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

[Takashi Iwai]

On Wed, 24 Feb 2021 10:27:13 +0100,
Jaroslav Kysela wrote:
> 
> 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

Those purposes are all fine.  But they don't need to be exposed for
user controls that can be abused.  That's the very concern.


Takashi