staging: bcm2835-audio: Fix draining behavior regression

The PCM draining behavior got broken since the recent refactoring, and
this turned out to be the incorrect expectation of the firmware
behavior regarding "draining".  While I expected the "drain" flag at
the stop operation would do processing the queued samples, it seems
rather dropping the samples.

As a quick fix, just drop the SNDRV_PCM_INFO_DRAIN_TRIGGER flag, so
that the driver uses the normal PCM draining procedure.  Also, put
some caution comment to the function for future readers not to fall
into the same pitfall.

Fixes: d7ca3a71 ("staging: bcm2835-audio: Operate non-atomic PCM ops")
BugLink: https://github.com/raspberrypi/linux/issues/2983
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://lore.kernel.org/r/20190914152405.7416-1-tiwai@suse.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c

@@ -12,7 +12,7 @@
 static const struct snd_pcm_hardware snd_bcm2835_playback_hw = {
 	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |
 		 SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID |
-		 SNDRV_PCM_INFO_DRAIN_TRIGGER | SNDRV_PCM_INFO_SYNC_APPLPTR),
+		 SNDRV_PCM_INFO_SYNC_APPLPTR),
 	.formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,
 	.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,
 	.rate_min = 8000,
@@ -29,7 +29,7 @@ static const struct snd_pcm_hardware snd_bcm2835_playback_hw = {
 static const struct snd_pcm_hardware snd_bcm2835_playback_spdif_hw = {
 	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |
 		 SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID |
-		 SNDRV_PCM_INFO_DRAIN_TRIGGER | SNDRV_PCM_INFO_SYNC_APPLPTR),
+		 SNDRV_PCM_INFO_SYNC_APPLPTR),
 	.formats = SNDRV_PCM_FMTBIT_S16_LE,
 	.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_44100 |
 	SNDRV_PCM_RATE_48000,

drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c

@@ -289,6 +289,7 @@ int bcm2835_audio_stop(struct bcm2835_alsa_stream *alsa_stream)
 					 VC_AUDIO_MSG_TYPE_STOP, false);
 }
 
+/* FIXME: this doesn't seem working as expected for "draining" */
 int bcm2835_audio_drain(struct bcm2835_alsa_stream *alsa_stream)
 {
 	struct vc_audio_msg m = {