On Thu, 2015-09-03 at 09:08 +0200, Takashi Iwai wrote:
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?
Yes it is odd to what we would normally expect. Due to our HW design, when irq comes, audio HW actually has collected a full period of data, but there is a buffer between the audio HW and memory, so at that moment some samples are still in the buffer, not on the memory. Add a small delay between triggering capture HW and enabling IRQ can also fix this, although I think changing the avail_min should be better.
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.
Sounds a little dangerous...How should I verify this? It this problem platform dependent?
thanks,
Takashi
Signed-off-by: Koro Chen koro.chen@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;}
1.7.9.5
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