How to create an UART interface in device tree

[PSchw.16]

Hello,

I would like to use the UART7 with A7 on the STM32MP157C-DK2 with the openstlinux-20-02-19 ecosystem. Therefore i tried to include/enable the UART interface in the device tree but it is not working correctly.

I configured UART7 in the stm32mp157a-dk1.dts file by setting the status "okay":

&uart7 {

   pinctrl-names = "default";

   pinctrl-0 = <&uart7_pins_a>;

   status = "okay";

};

and in the stm32mp157.c.dtsi file by setting the status "okay":

uart7: serial@40018000 {

         compatible = "st,stm32h7-uart";

         reg = <0x40018000 0x400>;

         clocks = <&rcc UART7_K>;

         resets = <&rcc UART7_R>;

         status = "okay";

      };

An alias was already given:

aliases {

      serial0 = &uart4;

      serial1 = &usart3;

      serial2 = &uart7;

   };

I ran bitbake again,flashed the new image to the board and ran the dmesg command.

The following output was generated:

root@stm32mp1-disco:~# dmesg | grep ttySTM*

[   0.000000] Kernel command line: root=/dev/mmcblk0p6 rootwait rw console=ttySTM0,115200

[   0.531634] 4000e000.serial: ttySTM3 at MMIO 0x4000e000 (irq = 24, base_baud = 4000000) is a stm32-usart

[   0.531934] serial serial0: tty port ttySTM3 registered

[   0.533538] 40010000.serial: ttySTM0 at MMIO 0x40010000 (irq = 25, base_baud = 4000000) is a stm32-usart

[   1.476144] console [ttySTM0] enabled

[   5.672859] systemd[1]: Unnecessary job for dev-ttySTM0.device was removed.

It seems that UART7 is not registered nor enabled.

Does Yocto compile a .dtb file or do I have to take additional steps?

I kindly ask you for support. Best wishes

[KnarfB]

Do you see any USART related error messages in dmesg?

Yocto should re-build the device tree. Here you find more about that: https://stackoverflow.com/questions/38917745/quick-rebuild-of-device-tree-only-with-yocto-bitbake

You should check the timestamps of relevant files to check if they were rebuild.

Your .dts source looks reasonable. Similar code works on my DK2 (using buildroot).

[JeanPhilippeR]

Dear @PSchw.16​ 

Please have a look to the wiki page https://wiki.st.com/stm32mpu/wiki/Trace_and_debug_scenario_-_UART_issue to help for debugging.

Regards

[PSchw.16]

Hello KnarfB,

thank you for your fast response.

I rebuilt the device tree and the process seemed to work properly now. The timestamps of the files are up to date.

I tried flashing the SDcard with "FlashLayout_sdcard_stm32mp157c-dk2-trusted.tsv" via STCubeProgrammer, but it seems to me that something went wrong:

After flashing the SDcard the .dtb-files on the board were not updated although the .dtb-files on the host PC are up to date.

-rw-r--r-- 1 root root  71751 Mar 9 2018 stm32mp157a-dk1.dtb

-rw-r--r-- 1 root root  73626 Mar 9 2018 stm32mp157c-dk2-a7-examples.dtb

-rw-r--r-- 1 root root  74162 Mar 9 2018 stm32mp157c-dk2-m4-examples.dtb

-rw-r--r-- 1 root root  73578 Mar 9 2018 stm32mp157c-dk2.dtb

So the mistake may be the way I flash my device.

I would be very thankful if you could provide the way you flashed/updated your device tree.

[PSchw.16]

Hello JeanPhilippeR,

thank you for answering my question. I tried to follow the steps given in the article to check the UART probe and I got the following outputs:

root@stm32mp1-disco:~# cat /proc/device-tree/soc/serial\@40018000/status

disabled

root@stm32mp1-disco:~# dmesg | grep 40018000

root@stm32mp1-disco:~# ls -l /proc/device-tree/aliases/*

-r--r--r-- 1 root root 23 Aug 18 01:38 /proc/device-tree/aliases/ethernet0

-r--r--r-- 1 root root 8 Aug 18 01:38 /proc/device-tree/aliases/name

-r--r--r-- 1 root root 21 Aug 18 01:38 /proc/device-tree/aliases/serial0

-r--r--r-- 1 root root 21 Aug 18 01:38 /proc/device-tree/aliases/serial1

-r--r--r-- 1 root root 21 Aug 18 01:31 /proc/device-tree/aliases/serial2

-r--r--r-- 1 root root 21 Aug 18 01:38 /proc/device-tree/aliases/serial3

root@stm32mp1-disco:~# cat /proc/device-tree/aliases/serial2

/soc/serial@40018000

Serial2 is correctly associated with UART7 (@4001800) but the status of @4001800 is "disabled"...am I missing something to enable the UART7?

What surprises me is the fact that there is somehow a serial3 although I haven't defined this alias in the corresponding .dts file.

I hope you might have an idea what is going wrong and I am looking forward to your answer.

Regards

[JeanPhilippeR · Accepted solution]

Hello @PSchw.16​ 

You can check for the dtb build result with the following command on the host PC, in the deploy image directory of the yocto build:

$ dtc -I dtb -O dts stm32mp157c-dk2.dtb

It will give you the dt values in human format.

You can then check for the status of the serial@40018000 node.

Following the result, it will be build issue, or dt apply issue at kernel boot.

Regards

[PSchw.16]

Hello @JeanPhilippeR​ ,

after running your recommended command I was able to identify the error as a build issue.

I converted the .dtb file into a text file with "$ dtc -I dtb -O dts -o <filename> and set the status of the UART7 from "disabled" to "okay".

Afterwards I converted my text file back into a .dtb file with "$ dtc -I dtc -O dtb -o <filename>" and replaced the incorrect .dtb files on my board.

Now the serial interface ttySTM2 exists and can be configured.

Thank you so much for your support.

Best wishes