[alsa-devel] [RFC 0/4] FIFO caused playback delay (latency) handling in soc

[Peter Ujfalusi]

Hello,

There has been discussion in alsa-devel in this subject several times,
but no actual patches has been sent.

Based on the available information the latency caused by the HW buffer
on some systems can be handled by updating the runtime->delay.

It has been discussed, that the runtime->delay can also be updated
dynamically to show more accurate delay.

To further complicate things, in ASoC we could have more buffer in the
chain. To handle this we need soc level support.

This RFC series tries to do that in soc by:
- introducing a pcm_pointer wrapper
- in this wrapper we call the original pcm_pointer functions to get the
  DMA pointer
- introducing a new interface in dai_ops to ask the delay from the dais
- adding the cpu_dai and codec_dai returned delay to form the actual
  delay
- update the runtime->delay with this value.

With this approach none of the existing drivers need change, but they
can add support for specifying the FIFO caused delay.

In this series on top of the core changes the omap(3) code is updated
to take this delay reporting into use.
I have not added the support to the tlv320dac33 codec driver, since it
needs a bit more work, but along the same line it can be done, and if
the tlv320dac33 is hooked to omap McBSP than applications can know the
whole delay/latency on that path.

Since this is for RFC, I have based it on sound-2.6 topc/asoc branch,
which does not have the McBSP regcache nor the sidetone patches, but I
would like to get feedback on this method first than create a proper
series.

I have left out linux-omap from this RFC round, since the problem
is ALSA related.

Thank you,
Péter Ujfalusi

---
Peter Ujfalusi (4):
  ASoC: core: soc level wrapper for pcm_pointer callback
  ASoC: core: Add delay operation to snd_soc_dai_ops
  OMAP3: McBSP: Add interface for transmit FIFO state query
  ASoC: OMAP3: Report delay on playback caused by the internal FIFO

 arch/arm/plat-omap/include/plat/mcbsp.h |    4 +++
 arch/arm/plat-omap/mcbsp.c              |   27 ++++++++++++++++++++++++
 include/sound/soc-dai.h                 |    6 +++++
 sound/soc/omap/omap-mcbsp.c             |   26 +++++++++++++++++++++++
 sound/soc/soc-core.c                    |   34 ++++++++++++++++++++++++++++++-
 5 files changed, 96 insertions(+), 1 deletions(-)