[alsa-devel] Writing a DMA-less PCM audio driver

Jaroslav Kysela perex at perex.cz
Tue Oct 18 20:16:20 CEST 2011

Date 18.10.2011 17:56, David Jander wrote:
> Hi all,
> I am writing a PCM audio driver for a piece of (embedded) hardware that has no
> DMA, only a FIFO capable of holding 2048 samples. I was trying to use a
> kthread with high realtime (SCHED_FIFO) priority to keep the FIFO filled, and
> sleep with schedule_timeout(x), where "x" is depending on the current FIFO fill
> level. The same thread also calls snd_pcm_period_elapsed() on every completed
> period. It seems to work, but I get sporadic audio skips and I am trying to
> figure out where they come from. Most of the time schedule_timeout(1) takes
> almost exactly 1ms (1 jiffy at HZ=1000) to complete, but sometimes it takes up
> to 30ms. It seems related to file-IO (via NFS) happening on the system, but
> the effect is far bigger if the program that is playing the audio (mplayer)
> itself is producing file-IO. When using a large cache parameter on mplayer
> (effectively preloading the entire MP3 file into RAM), the problem is almost
> gone.
> Before deciding whether I should debug the network driver to see if it could
> produce such tremendous amounts of latency, is there a better way to solve this
> problem? Are there any other DMA-less audio drivers I could look at as an
> example?
> Any suggestion is welcome.

Any pure PCMCIA driver has to handle FIFOs (see the sound/pcmcia tree).
They use hardware interrupts for a better timing.


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.

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