12 July 2020

# Kernel modules, device drivers and Device Tree

by Mike Krinkin

I continue going through Bootlin training materials on embedded systems and Linux Kernel. In the previous post I covered the environment setup, so now we should be able to access the board and share files between the board and the host.

In this article I’m going to try to actually create a few simple Linux Kernel modules, build them for the BeagleBone Black or BeagleBone Black Wireless board and test them on the actual hardware.

# Creating and building a module

I will start with the simplest module to show how to build the modules and then load them on the board. The module will do nothing useful, the only thing we’d need from it is to be able to verify that it loads and unloads successfully. In order to do that we will just print some log messages when the module loads and unloads.

Here is the code:

// SPDX-License-Identifier: GPL-2.0
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/utsname.h>

static int __init version_init(void)
{
utsname()->sysname,
utsname()->release,
utsname()->version);
return 0;
}

static void __exit version_exit(void)
{
}

module_init(version_init);
module_exit(version_exit);
MODULE_DESCRIPTION("Linux Kernel Version module");
MODULE_AUTHOR("Krinkin Mike <krinkin.m.u@gmail.com>");


There is just a couple of things in this code relevant for now:

• functions version_init and version_exit that are called when the module is loaded and unloaded correspondingly
• macroses module_init and module_exit is how we tell the kernel what function have to be called when the module is loaded and unloaded

Inside the version_init and version_exit function we simply print some messages to the kernel log using pr_alert.

We have the code ready and can move to the main part - building and loading the module.

Linux kernel uses a bit involved build system called Kbuild. Kbuild does not appear to be a completely new build system (like Bazel, Maven or CMake) and instead it’s more of an organization of [Make] files. Therefore it’s driven by makefiles that follow a certain structure. Let’s take a look at the Makefile for our simple module:

ifneq ($(KERNELRELEASE),) obj-m := version.o else KDIR ?= /lib/modules/uname -r/build default:$(MAKE) -C $(KDIR) M=$$PWD endif  The part relevant for Kbuild itself is just one line: obj-m := version.o  This one line magically tells the kernel build system that it should build a module and version.c is the only source file for the module. However as I mentioned before, kernel build system is a bit involved, so just calling make on that would not work. The build system is expected to build all kernel sources, even if they are part of a module, together. So individual make files are expeted to be included by the Kbuild when you run make in the top level directory of the kernel sources. That default target in the Makefile above executes a command that can build the module outside of the kernel source tree by itself. All you need to specify is the path to the kernel sources in the KDIR variable. In order to use this Makefile to compile for the BeagleBone Black we’d need to specify the architecture for the build, the toolchain and where the rest of the kernel code lives: export ARCH=arm export CROSS_COMPILE=arm-linux-gnueabi- export KDIR=/home/kmu/ws/linux make  NOTE: /home/kmu/ws/linux is where I store the Linux source code I used to build the kernel for the board. If build is successful we should see quite a few build artifacts in the directory with the module: ls -al total 328 drwxr-xr-x 3 kmu kmu 4096 Jul 12 14:58 . drwxr-xr-x 4 kmu kmu 4096 Jul 12 13:04 .. -rw-r--r-- 1 kmu kmu 8 Jul 12 13:07 built-in.a -rw-r--r-- 1 kmu kmu 163 Jul 12 13:07 .built-in.a.cmd -rw-r--r-- 1 kmu kmu 73044 Jul 12 12:39 .cache.mk -rw-r--r-- 1 kmu kmu 136 Jul 12 12:59 Makefile -rw-r--r-- 1 kmu kmu 40 Jul 12 13:07 modules.order -rw-r--r-- 1 kmu kmu 170 Jul 12 13:07 .modules.order.cmd -rw-r--r-- 1 kmu kmu 0 Jul 12 13:07 Module.symvers -rw-r--r-- 1 kmu kmu 215 Jul 12 13:07 .Module.symvers.cmd drwxr-xr-x 2 kmu kmu 4096 Jul 12 12:59 .tmp_versions -rw-r--r-- 1 kmu kmu 551 Jul 12 12:45 version.c -rw-r--r-- 1 kmu kmu 49848 Jul 12 13:07 version.dwo -rw-r--r-- 1 kmu kmu 21640 Jul 12 13:07 version.ko -rw-r--r-- 1 kmu kmu 284 Jul 12 13:07 .version.ko.cmd -rw-r--r-- 1 kmu kmu 40 Jul 12 13:07 version.mod -rw-r--r-- 1 kmu kmu 777 Jul 12 13:07 version.mod.c -rw-r--r-- 1 kmu kmu 147 Jul 12 13:07 .version.mod.cmd -rw-r--r-- 1 kmu kmu 33416 Jul 12 13:07 version.mod.dwo -rw-r--r-- 1 kmu kmu 10004 Jul 12 13:07 version.mod.o -rw-r--r-- 1 kmu kmu 26183 Jul 12 13:07 .version.mod.o.cmd -rw-r--r-- 1 kmu kmu 12944 Jul 12 13:07 version.o -rw-r--r-- 1 kmu kmu 30688 Jul 12 13:07 .version.o.cmd  The most important file for us at this stage is the version.ko. That’s the actual kernel module that we can load on the board: mkdir /home/kmu/ws/nfsroot/modules cp version.ko /home/kmu/ws/nfsroot/modules  NOTE: /home/kmu/ws/nfsroot is the path to the root file system that I share with the board over NFS. We can now access version.ko file on the board itslef and can even load it and check if the module works: cd /modules insmod version.ko rmmod version.ko dmes | tail [ 4.273057] TERM=linux [ 5.822049] g_ether gadget: packet filter 0e [ 5.822064] g_ether gadget: ecm req21.43 v000e i0000 l0 [ 33.778447] wlan-en-regulator: disabling [ 110.158443] random: crng init done [ 220.218673] version: loading out-of-tree module taints kernel. [ 220.225058] Linux version 5.8.0-rc3-next-20200703: #1 SMP Sat Jul 4 15:15:49 IST 2020 [ 240.576688] Goodbye, World!  If everything worked out, then we should see the log messages that our module prints during load and unload in the output of the dmesg command. You can find more details about Kbuild, the structure of Makefile for the Linux kernel modules and much more in the documentation in the kernel source code. The relevant parts are available in [Documentation/kbuild] (https://www.kernel.org/doc/html/latest/kbuild/index.html.) # Driver model Kernel modules allow to exectute almost arbitrary code in the kernel space. However for us the primary interest at this point is to create a driver. In the simplest terms the driver is just a piece of code that makes a device (real hardware device or some pseudo device) available to the users one way or another. Linux kernel provides a framework for creating device drivers. The goals of the framework is to separate platfrom dependent code from platform indpendent code, so that platform independent code can be reused between platforms. A typical example here would be some USB device. The same USB device (like a storage stick, keyboard, mouse, etc) can be used on x86, ARM or other architectures. While the details of implementation of USB support may change between platforms and boards, the driver for the same device ideally shoud not need to change. The driver model is organaized around three structures: Those structures are exteneded by driver developers normally using a particular way of implementing inheritance in C. This pattern is even documented in the container_of() secion of [Documentation/driver-api/driver-model/design-patterns.html.] (https://www.kernel.org/doc/html/latest/driver-api/driver-model/design-patterns.html) In practice while developing drivers using the supported buses, you will be interacting with the bus specific API and not with the three structures above directly. However the structure stays roughly the same: there will be bus specific driver and device structures. # Device Tree Bus is supposed to be responsible for discovering new devices and finding the right driver to handle the device. However, not all buses are capable of automatically discovering devices connected to it. In this case we need to tell the kernel what devices are there. One commonly used way to do that is Device Tree. Baiscally Device Tree statically describes the devices and connections between them. Compiled version of such description is provided to the kernel as input. The kernel parses and instantiates the devices described in the Device Tree. For the BeagleBone Black Wireless the top level Device Tree file lives in arch/arm/boot/dts/am335x-boneblack-wireless.dts in the Linux source code. This file is compiled into arch/arm/boot/dts/am335x-boneblack-wireless.dtb. The compiled file is what we provide to U-Boot via TFTP together with the kernel image itself. For a node in the Device Tree we can specify the compatible attribute. The string value of the attribute is used to find the right driver for the device. The way it works is that the driver itself encodes supported device identifiers that are checked against the value of the compatible attribute in the Device Tree. More information about using Device Tree in the Linux is available in the Documentation/devicetree # I2C Let’s throw in a bit of practice now to link all the pieces together. I2C is a very simple bus, physically it has just a couple of wires: one for data and one for clock signals. I2C is a master/slave bus, meaning that devices connected via I2C are not equal. All transactions are always initiated by the master, so no slave device will send any signals until asked. Each I2C device must have a unique address, which allows to connect multiple devices to the same bus. However the bus does not provide any capabilities to discover connected devices. Therefore, we’d need to provide some Device Tree description for the connected I2C devices. We will start with creating a new Device Tree file inside the arch/arm/boot/dts directory that contains exactly the same configuration as the Device Tree file for the BeagleBone Black Wireless board that I use. I’ll call the file am335x-boneblack-wireless-custom.dts: #include "am335x-boneblack-wireless-custom.dts"  To build this file we’d need to update the Makefile in the same directory: diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile index 74dd94e72848..e939150c4672 100644 --- a/arch/arm/boot/dts/Makefile +++ b/arch/arm/boot/dts/Makefile @@ -782,6 +782,7 @@ dtb-$(CONFIG_SOC_AM33XX) += \
am335x-bone.dtb \
am335x-boneblack.dtb \
am335x-boneblack-wireless.dtb \
+       am335x-boneblack-wireless-custom.dtb \
am335x-boneblue.dtb \
am335x-bonegreen.dtb \
am335x-bonegreen-wireless.dtb \


Now we should be able to build the kernel and generate the binary Device Tree:

cd /home/kmu/ws/linux
export ARCH=arm
export CROSS_COMPILE=arm-linux-gnueabi-
make dtbs


NOTE: again /home/kmu/ws/linux is where I store the kernel source code.

If everything is ok, we should have a compiled Device Tree file:

ls -l arch/arm/boot/dts/am335x-boneblack-wireless-custom.dtb
-rw-r--r-- 1 kmu kmu 62048 Jul 12 16:55 arch/arm/boot/dts/am335x-boneblack-wireless-custom.dtb


Since we didn’t make any changes to the Device Tree yet, the content of the compiled file should be very similar to the am335x-boneblack-wireless.dtb:

ls -l arch/arm/boot/dts/am335x-boneblack-wireless.dtb
-rw-r--r-- 1 kmu kmu 62048 Jul  4 15:15 arch/arm/boot/dts/am335x-boneblack-wireless.dtb


# I2C controller and Nunchuk device in Device Tree

Now let’s try to modify the Device Tree and introduce a configuration for a new I2C device. As a device for our experiments I will use the Nintendo Wiichuk, the device used in the Bootlin materials.

NOTE: in this post we will not communicate with the device itself, that will covered in the future posts. So it doesn’t really matter if you have a device or not at this point, I’m only refering to it here to be specific and setup a base for the future posts.

The device I2C address is 0x52. The address is hardcoded and enforced by the device itself. How do I know that the address is 0x52? This information is available in the Bootlin materials. Also, on the olimex.com where you can purchase the device there is a link to Arduino examples, where you can also find the I2C address of the device.

BeagleBone Black and BeagleBone Black Wireless have three I2C controllers. We will connect Wiichuk to the second I2C controller. This controller is not used for any builtin devices on the board and is not even properly configured in the default Device Tree, so we’d need to add the configuration for the controller and the device.

Let’s modify our custom Device Tree file to include the I2C controller node and a child node for the I2C controller with the Wiichuk device:

#include "am335x-boneblack-wireless.dts"

&i2c1 {
status = "okay";
clock-frequency = <100000>;

nunchuk@52 {
compatible = "nintendo,nunchuk";
reg = <0x52>;
};
};


i2c1 is a shortcut for the second I2C controller node in the Device Tree. It’s defined in the aliases section in the arch/arm/boot/dts/am33xx.dtsi file that is included indirectly by our Device Tree.

Normally Device Tree configuration is structured as a tree, thus the name. Creating an alias allows us to refer to a particular node without specifying the complete path to that node. That’s why in the Device Tree file above we could create the node for the second I2C controller directly without any parent nodes.

Also, Device Tree compiler merges trees together node by node. So it would be more correct to say that the file above does not create a node for the second I2C controller, but it modifes the existing node for the I2C controller adding status and clock-frequency properties as well as a child node nunchuk@52.

You may notice that the name of the node for the Wiichuk device has @52 suffix. This part of the name is called a unit address. Normally, the kernel should not depend on the value of the unit address. The unit address is mostly a convention that allows to generate a unique name for the Device Tree node. For example, in this particular case we used the I2C address of the Wiichuk device.

NOTE: even though unit address is not supposed to be used, it’s still available to the kernel code, so there is no hard restriction that prevents device drivers from depending on the unit address somehow.

This is by no means a complete I2C configuration, but let’s try to build it and test on the device:

cd /home/kmu/ws/linux
export ARCH=arm
export CROSS_COMPILE=arm-linux-gnueabi-
make dtbs
sudo cp arch/arm/boot/dts/am335x-boneblack-wireless-custom.dtb /var/lib/tftpboot/am335x-boneblack-wireless.dtb


Hopefully the board still can boot with the new compiled Device Tree file. After loading the board you should be able to check if it actually uses the newly build Device Tree file like this:

find /sys/firmware/devicetree -name "*nunchuk*"
/sys/firmware/devicetree/base/ocp/interconnect@48000000/segment@0/target-module@2a000/i2c@0/nunchuk@52


NOTE: it does not mean that the I2C controller or the device actually work. As a matter of fact we don’t even need to connect the Wiichuk to the board. This test just shows that the Device Tree used by the kernel contains the node for the Wiichuk device.

# I2C device driver

With the Device Tree in place we can now create a template of the I2C driver. At this stage the driver will not do anything useful and is only needed to check that our Device Tree is correct and the driver code is called by the kernel when it “detects” the device.

NOTE: as it was mentioned above I2C is a very simple bus and does not support enumerating devices connected to the bus. So when it comes to I2C detecting device means that the kernel found it in the Device Tree or somehow else was instructed that the device should be there.

I2C driver is required to prepare and register struct i2c_driver structure. This structure contains the information required to match the Device Tree nodes to the driver that can serve them and a few callbacks for the kernel to call during the device and device driver lifesycle.

For now we will use just two calbacks: probe and remove. Those are called when the device is “detected” and “removed”. In those callbacks we will do nothing except logging, so at this point it’s a bit too early to explain the parameters of the functions.

What is more important for us the is driver field of the struct i2c_driver structure. This is a field of type struct device_driver that we mentioned above. It contains parts common for all kinds of drivers (USB, I2C, PCI, etc). And information required to match Device Tree nodes to the device drivers fits that category.

of_match_table field of the struct device_driver points to the array of struct of_device_id that should hold the information to match against the compatible attribute in the Device Tree node. For example, for the Wiichuk in the device tree we specified:

nunchuk@52 {
compatible = "nintendo,nunchuk";
reg = <0x52>;
};


NOTE: the of prefix stands for Open Firmware. Open Firmware is a standard that defines Device Tree.

Therefore if we specify nintendo,nunchuk in the compatible attribute of the struct of_device_id the kernel will know that it’s this particular driver that handles the device specified in the Device Tree.

NOTE: the same driver can handle multiple different devices with different values for the compatible properly, therefore in the driver we don’t just have one struct of_device_id but an array of structures instead.

NOTE: we don’t specify the size of the array anywhere, instead to signal the kernel how many entires is there the array must end with a zero-filled entry.

The complete code is available in krinkinmu/bootlin repository. Here is the kernel module code:

// SPDX-License-Identifier: GPL-2.0
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/i2c.h>

static int wiichuk_i2c_probe(struct i2c_client *client,
const struct i2c_device_id *id)
{
(void) client;
(void) id;

return 0;
}

static int wiichuk_i2c_remove(struct i2c_client *client)
{
(void) client;

return 0;
}

static const struct of_device_id wiichuk_of_match[] = {
{ .compatible = "nintendo,nunchuk" },
{ },
};

MODULE_DEVICE_TABLE(of, wiichuk_of_match);

static const struct i2c_device_id wiichuk_i2c_id[] = {
{ "wiichuk_i2c", 0 },
{ },
};

MODULE_DEVICE_TABLE(i2c, wiichuk_i2c_id);

static struct i2c_driver wiichuk_i2c_driver = {
.driver = {
.name = "wiichuk_i2c",
.of_match_table = wiichuk_of_match
},
.probe = wiichuk_i2c_probe,
.remove = wiichuk_i2c_remove,
.id_table = wiichuk_i2c_id,
};

module_i2c_driver(wiichuk_i2c_driver);
MODULE_DESCRIPTION("Nintendo Wiichuk I2C driver");
MODULE_AUTHOR("Krinkin Mike <krinkin.m.u@gmail.com>");


It’s very similar to the code for the simple kernel module above. However you may notice that there is no init and exit functions. For common bus types kernel provides helper macroses for drivers if those init and exit functions are trivial.

For I2C such a helper macros is module_i2c_driver. All we need is to point the macros to the struct i2c_driver for our driver and it will generate the appropriate init and exit functions for our module.

You can also see in the code above the wiichuk_of_match array of struct of_device_id that is used to match Device Tree nodes against this driver as described above.

NOTE: the wiichuk_i2c_id array is used for purposes very similar to the wiichuk_of_match. It’s just a different mechanism to match the device with a driver that doesn’t use Device Tree. I’m not going to cover it here.

Makefile for the I2C driver is not different from the Makefile for the simple kernel module that was provided above (though, you may want to change the name of the C file with the code).

The procedure to build, share and load/unload the module is also exactly the same as with the simple kernel module that was shown at the very begining.