EBC Exercise 26 Device Drivers

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

Compiling
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$ git status 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 nothing to commit (working directory clean)
 * 1) On branch v3.8.13-bone28
 * 1) On branch hello1

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   in   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 linux-dev/KERNEL/drivers/char/ host$ git status # # # 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''' # # 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
 * 1) On branch hello1
 * 2) Changes not staged for commit:
 * 3)   (use "git add ..." to update what will be committed)
 * 4)   (use "git checkout -- ..." to discard changes in working directory)
 * 1) 	modified:  Kconfig
 * 2) 	modified:  Makefile
 * 1) Untracked files:
 * 2)   (use "git add ..." to include in what will be committed)
 * 1) 	examples/
 * 1) On branch hello1
 * 2) Changes to be committed:
 * 3)   (use "git reset HEAD ..." to unstage)
 * 1) 	modified:  Kconfig
 * 2) 	modified:  Makefile
 * 3) 	new file:  examples/Makefile
 * 4) 	new file:  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/linix-dev/KERNEL host$ source ~/crossCompileEnv.sh (Only need to run once per terminal session.) host$ make modules (or make -jX modules, where X = number of cores on host.)

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 Beagle beagle$ cd /lib/modules/3.8.13-bone28/kernel/drivers/char/ beagle$ mkdir examples beagle$ cd examples beagle$ mv ~/hello1.ko .

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

Now, insmod the module and check the log file.

beagle$ insmod hello1.ko beagle$ 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... beagle$ rmmod hello1 beagle$ 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 linux-dev/KERNEL/drivers/char host$ git status nothing to commit (working directory clean) Good, everything is committed. Now just host$ git format-patch v3.8.13-bone28 --stdout > hello1.patch 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.
 * 1) On branch hello1

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 will fail with an error like this: ERROR GOES HERE The reason for this is that the module is registered with the kernel on load with a command like this: register_chrdev(MAJOR_NUMBER, NAME, FILE_OPERATIONS*); 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  function unregisters the device (unlike the Listing). When compiling the  command be sure you are using the cross compiler for the ARM rather than the x86 compiler. If you sourced the crossCompilerEnv.sh file this should work:

host$ ${CROSS_COMPILE}gcc use-hello.c -o use-hello host$ file use-hello

The  command will tell you if you got the right compiler. Modify your  to make.

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  do?
 * What does  do?
 * Once your device is running try .  Do you see your device?

Optional Driver Work
Chapter 3 of Linux Device Drivers by Corbet, Rubini and Kroah-Hartman 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  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.

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  is read.

Reference
How to Write and Submit a Linux Kernel Patch