On 2/3/16 5:49 AM, Tim Sheridan wrote:
Hi,
I've been working on the Imagination Technologies AXD (audio DSP)
compress offload driver. One of the features of the DSP is scheduled
playback of audio using timestamps (that are obtained by an
application from userspace). A current problem with the compress
offload API is that there's no "blessed" way to get these timestamps
from tinycompress through to our compress offload driver.
Currently, I've added a SNDRV_COMPRESS_ENCODER_PTS value to the
sndrv_compress_encoder enum, and exposed this with an API in
tinycompress, handling this in my driver to pass it to the DSP. Does
this sound like a reasonable approach to go about adding compress
offload API support for this? Or is there some other part of the ALSA
API which would be more appropriate to use instead of this?
Not a simple problem I am afraid.
When we added support for compressed data the focus was really
elementary streams transferred over DMA - essentially the same model as
for PCM which doesn't support timestamps either. If you have an
application which deals with discontinuous buffers associated with
timestamps then the model is broken.
The only solution I can think of is to create a pseudo transport stream
with a headset containing the timestamp and the audio data inserted
between headers. I believe this is what you are suggesting?
Alternatively we could add an API that would match a decoded sample with
a timestamp but it's not clear how you would synchronize timestamp
information with the data stream (or rather there could be race
conditions leading to the timestamps being provided to the hardware too
late).
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