Difference between revisions of "EBC Exercise 26 Device Drivers"

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[[Category:ECE497]]
 
[[Category:ECE497]]
 +
{{YoderHead}}
  
Chapter 8 of the text [http://www.amazon.com/Embedded-Linux-Primer-Practical-Real-World/dp/0137017839] gives a nice example of a minimal device driver.  The purpose of this lab is to implement that driver.
+
Derek Molloy's excellent book Exploring BeagleBone [http://exploringbeaglebone.com/] has an Extra Content section [http://exploringbeaglebone.com/kernelprogramming/] on Linux Kernel Programming.  Part 1 [http://derekmolloy.ie/writing-a-linux-kernel-module-part-1-introduction/] is a nice example of a writing a minimal kernel module.
  
== Minimal Device Driver Example ==
+
Here are instructions for compiling the example on the Bone.
  
=== Compiling ===
+
== Cloning Source and Compiling ==
 +
You need to load the correct kernel headers on the bone before you can compile the driver.
  
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 <code>hello1.c</code> in <code>.../drivers/char/examples</code> 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.
+
  bone$ '''time sudo apt install linux-headers-`uname -r`'''
  
Note: There is a typo in Listing 8-2.
+
Note: Those are back quotes (top left on the keyboard, above the TAB key) around '''uname -r'''.  
  
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:
+
The '''uname''' command looks up the number of the kernel that's currently running.  The back quotes
 +
take that number and past it after '''linux-headers-''' and does an '''apt install''' on it.  A couple minutes later you have all the headers loaded.
  
host$ '''make modules'''
+
Now load the examples.
  
Mine took a while the first time as it compiles all the modules. The second it only took 31 seconds.
+
bone$ '''git clone https://github.com/derekmolloy/exploringBB.git'''
  
====Improvements to the Code in the Book====
+
Now you are ready to run the examples.
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:
 
<pre>
 
ERROR GOES HERE
 
</pre>
 
The reason for this is that the module is registered with the kernel on load with a command like this:
 
<pre>
 
register_chrdev(MAJOR_NUMBER, NAME, FILE_OPERATIONS*);
 
</pre>
 
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
 
<pre>
 
unregister_chrdev(MAJOR_NUMBER, NAME);
 
</pre>
 
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 ===
+
== Part 1: Introduction - Minimal Device Driver Example ==
 +
Change to the correct directory and make.
  
On the beagle edit <code>/lib/modules/2.6.32/modules.dep</code> and add
+
bone$ '''cd exploringBB/extras/kernel/hello'''
 
+
bone$ '''make'''
  /lib/modules/2.6.32/kernel/drivers/char/examples/hello1.ko:
+
make -C /lib/modules/5.10.145-ti-r55/build/ M=/home/debian/exploringBB/extras/kernel/hello modules
 +
make[1]: Entering directory '/usr/src/linux-headers-5.10.145-ti-r55'
 +
  CC [M]  /home/Debian/exploringBB/extras/kernel/hello/hello.o
 +
  MODPOST /home/Debian/exploringBB/extras/kernel/hello/Module.symvers
 +
  CC [M] /home/Debian/exploringBB/extras/kernel/hello/hello.mod.o
 +
  LD [M]  /home/Debian/exploringBB/extras/kernel/hello/hello.ko
 +
make[1]: Leaving directory '/usr/src/linux-headers-5.10.145-ti-r55'
 +
 +
real 0m11.890s
 +
user 0m7.320s
 +
sys 0m3.373s
 +
bone$ '''ls'''
 +
hello.c  hello.mod    hello.mod.o  Makefile      Module.symvers
 +
hello.ko hello.mod.c  hello.o      modules.order
  
Then copy the file <code>…/drivers/char/examples/hello1.ko</code> on the host computer to <code>/lib/modules/2.6.32/kernel/drivers/char/examples/</code> 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.
+
Your newly compiled kernel module is in '''hello.ko'''.
  
host$ '''cd …/drivers/char/examples'''
+
=== Inserting your module ===
host$ '''scp hello1.ko root@beagle:/lib/modules/2.6.32/kernel/drivers/char/examples'''
 
  
I suggest putting the <code>scp</code> command in the <code>Makefile</code> since you may use it several times while developing your code.
+
See if your module is there
 +
bone$ '''modinfo hello.ko'''
 +
filename:      /home/debian/exploringBB/extras/kernel/hello/hello.ko
 +
version:        0.1
 +
description:    A simple Linux driver for the BBB.
 +
author:        Derek Molloy
 +
license:        GPL
 +
srcversion:    0DD9FE0DE42157F9221E608
 +
depends:       
 +
name:          hello
 +
vermagic:      5.10.145-ti-r55 SMP preempt mod_unload modversions ARMv7 p2v8
 +
parm:          name: The name to display in /var/log/kern.log (charp)
  
Now, on the Beagle, modprobe the module and check the log file.
+
That looks good, now '''insmod''' the module and check the log file.
  
  beagle$ '''/sbin/modprobe hello1'''
+
  bone$ '''sudo insmod hello.ko'''
  beagle$ '''dmesg | tail -4'''
+
  bone$ dmesg -H | tail -1
 +
[  +2.857480] EBB: Hello world from the BBB LKM!
  
 
You should see your Init message.  And then...
 
You should see your Init message.  And then...
 
+
  bone$ '''sudo rmmod hello'''
  beagle$ '''/sbin/modprobe -r hello1'''
+
  bone$ '''dmesg -H | tail -2'''
  beagle$ '''dmesg | tail -4'''
+
[  +4.182591] EBB: Hello world from the BBB LKM!
 +
[  +3.542350] EBB: Goodbye world from the BBB LKM!
  
 
should show your Exit message.
 
should show your Exit message.
  
== Module Parameters ==
+
=== Passing Parameters ===
 +
You can pass parameters when inserting a module. 
  
Section 8.1.7 on page 211 of the text shows how to pass a parameter to a moduleModify your <code>hello1.c</code> to take a parameter as shown.
+
bone$ '''sudo insmod hello.ko name="Prof.Yoder"'''
 +
bone$ '''dmesg -H | tail -1'''
 +
[Sep24 16:53] EBB: Hello Prof.Yoder from the BBB LKM!
 +
bone$ '''sudo rmmod hello'''
 +
bone$ '''dmesg -H | tail -2'''
 +
[Sep24 16:53] EBB: Hello Prof.Yoder from the BBB LKM!
 +
  [Sep24 16:54] EBB: Goodbye Prof.Yoder from the BBB LKM!
  
Modify it so you can pass two parameters.
+
== Part 2: A Character Device ==
  
== Module Utilities ==
+
Part 2 of Molloy's example [http://derekmolloy.ie/writing-a-linux-kernel-module-part-2-a-character-device/] is a character device.  The code needs a slight change before it will run.
  
Play with the mod utilities listed in section 8.2 on page 212.
+
bone$ '''cd exploringBB/extras/kernel/ebbchar'''
  
== Driver Methods ==
+
Edit '''ebbchar.c''' and make the following changes.
  
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 <code>exit</code> function unregisters the device (unlike the Listing). When compiling the <code>use-hello</code> 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:
+
bone$ '''git diff ebbchar.c'''
 +
diff --git a/extras/kernel/ebbchar/ebbchar.c b/extras/kernel/ebbchar/ebbchar.c
 +
index 771b859..f869859 100644
 +
--- a/extras/kernel/ebbchar/ebbchar.c
 +
+++ b/extras/kernel/ebbchar/ebbchar.c
 +
@@ -142,7 +142,9 @@ static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *of
 +
  *  @param offset The offset if required
 +
  */
 +
  static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset){
 +
-  sprintf(message, "%s(%zu letters)", buffer, len);  // appending received string with its length
 +
  +  unsigned long ret;
 +
  +  // sprintf(message, "%s(%zu letters)", buffer, len);  // appending received string with its length
 +
+  ret = copy_from_user(message, buffer, len);
 +
  +  size_of_message = len;                // store the length of the stored message
 +
    printk(KERN_INFO "EBBChar: Received %zu characters from the user\n", len);
 +
    return len;
  
host$ '''${CROSS_COMPILE}gcc use-hello.c -o use-hello'''  
+
Add the lines starting with '''+''' and comment out the '''sprintf''' starting with '''-'''.
host$ '''file use-hello'''
 
  
The <code>file</code> command will tell you if you got the right compiler. Modify your <code>Makefile</code> to make <code>use-hello</code>.
+
Now make and insert.
  
Some questions...
+
bone$ '''make'''
* The major device number 234 is part of a range of unassigned numbersWhat is the range?
+
make -C /lib/modules/5.10.145-ti-r55/build/ M=/home/debian/exploringBB/extras/kernel/ebbchar modules
* What's the new line added to <code>hello_init</code> do?
+
make[1]: Entering directory '/usr/src/linux-headers-5.10.145-ti-r55'
* What does <code>mknod</code> do?
+
  CC [M]  /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.o
* Once your device is running try <code>$ cat /proc/devices</code>Do you see your device?
+
  MODPOST /home/Debian/exploringBB/extras/kernel/ebbchar/Module.symvers
 +
  CC [M] /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.mod.o
 +
  LD [M]  /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.ko
 +
make[1]: Leaving directory '/usr/src/linux-headers-5.10.145-ti-r55'
 +
cc testebbchar.c -o test
 +
 +
  bone$ '''sudo insmod ebbchar.ko'''
 +
bone$ '''dmesg -H | tail -4
 +
[Sep24 17:08] EBBChar: Initializing the EBBChar LKM
 +
[  +0.011910] EBBChar: registered correctly with major number 241
 +
[  +0.016113] EBBChar: device class registered correctly
 +
[  +0.010024] EBBChar: device class created correctly
  
=== Optional Driver Work ===
+
Now test it.
 +
bone$ '''sudo ./test'''
 +
Starting device test code example...
 +
Type in a short string to send to the kernel module:
 +
'''This is a test!'''
 +
Writing message to the device [This is a test!].
 +
Press ENTER to read back from the device...
 +
 +
Reading from the device...
 +
The received message is: [This is a test!]
 +
End of the program
 +
bone$ '''dmesg -H | tail -8'''
 +
[Sep24 17:08] EBBChar: Initializing the EBBChar LKM
 +
[  +0.011910] EBBChar: registered correctly with major number 241
 +
[  +0.016113] EBBChar: device class registered correctly
 +
[  +0.010024] EBBChar: device class created correctly
 +
[Sep24 17:09] EBBChar: Device has been opened 1 time(s)
 +
[  +9.333771] EBBChar: Received 15 characters from the user
 +
[  +1.185798] EBBChar: Sent 15 characters to the user
 +
[  +0.009778] EBBChar: Device successfully closed
  
Chapter 3 of ''Linux Device Drivers'' by Corbet, Rubini and Kroah-Hartman ([http://www.rose-hulman.edu/~yoder/Beagle/]) 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. 
+
Look over '''ebbchar.c''' and '''testebbchar.c''' to see how the user space and the kernel interact.
* 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)`
+
== Reference ==
+
 
* Test it with <code>use-hello.c</code> from page 222 of ''Embedded Linux Primer''.
+
[http://www.cnx-software.com/2011/08/19/how-to-write-and-submit-a-linux-kernel-patch/ How to Write and Submit a Linux Kernel Patch]
* 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 <code>insmod</code>.
 
  
Optional: Stretch time, I though these would be easy, but after reading up on them, they look rather involved.
+
{{YoderFoot}}
* How can your driver find what the minor device number is?
 
* Modify the driver to return some characters when <code>/dev/hello1</code> is read.
 

Latest revision as of 09:48, 26 December 2022

thumb‎ Embedded Linux Class by Mark A. Yoder


Derek Molloy's excellent book Exploring BeagleBone [1] has an Extra Content section [2] on Linux Kernel Programming. Part 1 [3] is a nice example of a writing a minimal kernel module.

Here are instructions for compiling the example on the Bone.

Cloning Source and Compiling

You need to load the correct kernel headers on the bone before you can compile the driver.

bone$ time sudo apt install linux-headers-`uname -r`

Note: Those are back quotes (top left on the keyboard, above the TAB key) around uname -r.

The uname command looks up the number of the kernel that's currently running. The back quotes take that number and past it after linux-headers- and does an apt install on it. A couple minutes later you have all the headers loaded.

Now load the examples.

bone$ git clone https://github.com/derekmolloy/exploringBB.git

Now you are ready to run the examples.

Part 1: Introduction - Minimal Device Driver Example

Change to the correct directory and make.

bone$ cd exploringBB/extras/kernel/hello
bone$ make
make -C /lib/modules/5.10.145-ti-r55/build/ M=/home/debian/exploringBB/extras/kernel/hello modules
make[1]: Entering directory '/usr/src/linux-headers-5.10.145-ti-r55'
  CC [M]  /home/Debian/exploringBB/extras/kernel/hello/hello.o
  MODPOST /home/Debian/exploringBB/extras/kernel/hello/Module.symvers
  CC [M]  /home/Debian/exploringBB/extras/kernel/hello/hello.mod.o
  LD [M]  /home/Debian/exploringBB/extras/kernel/hello/hello.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.10.145-ti-r55'

real	0m11.890s
user	0m7.320s
sys	0m3.373s
bone$ ls
hello.c   hello.mod    hello.mod.o  Makefile       Module.symvers
hello.ko  hello.mod.c  hello.o      modules.order

Your newly compiled kernel module is in hello.ko.

Inserting your module

See if your module is there

bone$ modinfo hello.ko
filename:       /home/debian/exploringBB/extras/kernel/hello/hello.ko
version:        0.1
description:    A simple Linux driver for the BBB.
author:         Derek Molloy
license:        GPL
srcversion:     0DD9FE0DE42157F9221E608
depends:        
name:           hello
vermagic:       5.10.145-ti-r55 SMP preempt mod_unload modversions ARMv7 p2v8 
parm:           name: The name to display in /var/log/kern.log (charp)

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

bone$ sudo insmod hello.ko
bone$ dmesg -H | tail -1

[ +2.857480] EBB: Hello world from the BBB LKM!

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

bone$ sudo rmmod hello
bone$ dmesg -H | tail -2
[  +4.182591] EBB: Hello world from the BBB LKM!
[  +3.542350] EBB: Goodbye world from the BBB LKM!

should show your Exit message.

Passing Parameters

You can pass parameters when inserting a module.

bone$ sudo insmod hello.ko name="Prof.Yoder"
bone$ dmesg -H | tail -1
[Sep24 16:53] EBB: Hello Prof.Yoder from the BBB LKM!
bone$ sudo rmmod hello
bone$ dmesg -H | tail -2
[Sep24 16:53] EBB: Hello Prof.Yoder from the BBB LKM!
[Sep24 16:54] EBB: Goodbye Prof.Yoder from the BBB LKM!

Part 2: A Character Device

Part 2 of Molloy's example [4] is a character device. The code needs a slight change before it will run.

bone$ cd exploringBB/extras/kernel/ebbchar

Edit ebbchar.c and make the following changes.

bone$ git diff ebbchar.c
diff --git a/extras/kernel/ebbchar/ebbchar.c b/extras/kernel/ebbchar/ebbchar.c
index 771b859..f869859 100644
--- a/extras/kernel/ebbchar/ebbchar.c
+++ b/extras/kernel/ebbchar/ebbchar.c
@@ -142,7 +142,9 @@ static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *of
  *  @param offset The offset if required
  */
 static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset){
-   sprintf(message, "%s(%zu letters)", buffer, len);   // appending received string with its length
+   unsigned long ret;
+   // sprintf(message, "%s(%zu letters)", buffer, len);   // appending received string with its length
+   ret = copy_from_user(message, buffer, len);
+   size_of_message = len;                 // store the length of the stored message
    printk(KERN_INFO "EBBChar: Received %zu characters from the user\n", len);
    return len;

Add the lines starting with + and comment out the sprintf starting with -.

Now make and insert.

bone$ make
make -C /lib/modules/5.10.145-ti-r55/build/ M=/home/debian/exploringBB/extras/kernel/ebbchar modules
make[1]: Entering directory '/usr/src/linux-headers-5.10.145-ti-r55'
  CC [M]  /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.o
  MODPOST /home/Debian/exploringBB/extras/kernel/ebbchar/Module.symvers
  CC [M]  /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.mod.o
  LD [M]  /home/Debian/exploringBB/extras/kernel/ebbchar/ebbchar.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.10.145-ti-r55'
cc testebbchar.c -o test

bone$ sudo insmod ebbchar.ko
bone$ dmesg -H | tail -4
[Sep24 17:08] EBBChar: Initializing the EBBChar LKM
[  +0.011910] EBBChar: registered correctly with major number 241
[  +0.016113] EBBChar: device class registered correctly
[  +0.010024] EBBChar: device class created correctly

Now test it.

bone$ sudo ./test
Starting device test code example...
Type in a short string to send to the kernel module:
This is a test!
Writing message to the device [This is a test!].
Press ENTER to read back from the device...

Reading from the device...
The received message is: [This is a test!]
End of the program
bone$ dmesg -H | tail -8
[Sep24 17:08] EBBChar: Initializing the EBBChar LKM
[  +0.011910] EBBChar: registered correctly with major number 241
[  +0.016113] EBBChar: device class registered correctly
[  +0.010024] EBBChar: device class created correctly
[Sep24 17:09] EBBChar: Device has been opened 1 time(s)
[  +9.333771] EBBChar: Received 15 characters from the user
[  +1.185798] EBBChar: Sent 15 characters to the user
[  +0.009778] EBBChar: Device successfully closed

Look over ebbchar.c and testebbchar.c to see how the user space and the kernel interact.

Reference

How to Write and Submit a Linux Kernel Patch




thumb‎ Embedded Linux Class by Mark A. Yoder