[alsa-devel] [RFC PATCH 3/4] ALSA: core: add report of max dma burst

[Takashi Iwai]

On Wed, 08 Jul 2015 12:10:35 +0200,
Pierre-Louis Bossart wrote:
> 
> Report how many bytes the DMA can burst before updating the hw_ptr.
> This can help fix two issues with stale data discussed many times over
> on the alsa-devel mailing list (refilling/reading ring buffer too late or
> rewinding too close to the hw_ptr)
> 
> FIXME:
> 1. this was copy/pasted/edited based on the fifo_size fields, not
> sure why we would need this IOCTL1

fifo_size would fit, but it means that it also changes the current
behavior.  I don't believe that currently there are many programs
relying on this value, but who knows.

> 2. we also need the ability to set the actual fifo size, if suppported,
> by the hardware, to negociate the prefetch amount between application
> and driver. By default the default should be max buffering to allow
> for lower power, but for low-latency use cases we may want to reduce
> the amount of prefetching.

Right.  So a hw_parmas field looks suitable for that purpose.


Takashi


> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
> ---
>  include/sound/pcm.h         |  2 ++
>  include/uapi/sound/asound.h |  5 +++--
>  sound/core/pcm_lib.c        | 19 +++++++++++++++++++
>  sound/core/pcm_native.c     |  7 +++++++
>  4 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
> index d5eff03..57c0571 100644
> --- a/include/sound/pcm.h
> +++ b/include/sound/pcm.h
> @@ -56,6 +56,7 @@ struct snd_pcm_hardware {
>  	unsigned int periods_min;	/* min # of periods */
>  	unsigned int periods_max;	/* max # of periods */
>  	size_t fifo_size;		/* fifo size in bytes */
> +	unsigned int max_dma_burst;     /* max DMA in-flight bytes, linked to hw_ptr precision/fuzziness */
>  };
>  
>  struct snd_pcm_substream;
> @@ -106,6 +107,7 @@ struct snd_pcm_ops {
>  #define SNDRV_PCM_IOCTL1_CHANNEL_INFO	2
>  #define SNDRV_PCM_IOCTL1_GSTATE		3
>  #define SNDRV_PCM_IOCTL1_FIFO_SIZE	4
> +#define SNDRV_PCM_IOCTL1_MAX_DMA_BURST	5
>  
>  #define SNDRV_PCM_TRIGGER_STOP		0
>  #define SNDRV_PCM_TRIGGER_START		1
> diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
> index b62b162..3dc049a 100644
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
> @@ -386,8 +386,9 @@ struct snd_pcm_hw_params {
>  	unsigned int msbits;		/* R: used most significant bits */
>  	unsigned int rate_num;		/* R: rate numerator */
>  	unsigned int rate_den;		/* R: rate denominator */
> -	snd_pcm_uframes_t fifo_size;	/* R: chip FIFO size in frames */
> -	unsigned char reserved[64];	/* reserved for future */
> +	snd_pcm_uframes_t fifo_size;	/* R: chip FIFO size in frames, indicates buffering after hw_ptr */
> +	unsigned int max_dma_burst;     /* R: max in-flight bytes, indicates buffering before hw_ptr */
> +	unsigned char reserved[60];	/* reserved for future */
>  };
>  
>  enum {
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index 7d45645..dc89aa0 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -1826,6 +1826,22 @@ static int snd_pcm_lib_ioctl_fifo_size(struct snd_pcm_substream *substream,
>  	return 0;
>  }
>  
> +static int snd_pcm_lib_ioctl_max_dma_burst(struct snd_pcm_substream *substream,
> +					      void *arg)
> +{
> +	struct snd_pcm_hw_params *params = arg;
> +
> +	/* FIXME: add sanity checks:
> +	 * max burst < ring buffer
> +	 * max burst >= min_period_size
> +	 * not sure if we can check against period sizes?
> +	 * any others ?
> +	 */
> +	params->max_dma_burst = substream->runtime->hw.max_dma_burst;
> +
> +	return 0;
> +}
> +
>  /**
>   * snd_pcm_lib_ioctl - a generic PCM ioctl callback
>   * @substream: the pcm substream instance
> @@ -1849,6 +1865,9 @@ int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,
>  		return snd_pcm_lib_ioctl_channel_info(substream, arg);
>  	case SNDRV_PCM_IOCTL1_FIFO_SIZE:
>  		return snd_pcm_lib_ioctl_fifo_size(substream, arg);
> +	case SNDRV_PCM_IOCTL1_MAX_DMA_BURST:
> +		return snd_pcm_lib_ioctl_max_dma_burst(substream, arg);
> +
>  	}
>  	return -ENXIO;
>  }
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index dd519b8..3d379b8 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -280,6 +280,7 @@ int snd_pcm_hw_refine(struct snd_pcm_substream *substream,
>  
>  	params->info = 0;
>  	params->fifo_size = 0;
> +	params->max_dma_burst = 0;
>  	if (params->rmask & (1 << SNDRV_PCM_HW_PARAM_SAMPLE_BITS))
>  		params->msbits = 0;
>  	if (params->rmask & (1 << SNDRV_PCM_HW_PARAM_RATE)) {
> @@ -437,6 +438,12 @@ int snd_pcm_hw_refine(struct snd_pcm_substream *substream,
>  				return changed;
>  		}
>  	}
> +	if (!params->max_dma_burst) {
> +		changed = substream->ops->ioctl(substream,
> +						SNDRV_PCM_IOCTL1_MAX_DMA_BURST, params);
> +		if (changed < 0)
> +			return changed;
> +	}
>  	params->rmask = 0;
>  	return 0;
>  }
> -- 
> 1.9.1
> 
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