[alsa-devel] [PATCH v2] ALSA: core: let low-level driver or userspace disable rewinds

[Subhransu S. Prusty]

From: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>

Add new hw_params flag to explicitly tell driver that rewinds will never
be used. This can be used by low-level driver to optimize DMA operations
and reduce power consumption. Use this flag only when data written in
ring buffer will never be invalidated, e.g. any update of appl_ptr is
final.

Note that the update of appl_ptr include both a read/write data
operation as well as snd_pcm_forward() whose behavior is not modified.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
Signed-off-by: Ramesh Babu <ramesh.babu at intel.com>
Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty at intel.com>
---

v1->v2: No change

Since the ack callback change is now dropped and "ALSA: pcm: conditionally
avoid mmap of control data" will be dropped with recent change "ALSA: pcm:
Suppress status/control mmap when ack ops is present", I think this patch 
can be merged independently.

 include/sound/pcm.h         | 1 +
 include/uapi/sound/asound.h | 1 +
 sound/core/pcm_native.c     | 8 ++++++++
 3 files changed, 10 insertions(+)

diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 79fedf517070..c1e2b87cd409 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -368,6 +368,7 @@ struct snd_pcm_runtime {
 	unsigned int rate_num;
 	unsigned int rate_den;
 	unsigned int no_period_wakeup: 1;
+	unsigned int no_rewinds:1;
 
 	/* -- SW params -- */
 	int tstamp_mode;		/* mmap timestamp is updated */
diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index fd41697cb4d3..c697ff90450d 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -365,6 +365,7 @@ struct snd_pcm_info {
 #define SNDRV_PCM_HW_PARAMS_NORESAMPLE	(1<<0)	/* avoid rate resampling */
 #define SNDRV_PCM_HW_PARAMS_EXPORT_BUFFER	(1<<1)	/* export buffer */
 #define SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP	(1<<2)	/* disable period wakeups */
+#define SNDRV_PCM_HW_PARAMS_NO_REWINDS	        (1<<3)	/* disable rewinds */
 
 struct snd_interval {
 	unsigned int min, max;
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index bf5d0f2acfb9..0e8b7ea0d38d 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -554,6 +554,8 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 	runtime->no_period_wakeup =
 			(params->info & SNDRV_PCM_INFO_NO_PERIOD_WAKEUP) &&
 			(params->flags & SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP);
+	runtime->no_rewinds =
+		(params->flags & SNDRV_PCM_HW_PARAMS_NO_REWINDS) ? 1 : 0;
 
 	bits = snd_pcm_format_physical_width(runtime->format);
 	runtime->sample_bits = bits;
@@ -2521,6 +2523,9 @@ static snd_pcm_sframes_t snd_pcm_playback_rewind(struct snd_pcm_substream *subst
 	if (frames == 0)
 		return 0;
 
+	if (runtime->no_rewinds)
+		return 0;
+
 	snd_pcm_stream_lock_irq(substream);
 	ret = do_pcm_hwsync(substream);
 	if (!ret)
@@ -2539,6 +2544,9 @@ static snd_pcm_sframes_t snd_pcm_capture_rewind(struct snd_pcm_substream *substr
 	if (frames == 0)
 		return 0;
 
+	if (runtime->no_rewinds)
+		return 0;
+
 	snd_pcm_stream_lock_irq(substream);
 	ret = do_pcm_hwsync(substream);
 	if (!ret)
-- 
1.9.1