[alsa-devel] [PATCH 1/7] ALSA: core: let low-level driver or userspace disable rewinds

[Takashi Iwai]

On Fri, 30 Sep 2016 14:43:24 +0200,
Subhransu S. Prusty wrote:
> 
> 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.
> 
> 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)

This is a general issue in the current PCM API design, and not
specific to this new bit.

> 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: Vinod Koul <vinod.koul at intel.com>
> Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty at 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 af1fb37..5344c16 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 609cadb..6828ed2 100644
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
> @@ -362,6 +362,7 @@ 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 c61fd50..be8617b 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -565,6 +565,8 @@ 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;
> @@ -2438,6 +2440,9 @@ 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;

Better to return an error instead?

>  	snd_pcm_stream_lock_irq(substream);
>  	switch (runtime->status->state) {
>  	case SNDRV_PCM_STATE_PREPARED:
> @@ -2486,6 +2491,9 @@ 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;

Ditto.


Takashi