ASoC: dmaengine: Document support for TX only or RX only streams

We intentionally do not return an error if we get a permanent failure
from dma_request_chan() in order to support systems which have TX only
or RX only channels. Add a comment documenting this.

Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/r/20201008161105.21804-1-broonie@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-generic-dmaengine-pcm.c

@@ -386,6 +386,11 @@ static int dmaengine_pcm_request_chan_of(struct dmaengine_pcm *pcm,
 			name = config->chan_names[i];
 		chan = dma_request_chan(dev, name);
 		if (IS_ERR(chan)) {
+			/*
+			 * Only report probe deferral errors, channels
+			 * might not be present for devices that
+			 * support only TX or only RX.
+			 */
 			if (PTR_ERR(chan) == -EPROBE_DEFER)
 				return -EPROBE_DEFER;
 			pcm->chan[i] = NULL;