[alsa-devel] PCM delay compensation

[Lennart Poettering]

On Tue, 07.10.08 17:32, Takashi Iwai (tiwai at suse.de) wrote:

> Hi,

Sorry for the overly long delay.
> 
> the patch below (to the latest sound git tree) adds the extra delay
> count for USB-audio driver.  This change will appear as the return
> value of snd_pcm_delay().
> 
> Could you check whether it's appropriate behavior you've wanted?

I have now tested this patch on the current 2.6.29-rc5 kernel. The
effect is that snd_pcm_delay() returns always increasing values, as if
the playback never proceeds. Most movie players which need that call
to synchronize video frames hence stall completely.

Lennart


> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
> index 40c5a6f..dcbdc60 100644
> --- a/include/sound/pcm.h
> +++ b/include/sound/pcm.h
> @@ -269,6 +269,7 @@ struct snd_pcm_runtime {
>  	snd_pcm_uframes_t avail_max;
>  	snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
>  	snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
> +	snd_pcm_sframes_t delay;	/* extra delay; typically FIFO size */
>  
>  	/* -- HW params -- */
>  	snd_pcm_access_t access;	/* access mode */
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index e61e125..df7c3fa 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -597,14 +597,15 @@ int snd_pcm_status(struct snd_pcm_substream *substream,
>  	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
>  		status->avail = snd_pcm_playback_avail(runtime);
>  		if (runtime->status->state == SNDRV_PCM_STATE_RUNNING ||
> -		    runtime->status->state == SNDRV_PCM_STATE_DRAINING)
> +		    runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
>  			status->delay = runtime->buffer_size - status->avail;
> -		else
> +			status->delay += runtime->delay;
> +		} else
>  			status->delay = 0;
>  	} else {
>  		status->avail = snd_pcm_capture_avail(runtime);
>  		if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)
> -			status->delay = status->avail;
> +			status->delay = status->avail + runtime->delay;
>  		else
>  			status->delay = 0;
>  	}
> @@ -2423,6 +2424,7 @@ static int snd_pcm_delay(struct snd_pcm_substream *substream,
>  			n = snd_pcm_playback_hw_avail(runtime);
>  		else
>  			n = snd_pcm_capture_avail(runtime);
> +		n += runtime->delay;
>  		break;
>  	case SNDRV_PCM_STATE_XRUN:
>  		err = -EPIPE;
> diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
> index 6e70ba4..7d5a103 100644
> --- a/sound/usb/usbaudio.c
> +++ b/sound/usb/usbaudio.c
> @@ -629,6 +629,7 @@ static int prepare_playback_urb(struct snd_usb_substream *subs,
>  	subs->hwptr_done += offs;
>  	if (subs->hwptr_done >= runtime->buffer_size)
>  		subs->hwptr_done -= runtime->buffer_size;
> +	runtime->delay += offs;
>  	spin_unlock_irqrestore(&subs->lock, flags);
>  	urb->transfer_buffer_length = offs * stride;
>  	if (period_elapsed)
> @@ -638,12 +639,20 @@ static int prepare_playback_urb(struct snd_usb_substream *subs,
>  
>  /*
>   * process after playback data complete
> - * - nothing to do
> + * - decrease the delay count again
>   */
>  static int retire_playback_urb(struct snd_usb_substream *subs,
>  			       struct snd_pcm_runtime *runtime,
>  			       struct urb *urb)
>  {
> +	unsigned long flags;
> +	int stride = runtime->frame_bits >> 3;
> +	int processed = urb->transfer_buffer_length / stride;
> +
> +	spin_lock_irqsave(&subs->lock, flags);
> +	if (processed > runtime->delay)
> +		runtime->delay -= processed;
> +	spin_unlock_irqrestore(&subs->lock, flags);
>  	return 0;
>  }
>  
> @@ -1542,6 +1551,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
>  	subs->hwptr_done = 0;
>  	subs->transfer_done = 0;
>  	subs->phase = 0;
> +	runtime->delay = 0;
>  
>  	/* clear urbs (to be sure) */
>  	deactivate_urbs(subs, 0, 1);
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Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
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