I have been wondering why DPCM needs special flag (= dpcm_playback/capture)
to use it. Below is the history why it was added to ASoC.

(A) In beginning, there was no dpcm_xxx flag on ASoC.
    It checks channels_min for DPCM, same as current non-DPCM.
    Let's name it as "validation check" here.

	if (rtd->dai_link->dynamic || rtd->dai_link->no_pcm) {
		if (cpu_dai->driver->playback.channels_min)
			playback = 1;
		if (cpu_dai->driver->capture.channels_min)
			capture = 1;

(B) commit 1e9de42f ("ASoC: dpcm: Explicitly set BE DAI link supported
    stream directions") force to use dpcm_xxx flag on DPCM. According to
    this commit log, this is because "Some BE dummy DAI doesn't set
    channels_min for playback/capture". But we don't know which DAI is it,
    and not know why it can't/don't have channels_min. Let's name it as
    "no_chan_DAI" here. According to the code and git-log, it is used as
    DCPM-BE and is CPU DAI. I think the correct solution was set
    channels_min on "no_chan_DAI" side, not update ASoC framework side. But
    everything is under smoke today.

	if (rtd->dai_link->dynamic || rtd->dai_link->no_pcm) {
		playback = rtd->dai_link->dpcm_playback;
		capture  = rtd->dai_link->dpcm_capture;

(C) commit 9b5db059 ("ASoC: soc-pcm: dpcm: Only allow playback/capture
    if supported") checks channels_min (= validation check) again. Because
    DPCM availability was handled by dpcm_xxx flag at that time, but some
    Sound Card set it even though it wasn't available. Clearly there's
    a contradiction here. I think correct solution was update Sound Card
    side instead of ASoC framework. Sound Card side will be updated to
    handle this issue later (commit 25612477 ("ASoC: soc-dai: set
    dai_link dpcm_ flags with a helper"))

	if (rtd->dai_link->dynamic || rtd->dai_link->no_pcm) {
		...
		playback = rtd->dai_link->dpcm_playback &&
			   snd_soc_dai_stream_valid(cpu_dai, ...);
		capture = rtd->dai_link->dpcm_capture &&
			   snd_soc_dai_stream_valid(cpu_dai, ...);

This (C) patch should have broken "no_chan_DAI" which doesn't have
channels_min, but there was no such report during this 4 years.
Possibilities case are as follows
	- No one is using "no_chan_DAI"
	- "no_chan_DAI" is no longer exist : was removed ?
	- "no_chan_DAI" is no longer exist : has channels_min ?

Because of these history, this dpcm_xxx is unneeded flag today. But because
we have been used it for 10 years since (B), it may have been used
differently. For example some DAI available both playback/capture, but it
set dpcm_playback flag only, in this case dpcm_xxx flag is used as
availability limitation. We can use playback_only flag instead in this
case, but it is very difficult to find such DAI today.

Let's add grace time to remove dpcm_playback/capture flag.

This patch don't use dpcm_xxx flag anymore, and indicates warning to use
xxx_only flag if both playback/capture were available but using only
one of dpcm_xxx flag, and not using xxx_only flag.

Link: https://lore.kernel.org/r/87edaym2cg.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patch.msgid.link/87seuyaahn.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>