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

[Takashi Iwai <tiwai@suse.de>]

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.

> 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 :)


Takashi