[PATCH v2] staging: bcm2835-audio: interpolate audio delay

[Kirill Marinushkin]

Hello Mike,

AFAIU, this patch is wrong. Please correct me, maybe I misunderstand something.

> The problem that this patch seeks to resolve is that when userland asks for
> the delay

The userspace asks not for delay, but for the pointer.
You modify the function, which is called `snd_bcm2835_pcm_pointer`. Here you are
supposed to increase `alsa_stream->pos` with the proper offset. Instead, you
imitate a delay, but in fact the delay is not increased.

So, the proper solution should be to fix the reported pointer. As a result,
userspace will recieve the correct delay, instead of these crazy 10 ms.

> FYI, there is
> a discussion of the effects of a downstream equivalent of this suggested patch
> at:
> https://github.com/raspberrypi/firmware/issues/1026#issuecomment-415746016.

Thank you for the link, it clarified for me what you try to achieve.

On 10/22/18 21:17, Mike Brady wrote:
> When the BCM2835 audio output is used, userspace sees a jitter up to 10ms
> in the audio position, aka "delay" -- the number of frames that must
> be output before a new frame would be played.
> Make this a bit nicer for userspace by interpolating the position
> using the CPU clock.
> The overhead is small -- an extra ktime_get() every time a GPU message
> is sent -- and another call and a few calculations whenever the delay
> is sought from userland.
> At 48,000 frames per second, i.e. approximately 20 microseconds per
> frame, it would take a clock inaccuracy of
> 20 microseconds in 10 milliseconds -- 2,000 parts per million --
> to result in an inaccurate estimate, whereas
> crystal- or resonator-based clocks typically have an
> inaccuracy of 10s to 100s of parts per million.
> 
> Signed-off-by: Mike Brady <mikebrady at eircom.net>
> ---
> Changes in v2 -- remove inappropriate addition of SNDRV_PCM_INFO_BATCH flag
> 
>  .../vc04_services/bcm2835-audio/bcm2835-pcm.c | 20 +++++++++++++++++++
>  .../vc04_services/bcm2835-audio/bcm2835.h     |  1 +
>  2 files changed, 21 insertions(+)
> 
> diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
> index e66da11af5cf..9053b996cada 100644
> --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
> +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
> @@ -74,6 +74,7 @@ void bcm2835_playback_fifo(struct bcm2835_alsa_stream *alsa_stream,
>  	atomic_set(&alsa_stream->pos, pos);
>  
>  	alsa_stream->period_offset += bytes;
> +	alsa_stream->interpolate_start = ktime_get();
>  	if (alsa_stream->period_offset >= alsa_stream->period_size) {
>  		alsa_stream->period_offset %= alsa_stream->period_size;
>  		snd_pcm_period_elapsed(substream);
> @@ -243,6 +244,7 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream)
>  	atomic_set(&alsa_stream->pos, 0);
>  	alsa_stream->period_offset = 0;
>  	alsa_stream->draining = false;
> +	alsa_stream->interpolate_start = ktime_get();
>  
>  	return 0;
>  }
> @@ -292,6 +294,24 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_substream *substream)
>  {
>  	struct snd_pcm_runtime *runtime = substream->runtime;
>  	struct bcm2835_alsa_stream *alsa_stream = runtime->private_data;
> +	ktime_t now = ktime_get();
> +
> +	/* Give userspace better delay reporting by interpolating between GPU
> +	 * notifications, assuming audio speed is close enough to the clock
> +	 * used for ktime
> +	 */
> +
> +	if ((ktime_to_ns(alsa_stream->interpolate_start)) &&
> +	    (ktime_compare(alsa_stream->interpolate_start, now) < 0)) {
> +		u64 interval =
> +			(ktime_to_ns(ktime_sub(now,
> +				alsa_stream->interpolate_start)));
> +		u64 frames_output_in_interval =
> +			div_u64((interval * runtime->rate), 1000000000);
> +		snd_pcm_sframes_t frames_output_in_interval_sized =
> +			-frames_output_in_interval;
> +		runtime->delay = frames_output_in_interval_sized;
> +	}
>  
>  	return snd_pcm_indirect_playback_pointer(substream,
>  		&alsa_stream->pcm_indirect,
> diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
> index e13435d1c205..595ad584243f 100644
> --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
> +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
> @@ -78,6 +78,7 @@ struct bcm2835_alsa_stream {
>  	unsigned int period_offset;
>  	unsigned int buffer_size;
>  	unsigned int period_size;
> +	ktime_t interpolate_start;
>  
>  	struct bcm2835_audio_instance *instance;
>  	int idx;
>