On Fri, 22 May 2009, Takashi Iwai wrote:
At Thu, 21 May 2009 14:44:27 -0400, Jon Smirl wrote:
I've been reading Lennart's article on glitch free audio....
On my scatter/gather DMA hardware I generate an interrupt at the end of each period. On the last period this causes ALSA to issue a trigger(STOP). However, there is a latency from when that interrupt is generated and when ALSA does the trigger(STOP). During that latency my hardware starts playing the next buffer (which contains stale data). Playing stale data causes a noise burst. I fixed this by programming my DMA hardware to not overrun appl_ptr.
There's another solution. ALSA already knows the stream ends mid-period and pads out the partial period with silence. If it simply padded out the next period with silence too the overrun problem would become non-audible and there would time to process trigger(STOP).
If padding out two periods at end of stream is possible, it then becomes possible for me to turn off interrupts in my audio driver.
This can be another interesting feature. For example, you could add a soft-mute padding to avoid a click noise at the end instead of a whole silence.
But, this kind of hack really depends on the hardware implementation. If a hardware has no DMA but require explicit writes, adding two periods is too much. Just a few samples would be enough.
I agree to leave such things to lowlevel drivers.
Or, in the case of normal PCI DMA transfer, how can it be stopped? That is, the stream is still running although it's claimed to be stopped. If a period size is big (say over one second), the stop at the next interrupt doesn't sound good.
I would suggest to improve drain() logic in kernel with snd_pcm_update_hw_ptr() calls and using proper timeouts for schedule_timeout().
Note that this situation were handled with dead and removed pcm tick API - although it required assistance (setup) from application.
Jaroslav
----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.