Difference between revisions of "EBC Exercise 26 Device Drivers"
m (→Minimal Device Driver Example: Switched with compiling) |
m (→Part 2: A Character Device: Added) |
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Line 81: | Line 81: | ||
[Sep24 16:54] EBB: Goodbye 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 [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. | |
− | + | ||
− | + | 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 = strlen(message); // 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/4.14.67-ti-r73/build/ M=/home/debian/exploringBB/extras/kernel/ebbchar modules | ||
+ | make[1]: Entering directory '/usr/src/linux-headers-4.14.67-ti-r73' | ||
+ | CC [M] /home/debian/exploringBB/extras/kernel/ebbchar/ebbchar.o | ||
+ | Building modules, stage 2. | ||
+ | MODPOST 1 modules | ||
+ | CC /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-4.14.67-ti-r73' | ||
+ | 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. | ||
== Driver Methods == | == Driver Methods == |
Revision as of 14:12, 24 September 2018
Embedded Linux Class by Mark A. Yoder
Derek Molloy's excelent 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.
Contents
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/4.14.67-ti-r73/build/ M=/home/debian/exploringBB/extras/kernel/hello modules make[1]: Entering directory '/usr/src/linux-headers-4.14.67-ti-r73' CC [M] /home/debian/exploringBB/extras/kernel/hello/hello.o Building modules, stage 2. MODPOST 1 modules CC /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-4.14.67-ti-r73' bone$ ls hello.c hello.mod.c hello.o modules.order hello.ko hello.mod.o Makefile Module.symvers
Your newly compiled kernel module is in hello.ko.
Inserting your module
See if your module is there
bone$ modinfo hello 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: 4.14.67-ti-r73 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$ 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$ 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 = strlen(message); // 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/4.14.67-ti-r73/build/ M=/home/debian/exploringBB/extras/kernel/ebbchar modules make[1]: Entering directory '/usr/src/linux-headers-4.14.67-ti-r73' CC [M] /home/debian/exploringBB/extras/kernel/ebbchar/ebbchar.o Building modules, stage 2. MODPOST 1 modules CC /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-4.14.67-ti-r73' 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.
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,192.168.7.2' (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 ([5]) 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.
Reference
How to Write and Submit a Linux Kernel Patch
Embedded Linux Class by Mark A. Yoder