[RFC PATCH (alsa-lib)] pcm: Modify check condition in snd_pcm_sw_params_set_avail_min

[Takashi Iwai]

On Thu, 03 Sep 2015 05:20:54 +0200,
Koro Chen wrote:
> 
> When we use a ping-ping buffer in capture, and if hw_ptr reported
> at IRQ is a little smaller than period_size:
> 
> |xxxxxxxxxxxxxxxxxxxxxxxxxxxx--|-----------------------------|
>                             hw_ptr < period_size

How this happens?  The period size is the size where irq (or wakeup)
wakes up for read/write.  Why the driver wakes up even if there is no
enough data?

> This available buffer will not be read since its size is smaller than
> avail_min (which is set to be period_size), and read thread continues
> to sleep. If the next hw_ptr is just a little larger than buffer_size,
> overrun occurs.
> 
> This could be resolved by setting a small avail_min to kernel,
> for example, 1, so read thread wakes up and reads every data at IRQ.
> But current alsa-lib only allows avail_min to be at least period_size.
> Remove the constraint and only check for zero case.

The restriction was introduced for avoiding CPU hogs with rate plugin
in many years ago.  avail_min=1 *might* work now because of the later
fix for rate plugin, but this must be verified.


thanks,

Takashi

> 
> Signed-off-by: Koro Chen <koro.chen at mediatek.com>
> ---
>  src/pcm/pcm.c |    8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
> index f5fc728..8492689 100644
> --- a/src/pcm/pcm.c
> +++ b/src/pcm/pcm.c
> @@ -5958,12 +5958,8 @@ int snd_pcm_sw_params_set_avail_min(snd_pcm_t *pcm, snd_pcm_sw_params_t *params,
>  #endif
>  {
>  	assert(pcm && params);
> -	/* Fix avail_min if it's below period size.  The period_size
> -	 * defines the minimal wake-up timing accuracy, so it doesn't
> -	 * make sense to set below that.
> -	 */
> -	if (val < pcm->period_size)
> -		val = pcm->period_size;
> +	if (!val)
> +		val = 1;
>  	params->avail_min = val;
>  	return 0;
>  }
> -- 
> 1.7.9.5
>