UART Async API support for STM32 Discovery Board

[rt-2pm2]

Hi Everyone,

Before opening a formal issue, I wanted to know if someone had some problem running the UART async API sample on a STM32.

I am trying to run the sample on a STM32 Discovery Board and I am getting an error at the point where the transmission request is issued. Precisely, the uart_tx() returns a -19 code for ENODEV.

For example, on an digital analyzer I see something like:

[00:00:05.000,000] \x1B[0m<inf> sample: Loop 0: Sending 3 packets\x1B[0m
[00:00:05.000,000] \x1B[1;31m<err> sample: Unknown error (-19)\x1B[0m
[00:00:05.000,000] \x1B[1;31m<err> sample: Unknown error (-19)\x1B[0m
[00:00:05.000,000] \x1B[1;31m<err> sample: Unknown error (-19)\x1B[0m
[00:00:05.000,000] \x1B[0m<inf> sample: RX is now enabled\x1B[0m

Best

Step to reproduce

1. Compile

west build -b stm32f4_disco samples/drivers/uart/async_api

2. Flash

west flash --runner openocd 

[ldomaigne]

I don't have the stm32f4_disco available, so I went for a nucleo_f411re instead.
Getting the async UART working for the STM32F4 boards isn't as straightforward as for the Nordic boards.
In particular, you need to configure DMA.
But the UART Async API sample should work fine.

In my case, I used USART1, mapped on PB_6 & PB_7.
USART2 is the default uart, so I needed to change zephyr,shell-uart accordingly.

As you might see with Kconfig search / menuconfig, the UART_ASYNC_API requires DMA to be configured for the STM32 boards.
So, I had to enable the DMA controller (dma2 in my case), and overlay the USART1 node to configure DMA transfer.
Finally, I looped TX/RX using the appropriate PINs on the morpho header.

In my case, the devicetree overlays looks like this:

/{
        chosen {
                zephyr,console=&usart2;
                zephyr,shell-uart = &usart1;
        };
};

&dma2 {
        status = "okay";
};

&usart1 {
        dmas =  <&dma2 7 4 STM32_DMA_PERIPH_TX STM32_DMA_FIFO_FULL>,
                <&dma2 2 4 STM32_DMA_PERIPH_RX STM32_DMA_FIFO_FULL>;
        dma-names = "tx", "rx";
};

Please check your board/Soc reference manual for the exact DMA configuration. See Related discussion.

Hope this helps!

[rt-2pm2]

/{
chosen {
zephyr,console=&usart2;
zephyr,shell-uart = &usart1;
};
};

&dma2 {
status = "okay";
};

&usart1 {
dmas = <&dma2 7 4 STM32_DMA_PERIPH_TX STM32_DMA_FIFO_FULL>,
<&dma2 2 4 STM32_DMA_PERIPH_RX STM32_DMA_FIFO_FULL>;
dma-names = "tx", "rx";
};

Hi @ldomaigne,

Sorry for the late response. Thank you very much, your informative response was highly appreciated. Indeed, the problem was related to the DMA, and your overlay fixed the issue I was having. Now the sample code works.

By the way, since I am still learning to master Zephyr, I didn't get how you spotted the dependency using the kconfig. I was looking into it, but the kconfig was not referring to the DMA. The information I was able to retrieve on the UART_ASYNC_API was:

name: UART_ASYNC_API
Prompt: Asynchronous UART API
Type: bool
Value: y

Help:

  This option enables asynchronous UART API.

Direct dependencies (=y):
     SERIAL_SUPPORT_ASYNC(=y)
  && SERIAL(=y)

Symbols currently n-selecting this symbol (no effect):
  - NRF_ETR_SHELL
  - MODEM_IFACE_UART_ASYNC

Kconfig definition, with parent deps. propagated to 'depends on'
================================================================

At drivers/serial/Kconfig:66
Included via /tmp/tempprj/zephyr/Kconfig:8 -> Kconfig.zephyr:50 -> drivers/Kconfig:81
Menu path: (Top) -> Device Drivers -> Serial drivers

  config UART_ASYNC_API
        bool "Asynchronous UART API"
        depends on SERIAL_SUPPORT_ASYNC(=y) && SERIAL(=y)
        help
          This option enables asynchronous UART API.

How did you find the dependency on the DMA using the tools built around the kconfig? Thanks

[ldomaigne]

Hi @rt-2pm2

my turn with the late answer... 😄

By the way, since I am still learning to master Zephyr, I didn't get how you spotted the dependency using the kconfig. I was looking into it, but the kconfig was not referring to the DMA. The information I was able to retrieve on the UART_ASYNC_API was:

Guess, I got lucky by picking the nucleo_f411re.. During the west build, I got the following warning:

warning: UART_ASYNC_API (defined at drivers/serial/Kconfig:66) was assigned the value 'y' but got
the value 'n'. Check these unsatisfied dependencies: SERIAL_SUPPORT_ASYNC (=n). 
See http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_UART_ASYNC_API and/or look up UART_ASYNC_API in 
the menuconfig/guiconfig interface.
The Application Development Primer, Setting Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful too.

Then I used menuconfig to work through to missing dependencies, and started my search...

Redoing the exercise with the stm32f4_disco unfortunately doesn't bring any warnings, because dma1 on that board is enabled in the devicetree, but not dma2. To discover the link, you need to look at what causes SERIAL_SUPPORT_ASYNC(=y) to be selected.

The way you can work through dependencies (apart from looking at the corresponding Kconfig) is using the search option (/) and the Symbol info (?)
So /UART_ASYNC_API shows all symbols CONFIG_ASYNC_API* (search uses regular expression)
Type then [ENTER] and ?

You see the direct dependencies:

Direct dependencies (=y):
     SERIAL_SUPPORT_ASYNC(=y)
  && SERIAL(=y)

Here both SERIAL_SUPPORT_ASYNC and SERIAL are selected. You can then re-iterate the steps using /SERIAL_SUPPORT_ASYNC to discover that UART_STM32 is selecting this option. Looking at UART_STM32, you see that DMA is selected if UART_ASYNC_API is selected etc.