[alsa-devel] safe support for rewind in ALSA
perex at perex.cz
Mon Feb 1 19:00:52 CET 2010
On Mon, 1 Feb 2010, pl bossart wrote:
> I'd like to reopen a thread on concerns with the current
> implementation of rewind in ALSA. This was already discussed on the
> mailing list last year without any progress.
I don't think that there have been no progress. The queued samples can be
stored to runtime->delay now, so snd_pcm_delay() returns a correct
The snd_pcm_rewind() and snd_pcm_forward() functions should operate in the
ring buffer and it's ok. See the USB driver for an example. Just add
support for runtime->delay to the lowlevel drivers and use snd_pcm_delay()
correctly in the user space and everything will work as expected.
In other words - for hardware with large FIFOs, the runtime->delay should
be used for queued samples and the hw_ptr in the ring buffer should be
increased as soon as the FIFO is filled with samples from the ring buffer.
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.
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