[PATCH] ASoC: core: use less strict tests for dailink capabilities

[Jerome Brunet]

On Fri 24 Jul 2020 at 21:05, Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com> wrote:

>> Again, this is changing the original meaning of the flag from "playback
>> allowed" to "playback required".
>>
>> This patch (or the orignal) does not explain why this change of meaning
>> is necessary ? The point I was making here [0] still stands.
>>
>> If your evil plan is to get rid of 2 of the 4 flags, why go through the
>> trouble of the changing the meaning and effect of one them ?
>
> My intent was to have a non-ambiguous definition.

I still fail to understand how it was ambiguous and how throwing an
error for something that used to work well so far is making things better.

Maybe there could be have been a better name for it, but what it did was
clear.

The flag is even (briefly) documented:
	/* DPCM capture and Playback support */
	unsigned int dpcm_capture:1;
	unsigned int dpcm_playback:1;

"Support" means the dai_link supports it, not that it is required for it
work. This is what was implemented.

>
> I don't know 'playback allowed' means. What is the point of using this flag
> if it may or may not accurately describe what is actually implemented? And
> how can we converge the use of flags since in the contrary 'playback_only'
> is actually a clear indication of what the link does. We've got to align on
> the semantics, and I really don't see the point of watering-down
> definitions. When things are optional or poorly defined, the confusion
> continues.

The problem is that commit b73287f0b074 ("ASoC: soc-pcm: dpcm: fix
playback/capture checks") has changed the semantic:
* without actually warning that it was doing so in the commit description
* breaking things for other who relied on the previous semantics

Previous semantics of the flag allowed to disable a stream direction on
a link which could have otherwise had it working, if the stream had it.
It added information/control on the link at least.

New flag semantics forces the flag and stream capabilities to be somehow
aligned. This is not clearing the confusion, this is redundant
information. How is this helping the framework or the users ?

>
> WFIW, my 'evil' plan was to rename 'dpcm_playback' as 'can_playback' (same
> for capture) and replace 'playback_only' by 'can_playback = 1; can_capture
> = 0'. So this first step was really to align them on the expected behavior
> and minimal requirements.

IMO the previous flag semantics was inverted yes, but aligned:

playback_only = 1 was the same as dpcm_capture = 0
capture_only = 1 was the same as dpcm_playback = 0

Having both *_only set does not make sense for a stream, same as having
none of dpcm_*

Having none of *_only flag means there is no restriction on the stream,
same as having both dpcm_* set.

This seems aligned to me.