i.MX6 ASRC audio support

[alexandersmirno]

Hi folks,

could anybody help me please with ASRC support for audio playback in i.MX6q-based board.

I use LTIB Linux kernel as a reference and wrote a driver for DAC, which is connected to SSI2. The code is derived from sgtl5000 and cs42888 drivers. I've added 2 dai links with names: "HiFi" and "HiFi_ASRC". The first link assumes to play the stream with sample rate it has, and the second - should use ASRC. "HiFi" configuration works, when I start the following command:

$ aplay -D "hw:0,0" my.wav

I see a data at oscilloscope and hear sound. But when I try to use "HiFi_ASRC":

$ aplay -D "hw:0,1" my.wav

I get an I/O error after some timeout. Moreover I see about 4-5 SDMA interrupts only in 'cat /proc/interrupts' after each execution, while for the first case there are about 100.

In my base driver implementation I used the following ASRC configuration:

        config.input_word_width = get_asrc_input_width(params);
        config.output_word_width = iprtd->p2p->p2p_width;
        config.pair = iprtd->asrc_index;
        config.channel_num = channel;
        config.input_sample_rate = rate;
        config.output_sample_rate = iprtd->p2p->p2p_rate;
        config.inclk = INCLK_NONE;
        config.outclk = OUTCLK_SSI2_TX;

But also I tried to use different combinations of clocks for 'inclk' and 'outclk', but result was the same.

So I have several question and any hints are highly appreciated:

1. How ASRC works, does it require both clocks: input and output? According to the RM, it uses input clock to process samples, but how the output clock is involved to this process?

2. How "asrc_clk" and "asrc_serial_clk" are used, should I set some magic rate for them?

3. Lets say I have an input stream with sample rate 44.1KHz and I want to play it over SSI2 at 48KHz rate. Which clocks should I use? Should I manually setup input clock rate to: F = 44100 * WORD_SIZE * CHANNELS Hz?

Thank you!

With best regards,

Alex

[alexsmirnov]

Hello again,

I got some status update. I've used the following clock setup:

        ...

        config.channel_num = 2;

        config.input_sample_rate = 48000;

        config.output_sample_rate = 48000;

        config.inclk = INCLK_SSI2_RX;
        config.outclk = OUTCLK_NONE;

Also I checked the ASRC input clock divider and prescaler, so the Fs_in = 48KHz. Then I tried to play 48KHz audio by using aplay and I saw no error, but no data was observed at DAC side. But on the other hand I saw lots of SDMA interrupts in 'cat /proc/interrupts'. If I set outclk to OUTCLK_SSI2_TX - the I/O problem will be seen.

So, I'd like to add new question to the list, what do INCLK_NONE and OUTCLK_NONE clocks mean?

Alex

[alexsmirnov · Accepted solution]

Hello again,

I'd like to share the results of my research, probably it will help somebody in future.

i.MX6 ASRC implementation has 2 interfaces:

• User space package: fsl-asla-plugins
• Kernel API

1. First interface doesn't require any kernel modifications. The only thing is required - add necessary line to ALSA config. All the information can be found in fsl-alsa-plugins-1.0.25.tar.gz in file asrcrate.txt. Below is example for 44100KHz:

Write in your ~/.asoundrc or /etc/asound.conf, such as

defaults.pcm.rate_converter "asrcrate"

pcm.dmix_44100 {

    type dmix

    ipc_key 5678293

    ipc_key_add_uid yes

    slave{

        pcm "hw:0,0"

        period_time 10000

        format S16_LE

        rate 44100

    }

}

pcm.asrc {

    type plug

    route_policy "average"

    slave.pcm "dmix_44100"

}

But, as mentioned in the text file, there are limitations in convertion rates.

2. Kernel API uses the same functions as the interface above, but calls them directly instead of using IOCTL. I used cs42888 codec as a reference code, so derived all ASRC parts and put them in my codec - but it didn't work due to problems mentioned in previous posts.

The first approach works and completely fits my requirements, so I stopped digging kernel part here.

Alex