ASoC: macaudio: constrain frontend channel counts

In order to support the wide range of audio arrangements possible
on this platform in a generic way, it is necessary for the frontend
PCMs to be populated with enough TDM slots to cover all intended use
cases. Userspace therefore attempts to open "phantom" channels
when a frontend has more channels than its associated backend, which
results in garbled audio samples and dropped frames.

We must therefore dynamically constrain the frontends when they
are started to ensure that userspace can never open more channels
than are present on the hardware being represented by the
frontend in question.

Signed-off-by: James Calligeros <[email protected]>

Author: chadmed

sound/soc/apple/macaudio.c

@@ -52,6 +52,7 @@ struct macaudio_snd_data {
 	int jack_plugin_state;
 
 	bool has_speakers;
+	unsigned int max_channels;
 
 	struct macaudio_link_props {
 		/* frontend props */
@@ -345,6 +346,10 @@ static int macaudio_parse_of(struct macaudio_snd_data *ma)
 		ncodecs_per_cpu = num_codecs / num_bes;
 		nchannels = num_codecs * (speakers ? 1 : 2);
 
+		/* Save the max number of channels on the platform */
+		if (nchannels > ma->max_channels)
+			ma->max_channels = nchannels;
+
 		/*
 		 * If there is a single speaker, assign two channels to it, because
 		 * it can do downmix.
@@ -455,6 +460,25 @@ static int macaudio_dpcm_hw_params(struct snd_pcm_substream *substream,
 	return 0;
 }
 
+static int macaudio_fe_startup(struct snd_pcm_substream *substream)
+{
+
+	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
+	struct macaudio_snd_data *ma = snd_soc_card_get_drvdata(rtd->card);
+	int ret;
+
+	/* The FEs must never have more channels than the hardware */
+	ret = snd_pcm_hw_constraint_minmax(substream->runtime,
+					SNDRV_PCM_HW_PARAM_CHANNELS, 0, ma->max_channels);
+
+	if (ret < 0) {
+		dev_err(rtd->dev, "Failed to constrain FE %d! %d", rtd->dai_link->id, ret);
+		return ret;
+		}
+
+	return 0;
+}
+
 static int macaudio_fe_hw_params(struct snd_pcm_substream *substream,
 				   struct snd_pcm_hw_params *params)
 {
@@ -495,6 +519,7 @@ static void macaudio_dpcm_shutdown(struct snd_pcm_substream *substream)
 }
 
 static const struct snd_soc_ops macaudio_fe_ops = {
+	.startup	= macaudio_fe_startup,
 	.shutdown	= macaudio_dpcm_shutdown,
 	.hw_params	= macaudio_fe_hw_params,
 };