STM32MP1 and Codec Running as Slave with si5351B as Master clock on SAI

[MToum.1]

Hello,

I am wondering what could be the best way to write a device tree that sets the STM32MP1 as SAI SLAVE with a CODEC also running as SAI slave. The HW design I am studying has the following specs:

1) STM32MP1 with ALSA and one SAI interface.

2) A codec chipset running as SAI slave.

3) The Si5351B part running as a MASTER clock providing the SAI bit and frame (L/R) clocks to both the SoC and CODEC.

I understand such a setup is a little out of the norm. How can the device tree be modified on the SAI side to avoid the STM32MP1 to become a MASTER. The snippet of a possible device tree is below.

How can  I force the STM32MP1 to be a SLAVE on SAI?

I would imagine writing the device tree as follows:

&sai1{
    pinctrl-names = "default", "sleep";
    pinctrl-0 = <&sai1b_pins_mx>;
    pinctrl-1 = <&sai1b_sleep_pins_mx>;
    status = "okay";

    /* USER CODE BEGIN sai1 */
    /* Specify external clock sources for SAI1
     We MUST add the clock-names according to the doc. */
    clocks = <&rcc SAI1>, <&rcc PLL3_Q>, <&rcc PLL3_R>, <&si5351b_clk0>, <&si5351b_clk1>;
    clock-names = "pclk", "x8k", "x11k", "sclk", "lrclk";
    /*  USER CODE END sai1 */


    sai1b:audio-controller@4400a024{
       status = "okay";
       /* USER CODE BEGIN sai1b */
       dma-names = "tx";
       mode = "i2s-slave"; /* Set SAI interface to slave mode */
       sai1b_port: port {
          sai1b_endpoint: endpoint {
             remote-endpoint = <&CODEC_tx_endpoint>;
             format = "i2s";
             dai-tdm-slot-num = <2>;
             dai-tdm-slot-width = <32>;
             bitclock-master = <0>; /* SAI is not the bit clock master */
             frame-master = <0>; /* SAI is not the frame sync master */
             /* Synchronization configuration */
             st,sync = "lrclk"; /* Synchronization signal source */

          };
       };
       /* USER CODE END sai1b */
    };
};

The couple of tests performed on a target device are showing that a similar device tree as above still put the STM32MP1 as a MASTER. 

Please comment on this.

I am also reading the driver code and the notion of EXTERNAL is unclear to me, are we talking about a signal External to SAI or External to the SoC?

The code reference for the ST driver is: https://github.com/STMicroelectronics/linux/blob/v5.15-stm32mp/sound/soc/stm/stm32_sai.c and https://github.com/STMicroelectronics/linux/blob/v5.15-stm32mp/sound/soc/stm/stm32_sai_sub.c

[Christophe Guibout · Accepted solution]

Hello @MToum.1,

You need to remove these two lines from SAI DT entry to configure it as slave:

frame-master;

bitclock-master; 

Feel free to look into the SAI DT configuration wiki page.

The following page could help as well : Soundcard configuration.

BR,

Christophe