Re: [RFC 2/2] ASoC: rt5670: Add LED trigger support

[Jaroslav Kysela <perex@perex.cz>]

Dne 24. 02. 21 v 9:52 Takashi Iwai napsal(a):
> On Wed, 24 Feb 2021 09:14:41 +0100,
> Jaroslav Kysela wrote:
>>
>> Dne 24. 02. 21 v 8:12 Takashi Iwai napsal(a):
>>> On Tue, 23 Feb 2021 21:56:16 +0100,
>>> Jaroslav Kysela wrote:
>>>>
>>>> Dne 23. 02. 21 v 17:20 Takashi Iwai napsal(a):
>>>>>>> Of course, this implementation would make the integration much easier,
>>>>>>> and that's a big benefit.  So I have a mixed feeling and not decided
>>>>>>> yet whether we should go for it right now...
>>>>>>
>>>>>> I think that we can reconsider the LED handling implementation later, when
>>>>>> someone brings something better on the table.
>>>>>
>>>>> What worried me is the plan to expose this capability to user-space.
>>>>> If it's only a kernel-internal, we can fix it in the kernel and
>>>>> nothing else broken, but if it's a part of API, that's not easy.
>>>>>
>>>>> So, if any, I'd like to avoid exposing to the user-space at first.
>>>>> (But then it comes to the question how to deal with a case like AMD
>>>>> ACP...)
>>>>
>>>> I tried to propose a complete solution and the ACP was one strong reason for
>>>> this kernel / user space API. So without the user space support, it's just
>>>> a half solution for known issues.
>>>>
>>>> Frankly, I don't see any drawback or a problem even if we remove this API
>>>> later.
>>>
>>> Removing the user-space API is absolutely no-go.  The only exception
>>> would be either the case really no one uses it or it's too buggy and
>>> unfixable.
>>
>> This is a special case. Even if those LED bits are ignored by kernel in
>> future, we expect to be replaced with another layer. Thus the functionality
>> must be retained.
> 
> Well, we cannot know whether the replacement really happens or
> happened, and hence we never kill the old one.  That's the problem.
> 
>>>> The LED group bits are just informal for the user space and it's
>>>> expected to create the user controls tied to this LED functionality only in
>>>> alsa-lib/plugins at the moment. The kernel may return an error when the user
>>>> space tries to set those new bits when the API is deprecated and I believe
>>>> that the hardware design faults like AMD ACP (without the hardware mute) are rare.
>>>
>>> The experience tells us that users are creative enough to (ab)use a
>>> new ABI in any unexpected ways, and we have no control for it.  So
>>> it's not about how alsa-lib is implemented but rather how ABI could be
>>> abused :)
>>
>> Ok, I don't have other ideas. I don't agree with your argumentation for this
>> particular case, where the functionality is marginal. Ideally, the AMD driver
>> may be recoded to use double-buffering and software mute switch, so we should
>> handle everything in the kernel space.
> 
> My argument is that we're trying to add too much freedom just for this
> "marginal" problem.  Honestly speaking, I would feel rather more
> comfortable if it were a kernel control element that just does trigger
> the LED like the original patch from AMD guys.  Then you cannot do
> much wrong.  OTOH, creating a virtual capture switch and let alsa-lib
> handling the software mute, while PA should ignores the soft-mute but

We can force the softvol even if PA set the skip flag for this particular PCM
stream.

> dealing only with the assigned mute LED...  Sounds too complex to me.

It seems that you misunderstood the number of issues which my code is trying
to resolve:

1) set LED based on state from multiple cards (so you cannot trigger LED
inside single driver / single control element); we need one arbiter; this is
the main argument
2) unifies the audio LED interface
3) reduce the hardware driver code

					Jaroslav

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.