of code to your module's exit function

unregister_chrdev(MAJOR_NUMBER, NAME);

This will properly unregister the module from the kernel and allow it to be inserted and removed from the kernel at will without restarting your system in between.

Listing 8-10 also uses the ioctl field in the file_operations struct. Newer kernels have removed this. If the code from the listing complains about ioctl being an unknown field, use unlocked_ioctl in its place:

struct file_operations hello_fops = {
    owner:           THIS_MODULE,
    read:            hello_read,
    write:           hello_write,
    unlocked_ioctl:  hello_ioctl,
    open:            hello_open,
    release:         hello_release,

Driver Methods

Section 8.3 on page 217 gives a longer example of how to use the file interface with modules. Implement the example. Be sure to fix the unsigned int format error, and make sure your exit function unregisters the device (unlike the Listing). I've created a build.sh script based on Section 8.5 of the text that makes building much faster.

host$ cd exercises/modules
host$ ./build.sh
make: Entering directory `/home/yoder/BeagleBoard/linux-dev/KERNEL'
  CC [M]  /home/yoder/BeagleBoard/exercises/modules/hello1.o
  CC [M]  /home/yoder/BeagleBoard/exercises/modules/hello2.o
  CC [M]  /home/yoder/BeagleBoard/exercises/modules/hello3.o
  Building modules, stage 2.
  MODPOST 3 modules
  CC      /home/yoder/BeagleBoard/exercises/modules/hello1.mod.o
  LD [M]  /home/yoder/BeagleBoard/exercises/modules/hello1.ko
  CC      /home/yoder/BeagleBoard/exercises/modules/hello2.mod.o
  LD [M]  /home/yoder/BeagleBoard/exercises/modules/hello2.ko
  CC      /home/yoder/BeagleBoard/exercises/modules/hello3.mod.o
  LD [M]  /home/yoder/BeagleBoard/exercises/modules/hello3.ko
make: Leaving directory `/home/yoder/BeagleBoard/linux-dev/KERNEL'
Warning: Permanently added 'bone,' (RSA) to the list of known hosts.
hello1.ko                                     100% 3571     3.5KB/s   00:00    
hello2.ko                                     100% 4344     4.2KB/s   00:00    
hello3.ko                                     100% 6097     6.0KB/s   00:00 

It just compiled all three modules and scp'ed them to the bone. A quick listing shows many intermediate files were created.

host$ ls -a
.               .hello1.mod.o.cmd  .hello2.mod.o.cmd  .hello3.mod.o.cmd
..              hello1.o           hello2.o           hello3.o
build.sh        .hello1.o.cmd      .hello2.o.cmd      .hello3.o.cmd
hello1.c        hello2.c           hello3.c           Makefile
hello1.ko       hello2.ko          hello3.ko          modules.order
.hello1.ko.cmd  .hello2.ko.cmd     .hello3.ko.cmd     Module.symvers
hello1.mod.c    hello2.mod.c       hello3.mod.c       .tmp_versions
hello1.mod.o    hello2.mod.o       hello3.mod.o

Clean the extra files up with

host$ ./clean.sh

Now go to the Beagle and move the .ko file to the right place.

beagle$ cd /lib/modules/3.8.13-bone28/kernel/drivers/char/examples
beagle$ cp ~/hello3.ko .

Rebuild the dependency file and insert the module.

beagle$ depmod -a
beagle$ modprobe hello3

Create a node for the module and test it.

beagle$ mknod /dev/hello3 c 234 0
beagle$ cat /dev/hello3 
beagle$ dmesg | tail -4
[   71.219651] [drm:output_poll_execute], [CONNECTOR:5:HDMI-A-1] status updated from 2 to 2
[   76.099158] hello_open: successful
[   76.099260] hello_read: returning zero bytes
[   76.099294] hello_release: successful

It working!

Some Questions

  • The major device number 234 is part of a range of unassigned numbers. What is the range?
  • What's the new line added to hello_init do?
  • What does mknod do?
  • Once your device is running try $ cat /proc/devices. Do you see your device?

Optional Driver Work

Chapter 3 of Linux Device Drivers by Corbet, Rubini and Kroah-Hartman ([2]) gives some more details on device drivers. Our text uses an older, static, method for major device number allocation. The book, referenced above, uses the newer dynamic allocation.

  • Convert the example in our text to use the newer method. It's only a couple of additional lines, but you will have to read the book to know how to do it.
  • Modify the scull_load script (call it hello_load) on page 47, of chapter 3, to load your module. Hint: the back quotes are missing in this line in the text:
major=`awk "\\$2==\"$module\" {print \\$1}" /proc/devices)`

  • Test it with use-hello.c from page 222 of Embedded Linux Primer.
  • Write a hello_unload script that will rmmod the driver and remove the nodes in /dev
  • Modify hello.c to pass the major device number in as a parameter during insmod.

Optional: Stretch time, I though these would be easy, but after reading up on them, they look rather involved.

  • How can your driver find what the minor device number is?
  • Modify the driver to return some characters when /dev/hello1 is read.


How to Write and Submit a Linux Kernel Patch

thumb‎ Embedded Linux Class by Mark A. Yoder