[Yocto-STM32MP1] 2. Creating a kernel device tree patch, enabling i2c

Translated from the Vietnamese original; wording may still be rough. Read the original →

In this post I show you how to create a kernel device tree patch to enable i2c5 on the board

1. Documents and hardware

Read the vendor's User Manual ; page 8 has the Hardware block diagram, showing the connections currently routed out from the chip

We see i2c1 and i2c4 are already used for other peripherals

Continue to page 31 to read about the GPIOs in more detail

So we see pins PA12 and PA11 are routed out; they are the SDA and SCL pins of i2c5 on the board.

Now let's check whether i2c5 is enabled, similar to the post [Yocto-BBB] 8. Connecting an SSD1306 0.96 inch OLED over I2C . On core-image-minimal we check

bash

root@stm32mp1:~# ls -l /dev/i2c*
crw-------    1 root     root       89,   0 Jan  1  2000 /dev/i2c-0
crw-------    1 root     root       89,   1 Jan  1  2000 /dev/i2c-1
crw-------    1 root     root       89,   2 Jan  1  2000 /dev/i2c-2
root@stm32mp1:~# ls /sys/bus/i2c/devices/
0-0039  0-004a  1-0028  1-0033  i2c-0   i2c-1   i2c-2
root@stm32mp1:~# ls -l /sys/bus/i2c/devices/
lrwxrwxrwx    1 root     root             0 Jan  1  2000 0-0039 -> ../../../devices/platform/soc/5c007000.bus/40012000.i2c/i2c-0/0-0039
lrwxrwxrwx    1 root     root             0 Jan  1  2000 0-004a -> ../../../devices/platform/soc/5c007000.bus/40012000.i2c/i2c-0/0-004a
lrwxrwxrwx    1 root     root             0 Jan  1  2000 1-0028 -> ../../../devices/platform/soc/5c007000.bus/5c002000.i2c/i2c-1/1-0028
lrwxrwxrwx    1 root     root             0 Jan  1  2000 1-0033 -> ../../../devices/platform/soc/5c007000.bus/5c002000.i2c/i2c-1/1-0033
lrwxrwxrwx    1 root     root             0 Jan  1  2000 i2c-0 -> ../../../devices/platform/soc/5c007000.bus/40012000.i2c/i2c-0
lrwxrwxrwx    1 root     root             0 Jan  1  2000 i2c-1 -> ../../../devices/platform/soc/5c007000.bus/5c002000.i2c/i2c-1
lrwxrwxrwx    1 root     root             0 Jan  1  2000 i2c-2 -> ../../../devices/platform/soc/5c007000.bus/40012000.i2c/i2c-0/i2c-2

What we need to look at is the i2c addresses. So there are 2 i2c buses enabled

bash

40012000.i2c
40012000.i2c
5c002000.i2c
5c002000.i2c
40012000.i2c
5c002000.i2c
40012000.i2c

Go to the Reference manual, Table 9 in 2.5.2 Memory Map and register boundary addresses. We see these are exactly i2c1 and i2c4 mentioned above.

So now we need to enable i2c5

2. Checking the device tree after the build

2.1 Creating the patch

Let's check the set of device tree outputs to see what we have

bash

zk47@zk47-ltu:~/Learning/yocto-mp1/build-mp1$ find . -name "stm32mp157d-dk1.dts"
./tmp/work/x86_64-linux/tf-a-tools-native/v2.10.13-stm32mp-r2/git/fdts/stm32mp157d-dk1.dts
./tmp/work/x86_64-linux/tf-a-tools-native/v2.10.13-stm32mp-r2/git/.pc/0001-v2.10-stm32mp-r2.patch/fdts/stm32mp157d-dk1.dts
./tmp/work/x86_64-linux/u-boot-tools-stm32mp-native/v2023.10-stm32mp-r2/git/.pc/0001-v2023.10-stm32mp-r2.patch/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work/x86_64-linux/u-boot-tools-stm32mp-native/v2023.10-stm32mp-r2/git/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work/stm32mp1-poky-linux-gnueabi/optee-os-stm32mp/4.0.0-stm32mp-r2/git/core/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work/stm32mp1-poky-linux-gnueabi/optee-os-stm32mp/4.0.0-stm32mp-r2/git/.pc/0001-4.0.0-stm32mp-r2.patch/core/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/uboot-source/.pc/0001-v2023.10-stm32mp-r2.patch/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/uboot-source/arch/arm/dts/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/kernel-source/.pc/0001-v6.6-stm32mp-r2.patch/arch/arm/boot/dts/st/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/kernel-source/arch/arm/boot/dts/st/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/tfa-source/fdts/stm32mp157d-dk1.dts
./tmp/work-shared/stm32mp1/tfa-source/.pc/0001-v2.10-stm32mp-r2.patch/fdts/stm32mp157d-dk1.dts

We look at the main files and check whether anything relates to i2c5

bash

/home/zk47/Learning/yocto-mp1/build-mp1/tmp/work-shared/stm32mp1/kernel-source/arch/arm/boot/dts/st/stm32mp157d-dk1.dts

/home/zk47/Learning/yocto-mp1/build-mp1/tmp/work-shared/stm32mp1/kernel-source/arch/arm/boot/dts/st/stm32mp157d-dk1.dts

And then

bash

/home/zk47/Learning/yocto-mp1/build-mp1/tmp/work-shared/stm32mp1/kernel-source/arch/arm/boot/dts/st/stm32mp15xx-dkx.dtsi

In this file we see

The current status is disabled, so we need to change it to "okay"

So we add the following block to the file stm32mp157d-dk1.dts

bash

&i2c5 {
	pinctrl-names = "default", "sleep";
	pinctrl-0 = ;
	pinctrl-1 = ;
	i2c-scl-rising-time-ns = ;
	i2c-scl-falling-time-ns = ;
	clock-frequency = ;
	status = "okay";
};

We use a trick: copy that file into a folder and duplicate it. Name the original stm32mp157d-dk1.dts.orig, and the edited copy with the i2c5 node stm32mp157d-dk1.dts.

Now we run the command

bash

git diff --no-index stm32mp157d-dk1.dts.orig stm32mp157d-dk1.dts > 0001-add-i2c-userspace-sts.patch

However, this patch uses paths relative to the current folder, so we need to fix the path to match the linux source. We change the patch to the following

So we have the patch; now we need to create a meta-layer and a kernel recipe

2.2 Creating the meta-layer and kernel recipe

Here I create meta-stm32mp1 for customisations, with the following folder structure

But we don't know the name of the kernel recipe yet, which we need in order to add a bbappend for it. So we check with the command

bash

zk47@zk47-ltu:~/Learning/yocto-mp1/build-mp1$ oe-pkgdata-util lookup-recipe kernel
linux-stm32mp

Okay, now we create linux-stm32mp_%.bbappend and that's it

bash

zk47@zk47-ltu:~/Learning/yocto-mp1/build-mp1$ cat /home/zk47/Learning/yocto-mp1/meta-stm32mp1/recipes-kernel/linux/linux-stm32mp_%.bbappend
FILESEXTRAPATHS_prepend := "${THISDIR}/stm32mp1:"
SRC_URI += "file://0001-add-i2c-userspace-dts.patch"

And to make debugging i2c easier, we add the following to core-image-minimal.bbappend

bash