On 13/3/20 9:55 am, Pierre-Louis Bossart wrote:
On 3/11/20 5:54 PM, Matt Flax wrote:
Hi there,
A large number of audio codecs allow different formats for playback
and capture. This becomes very useful when there are different
latencies between playback and capture hardware data lines. For
example digital isolation chips typically have a 1 bit delay in
propagation as the bit clock rate gets faster for higher sample
rates. By setting the capture and playback formats to differ by one
or two bit clock cycles, the delay problem is solved.
There doesn't seem to be a simple way to detect stream direction in
the codec driver's set_fmt function.
The snd_soc_runtime_set_dai_fmt function :
https://github.com/torvalds/linux/blob/master/sound/soc/soc-core.c#L1480
calls the snd_soc_dai_set_fmt function :
https://github.com/torvalds/linux/blob/master/sound/soc/soc-dai.c#L101
which calls the set_fmt function :
https://github.com/torvalds/linux/blob/master/include/sound/soc-dai.h#L189
The snd_soc_dai_ops set_fmt function is defined as :
int (*set_fmt)(struct snd_soc_dai *dai, unsigned int fmt);
Is there a simple way to find the stream direction from a snd_soc_dai ?
If the stream direction can be detected then the playback and capture
formats can be set independently for the codec.
It there a different way to set the playback and capture formats for
the codec independently at runtime, depending on the sample rate ?
FWIW I remember a discussion in the past on how to deal with
interfaces that may have different clocks sources for capture and
playback (typically with the 6-pin version of I2S/TDM), and the answer
was: use two dais, with one dealing with capture and the other with
playback.
I would bet this applies for the format as well. If you use a DAI that
can do both directions, then indeed there's no obvious way to specify
that formats or clock ownership could be different between the two
directions.
It would probably make sense anyway to have a representation with two
dais, e.g. the codec capture dai receives data from somewhere and the
codec playback dai forwards it to another destination.
I think I get it ...
This approach would keep extra stream selective functionality out of
soc-dai.c. That is probably a good thing for the simplicity of the core.
A machine driver could then call snd_soc_dai_set_fmt passing in the
correct codec_dai from the codec_dais array for the stream they want to
operate on.
Matt
My 2 cents
-Pierre
