ALSA: pcm - add support for hostless audio

Allow some PCM devices to be hostless, i.e. there is no PCM data transferred
to or from the host CPU. This can be used to minimise power on systems since
the CPU can idle/sleep during the PCM device operation (e.g. a phone call
where the DAI is between a MODEM and DSP)

TODO: cleanup, look at adding a read/write blocker.

Singed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>

include/sound/pcm.h

@@ -490,6 +490,7 @@ struct snd_pcm_substream {
 #endif /* CONFIG_SND_VERBOSE_PROCFS */
 	/* misc flags */
 	unsigned int hw_opened: 1;
+	unsigned int hw_no_buffer: 1; /* substream may not have a buffer */
 };
 
 #define SUBSTREAM_BUSY(substream) ((substream)->ref_count > 0)

sound/core/pcm_lib.c

@@ -2115,6 +2115,9 @@ static int pcm_sanity_check(struct snd_pcm_substream *substream)
 	struct snd_pcm_runtime *runtime;
 	if (PCM_RUNTIME_CHECK(substream))
 		return -ENXIO;
+	/* TODO: consider and -EINVAL here */
+	if (substream->hw_no_buffer)
+		snd_printd("%s: warning this PCM is host less\n", __func__);
 	runtime = substream->runtime;
 	if (snd_BUG_ON(!substream->ops->copy && !runtime->dma_area))
 		return -EINVAL;

sound/core/pcm_native.c

@@ -1024,6 +1024,7 @@ static int snd_pcm_pre_start(struct snd_pcm_substream *substream, int state)
 	if (runtime->status->state != SNDRV_PCM_STATE_PREPARED)
 		return -EBADFD;
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
+	    !substream->hw_no_buffer &&
 	    !snd_pcm_playback_data(substream))
 		return -EPIPE;
 	runtime->trigger_tstamp_latched = false;