ALSA: pcm: introduce INFO_NO_REWINDS flag

When the hardware can only deal with a monotonically increasing
appl_ptr, this flag can be set. In case the application requests a
rewind, snd_pcm_rewind() will not return an error code but signal that
the appl_ptr was not modified.

Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>

include/uapi/sound/asound.h

@@ -300,7 +300,7 @@ typedef int __bitwise snd_pcm_subformat_t;
 #define SNDRV_PCM_INFO_HAS_LINK_ESTIMATED_ATIME    0x04000000  /* report estimated link audio time */
 #define SNDRV_PCM_INFO_HAS_LINK_SYNCHRONIZED_ATIME 0x08000000  /* report synchronized audio/system time */
 #define SNDRV_PCM_INFO_EXPLICIT_SYNC	0x10000000	/* needs explicit sync of pointers and data */
-
+#define SNDRV_PCM_INFO_NO_REWINDS	0x20000000	/* hardware can only support monotonic changes of appl_ptr */
 #define SNDRV_PCM_INFO_DRAIN_TRIGGER	0x40000000		/* internal kernel flag - trigger in drain */
 #define SNDRV_PCM_INFO_FIFO_IN_FRAMES	0x80000000	/* internal kernel flag - FIFO size is in frames */
 

sound/core/pcm_native.c

@@ -2897,6 +2897,8 @@ static snd_pcm_sframes_t snd_pcm_rewind(struct snd_pcm_substream *substream,
 
 	if (frames == 0)
 		return 0;
+	if (substream->runtime->info & SNDRV_PCM_INFO_NO_REWINDS)
+		return 0;
 
 	snd_pcm_stream_lock_irq(substream);
 	ret = do_pcm_hwsync(substream);