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

[Jerome Brunet]

On Mon 27 Jul 2020 at 16:13, Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com> wrote:

> On 7/27/20 4:42 AM, Jerome Brunet wrote:
>>
>> 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.
>
> Makes no sense to me to have information that's useless.

Maybe. That's not point
The point is
* No explanation has been provided so far about why throwing an error
  like done here (or in the previous change) makes it more usefull.
  The semantic change just make it redundant with the information
  coming from the DAI caps. The new semantic makes the flag even more
  useless.
  
* Throwing an error like break cards that used to work nicely for no
  gain
  
* This adds yet another level of complexity that was not necessary
  before (snd_soc_dai_link_set_capabilities())

> What does 'no restrictions' on a stream mean?

I thought the code was fairly simple but I can explain
- A dai_link has 2 stream directions. The direction can be enabled
  if the DAIs on the link supports it.
- A direction could be forcefully disabled at the dai_link level using
  those flags (restrict the direction). I suppose to give more control
  to the card driver.

I did not write that code, I have no idea if those flags are any use to
anyone. 

> 'anything goes' is not a scalable design principle.

What does scalability has to do with the matter ?

In the end, I'm just asking to drop the error condition you added.

You want to rework/remove some flags, I think it is a great idea.
I even willing to help out, but not in a way that makes things complex
and redundant.