Re: [PATCH 09/10] ASoC: Intel: sof_sdw: Add support for Cirrus Logic CS35L56

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

>>> +                      card->components);
>>> +    if (!card->components)
>>> +        return -ENOMEM;
>>> +
>>> +    ret = snd_soc_dapm_new_controls(&card->dapm,
>>> +                    cs35l56_sof_widgets,
>>> ARRAY_SIZE(cs35l56_sof_widgets));
>>> +    if (ret) {
>>> +        dev_err(card->dev, "Widgets add failed: %d\n", ret);
>>> +        return ret;
>>> +    }
>>> +
>>> +    ret = snd_soc_dapm_add_routes(&card->dapm, cs35l56_sof_map, count);
>>> +    if (ret) {
>>> +        dev_err(card->dev, "Map add %d failed: %d\n", count, ret);
>>> +        return ret;
>>> +    }
>>> +
>>> +    /* Enable one feedback TX per amp on different slots */
>>> +    for_each_rtd_codec_dais(rtd, i, codec_dai) {
>>> +        ret = snd_soc_dai_set_tdm_slot(codec_dai, 0x3, 1 << i, 4, 16);
>>
>> TDM slots? Not getting how this would work with SoundWire?
>>
> 
> Strictly speaking Soundwire is TDM (the frame time is divided up into
> slots for each sample...).
> 
> The problem is if you have N amps on the dailink all feeding back audio
> on the same bus. Their DP slots are all programmed to the same positions
> in the frame, same as for the playback. So you have 4 amps all trying to
> send 6 audio channels in the same positions in the frame and you'll just
> get a ton of bus clash interrupts.
> 
> So we use the set_tdm_slot() like we do with I2S TDM to set which slots
> are active for each amp.
> 
> I can't see that there's any obvious "generic" way that the manager code
> can automatically figure out how many channels to enable on each amp and
> what order to map them, so we do it here. Just as with I2S TDM - you
> have many slots and many codecs but the machine driver has to tell it
> how to map those.

IIRC Bard did the same thing recently, and the order of the feedback
channels is really defined by the order in which the peripheral devices
are added in the dailink. See
https://github.com/thesofproject/linux/pull/4108

There's also another open related to the number of channels, we need to
patch what the CPU DAI can handle, see
https://github.com/thesofproject/linux/pull/4136 or
https://github.com/thesofproject/linux/pull/4134