[alsa-devel] [RFC PATCH 1/4] ALSA: core: let low-level driver or userspace disable rewinds

[Pierre-Louis Bossart]

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.

Caveat: there is currently no way to query capabilities without
opening a pcm stream, so applications might need to serially
open all exposed devices, check what they support by looking at
hw_params->info and close them (this is what PulseAudio does so
might not be an issue)

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
---
 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 691e7ee..25310b7 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -370,6 +370,7 @@ 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 a45be6b..b62b162 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -356,6 +356,7 @@ typedef int snd_pcm_hw_param_t;
 #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 d126c03..a70e52d 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -543,6 +543,8 @@ 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;
 
 	bits = snd_pcm_format_physical_width(runtime->format);
 	runtime->sample_bits = bits;
@@ -2428,6 +2430,9 @@ 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);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_PREPARED:
@@ -2476,6 +2481,9 @@ 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);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_PREPARED:
-- 
1.9.1