2010/6/28 David Dillow <dave@xxxxxxxxxxxxxx> > > > On Mon, 2010-06-28 at 08:37 +0800, Raymond Yau wrote: > > 2010/6/28 David Dillow <dave@xxxxxxxxxxxxxx> > > > Perhaps documentation that recommends setting avail_min to 1 when using > > > read/write to avoid this issue would be helpful, in lieu of changing > the > > > ALSA code or giving concrete guarantees. > > > > > > Dave > > > > > > > > Refer to > > http://article.gmane.org/gmane.linux.alsa.devel/69333/match=avail_min > > > > avail_min cannot be less than period size , however set_avail_min() did > not > > return any error when the value is less than period size and this mislead > > the programmers to believe that they can set avail_min to 1 > > They used to be able to, and it would work. You likely cannot get > latency that low, but anyone that was expecting for that to magically > change the way hardware works is fooling themselves. avail_min is like > the timeout argument to poll() -- you're guaranteed at least that value, > but you may get something much larger. > > I understand the rationale given for patch b0b7d0 to alsa-lib, but > limiting avail_min in this way removed a workaround for the muddled > snd_pcm_read/write() semantics. I suppose it is still possible, if > someone wants to color outside libasound -- the kernel still allows it. > > > if you want low latency, you need to use small period size > > Small period sizes are often needed for low latency -- 10ms is common > for VoIP -- but if you are stuck with a device that can only do two > periods per buffer, the current read/write code will cause unstable > audio with frequent overruns. You can add another round trip to the > kernel with snd_pcm_wait(), but that's suboptimal and should not be > required. > > You can get reasonably good stability on limited devices with avail_min > == 1, or with a read/write that doesn't force you to wait two periods > when trying to work with one. > > Dave > > The audio data record at the sample rate by ac97codec through ac97link to ac97 controller e.g. cs46xx use snd_pcm_indirect_capture_transfer() it is unlikely to set avail_min or minimum period size to a value less than the PCI brust size if your driver are using DMA transfer you cannot set avail_min to 1 unless your period_size is 1 because if (val < pcm->period_size) val = pcm->period_size; http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=b0b7d0280f977dee1bbff0a1b4cd0c50068d2371;hp=1cf37d72c4e77fac55f2b1792bdcf12744a5a51a int snd_pcm_sw_params_set_avail_min(snd_pcm_t *pcm, snd_pcm_sw_params_t *params, snd_pcm_uframes_t val) #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; params->avail_min = val; return 0; } _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
