ASoc: soc_core.c stream direction from snd_soc_dai

[Pierre-Louis Bossart]

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.

My 2 cents
-Pierre