[alsa-devel] [PATCH 4/6] alsa-lib:pcm: fix the hw_ptr update until the boundary available.

[Takashi Iwai]

On Fri, 30 Dec 2016 07:29:27 +0100,
sutar.mounesh at gmail.com wrote:
> 
> From: mahendran.k <mahendran.kuppusamy at in.bosch.com>
> 
> For long time test case, the slave_hw_ptr will exceed the boundary
> and wraparound the slave_hw_ptr. This slave boundary wraparound will
> cause the rate->hw_ptr to wraparound irrespective of the rate->boundary
> availability and due to that the available size goes wrong.
> 
> Hence, To get the correct available size,
> - Its necessary to increment the rate->hw_ptr upto the rate->boundary
> and then wraparound the rate->hw_ptr.
> - While handling fraction part of slave period, rounded value will be
> introduced by input_frames(). To eliminate rounding issue on rate->hw_ptr,
> subtract last rounded value from rate->hw_ptr and add new rounded value
> of present slave_hw_ptr fraction part to rate->hw_ptr.
> 
> Signed-off-by: mahendran.k <mahendran.kuppusamy at in.bosch.com>
> Signed-off-by: Mounesh Sutar <mounesh_sutar at mentor.com>

Thanks, it's been a long-standing issue, indeed.

I applied this one (again) with a slight coding style fix now.
But...

> diff --git a/src/pcm/pcm_rate.c b/src/pcm/pcm_rate.c
> index 1f830dd..5bee77c 100644
> --- a/src/pcm/pcm_rate.c
> +++ b/src/pcm/pcm_rate.c
> @@ -50,7 +50,7 @@ typedef struct _snd_pcm_rate snd_pcm_rate_t;
>  
>  struct _snd_pcm_rate {
>  	snd_pcm_generic_t gen;
> -	snd_pcm_uframes_t appl_ptr, hw_ptr;
> +	snd_pcm_uframes_t appl_ptr, hw_ptr, last_slave_hw_ptr;
>  	snd_pcm_uframes_t last_commit_ptr;
>  	snd_pcm_uframes_t orig_avail_min;
>  	snd_pcm_sw_params_t sw_params;
> @@ -563,14 +563,28 @@ static inline void snd_pcm_rate_sync_hwptr0(snd_pcm_t *pcm, snd_pcm_uframes_t sl
>  {
>  	snd_pcm_rate_t *rate = pcm->private_data;
>  
> +	snd_pcm_sframes_t slave_hw_ptr_diff = slave_hw_ptr - rate->last_slave_hw_ptr;
> +	snd_pcm_sframes_t last_slave_hw_ptr_frac;
> +
>  	if (pcm->stream != SND_PCM_STREAM_PLAYBACK)
>  		return;
> -	/* FIXME: boundary overlap of slave hw_ptr isn't evaluated here!
> -	 *        e.g. if slave rate is small... 
> -	 */
> -	rate->hw_ptr =
> -		(slave_hw_ptr / rate->gen.slave->period_size) * pcm->period_size +
> -		rate->ops.input_frames(rate->obj, slave_hw_ptr % rate->gen.slave->period_size);
> +
> +	if (slave_hw_ptr_diff < 0)
> +		slave_hw_ptr_diff += rate->gen.slave->boundary; /* slave boundary wraparound */
> +	else if (slave_hw_ptr_diff == 0)
> +		return;
> +	last_slave_hw_ptr_frac = rate->last_slave_hw_ptr % rate->gen.slave->period_size;
> +	/* While handling fraction part fo slave period, rounded value will be introduced by input_frames().
> +	 * To eliminate rounding issue on rate->hw_ptr, subtract last rounded value from rate->hw_ptr and
> +	 * add new rounded value of present slave_hw_ptr fraction part to rate->hw_ptr. Hence,
> +	 * rate->hw_ptr += [ (no. of updated slave periods * pcm rate period size) -
> +	 * 				fractional part of last_slave_hw_ptr rounded value +
> +	 * 				fractional part of updated slave hw ptr's rounded value ] */
> +	rate->hw_ptr += (
> +			(((last_slave_hw_ptr_frac + slave_hw_ptr_diff) / rate->gen.slave->period_size) * pcm->period_size) -
> +			rate->ops.input_frames(rate->obj, last_slave_hw_ptr_frac) +
> +			rate->ops.input_frames(rate->obj, (last_slave_hw_ptr_frac + slave_hw_ptr_diff) % rate->gen.slave->period_size));
> +	rate->last_slave_hw_ptr = slave_hw_ptr;

.... this ended up with a substantial increase of computation.
Can we optimize, e.g. doing this conditionally only for the
overwrapping case?


thanks,

Takashi