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
Follow the 5 steps given in Section 8.1.4 on page 205. You can get a copy of Listing 8-1 here (ECE497_Listings_for_Embedded_Linux_Primer_Chapter_8). 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.

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$ 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 31 seconds.

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.

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, };

Moving to Beagle
On the beagle edit  and add

/lib/modules/2.6.32/kernel/drivers/char/examples/hello1.ko:

Then copy the file  on the host computer to   on your Beagle. This can be done with a single command though you may have to mkdir the char/examples directory on the Beagle first.

host$ cd …/drivers/char/examples host$ scp hello1.ko root@beagle:/lib/modules/2.6.32/kernel/drivers/char/examples

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

Now, on the Beagle, add the module to modules.dep

beagle$ cd /lib/modules/2.6.32 beagle$ gedit modules.dep

Add the path to your new module and save. You will also have to move the modules.dep.bin, otherwise the beagle will read from it rather than the modules.dep.

beagle$ mv modules.dep.bin modules.dep.bin.orig

Now, modprobe the module and check the log file.

beagle$ modprobe hello1 beagle$ dmesg | tail -4

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

beagle$ modprobe -r hello1 beagle$ dmesg | tail -4

should show your Exit message.

Module Parameters
Section 8.1.7 on page 211 of the text shows how to pass a parameter to a module. Modify your  to take a parameter as shown.

Modify it so you can pass two parameters.

Module Utilities
Play with the mod utilities listed in section 8.2 on page 212.

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