EBC Exercise 26 Device Drivers

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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

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 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.

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$ make modules

Mine took a while the first time as it compiles all the modules. The second it only took 31 seconds.

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.

Moving to Beagle

On the beagle edit /lib/modules/2.6.32/modules.dep and add

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

Then copy the file …/drivers/char/examples/hello1.ko on the host computer to /lib/modules/2.6.32/kernel/drivers/char/examples/ 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 scp command in the Makefile since you may use it several times while developing your code.

Now, on the Beagle, modprobe the module and check the log file.

beagle$ /sbin/modprobe hello1
beagle$ dmesg | tail -4

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

beagle$ /sbin/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 hello1.c 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 exit function unregisters the device (unlike the Listing). When compiling the use-hello 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 file command will tell you if you got the right compiler. Modify your Makefile to make use-hello.

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?

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.