[alsa-devel] [PATCH] ASoc: Handle multiple codecs with split playback / capture

[Ricard Wanderlof]

On Mon, 17 Aug 2015, Mark Brown wrote:

> On Mon, Aug 17, 2015 at 10:02:29AM +0200, Ricard Wanderlof wrote:
> > On Fri, 14 Aug 2015, Mark Brown wrote:
> > > On Thu, Aug 13, 2015 at 04:12:56PM +0200, Ricard Wanderlof wrote:
> 
> > Should the commit message have any information as to which specific 
> > use case led to the creation of the patch?
> 
> That's often good to mention, yes.

Ok, will do.

> > > > +	return !!codec_stream->rates;
> 
> > > The !! is redundant here.
> 
> > I still felt it made sense as rates is an int and the function 
> > actually returns a bool, i.e. the !! indicates to a human reader that 
> > the author of the function is aware that rates is not a bool yet the 
> > function is declared as one.
> 
> It's sufficiently obvious that you're just checking if rates has anything 
> set at all here I think.

Ok. I will change it then.

> > > > +		/* Skip codecs which don't support the current stream 
> type. */
> > > > +		if (!snd_soc_dai_stream_valid(codec_dai, 
> substream->stream))
> > > > +			continue;
> 
> > > >  		/* copy params for each codec */
> > > >  		codec_params = *params;
> 
> > > We're still calling hw_params() for all CODECs, even ones that 
> > > happen to not be suitable...  is that sensible?
> 
> > Hm, how so, the soc_dai_hw_params(..., codec_dai) is further on inside 
> > the loop so the continue skips over it?
> 
> > However, if nothing matches, soc_dai_hw_params(..., cpu_dai) would 
> > still be called, just after the loop, which might not make sense.
> 
> I think that's the bit I was thinking of, sorry.

[in light of this, rephrasing Mark's comment to:]
We're still calling soc_dai_hw_params() even if no CODEC DAI matches.
> > > ...  is that sensible? It might be in a 
> > > case where the CODEC for one direction clocks everything but I can 
> > > see problems occurring if the transmit and receive paths are 
> > > separately clocked which is going to be quite common for things like 
> > > i.MX systems.  If the two are tied together there's going to be 
> > > constraints between the two directions.
> 
> > > I'd expect that we could take a pretty good guess as to what the 
> > > best thing to do is based on the symmetry constraints we have for 
> > > the link but I think we do want to think about this early.
> 
> > I guess it would be better just to 'goto out' after the loop if a 
> > suitable codec could not be found.
> 
> Well, there's two things here: one is if we've got a completely invalid 
> configuration with no CODECs being configured and the other is if we 
> should only be configuring matching CODECs in the first place.  For 
> systems where the LRCLK and BCLK are shared between playback and capture 
> we may want to always configure them even if the CODEC driving the clocks 
> isn't the one handling the audio.

Considering the first thing, i.e. a completely invalid configuration, such 
as trying to play back on a capture-only DAI link, then I would expect 
that would be taken care of long before we get to even attempt to 
configure anything, resulting in something like 'Unknown PCM' at the user 
level. We could of course bail out of this function before attempting to 
set up the CPU DAI in the event that that type of situation occurs anyway. 
I can add code to do just that.

The second thing is which codecs to configure. Should we attempt to 
configure all of them, even those that don't match the stream type, or (as 
the patch is currently written) only the ones that match the stream type. 
Let me see if I get the issues right here:

In the former case, IIUC, this might be needed if we have a codec that is 
generating clocks even though it is not actually used for the transfer, 
e.g. a playback codec generating LRCLK and/or BCLK for the whole link, 
including the capture codec. In this case we would want to set up both 
codecs even though only one is actually used.

In the latter case there might be systems which need to set up different 
clocking configurations for playback and capture on the CPU DAI, in which 
case we only want to configure matching codecs.

So how do we differentiate between these two cases? It seems to me that we 
cannot determine this using software means as it is really part of how the 
hardware is designed, so it must either be specified in the machine driver 
or via DT.

Since the current situation is that we can't support separate playback 
and capture codecs on a DAI link at all without this patch, my suggestion 
is to let the patch handle the case at hand, and add a comment as to which 
considerations have been made regarding which codecs are to be configured 
in soc_pcm_hw_params(), so that when the need arises to handle cases with 
different CPU DAI settings for both directions that can be handled 
appropriately.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30