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

From: Takashi Iwai <tiwai@suse.de>
To: Jaroslav Kysela <perex@perex.cz>
Cc: Oder Chiou <oder_chiou@realtek.com>,
	alsa-devel@alsa-project.org, Liam Girdwood <lgirdwood@gmail.com>,
	Hans de Goede <hdegoede@redhat.com>,
	Mark Brown <broonie@kernel.org>, Bard Liao <bard.liao@intel.com>
Subject: Re: [RFC 2/2] ASoC: rt5670: Add LED trigger support
Date: Thu, 25 Feb 2021 12:00:53 +0100	[thread overview]
Message-ID: <s5hzgzspg2i.wl-tiwai@suse.de> (raw)
In-Reply-To: <3c262ea6-2313-3af8-60ae-d59ae8be262d@perex.cz>

On Wed, 24 Feb 2021 18:57:42 +0100,
Jaroslav Kysela wrote:
> 
> Dne 24. 02. 21 v 13:42 Takashi Iwai napsal(a):
> > On Wed, 24 Feb 2021 13:08:55 +0100,
> > Jaroslav Kysela wrote:
> >>
> >> Dne 24. 02. 21 v 12:43 Takashi Iwai napsal(a):
> >>
> >>>>> So far, a user control is merely storing the value, let read/write via
> >>>>> the control API.  That's all, and nothing wrong can happen just by
> >>>>> that.  Now if it interacts with other subsystem...
> >>>>>
> >>>>> A more serious concern is rather the fragility of the setup; for
> >>>>> enabling the mute LED control, you'd have to create a new user-space
> >>>>> control, the function of the control has to be ignored by some
> >>>>> application and some not, etc.  This has to be done on each machine
> >>>>
> >>>> You're using "ignore", but as I explained before, the user space switch will
> >>>> be used in the whole chain:
> >>>>
> >>>> capture stream ->
> >>>>   alsa-lib mute switch / silence PCM stream ->
> >>>>   PA mute switch / silence PCM stream
> >>>>
> >>>> So PA can use this switch like the traditional hardware mute switch.
> >>>
> >>> Does it mean PA would work as of now without any change?  Or does it
> >>> need patching?
> >>
> >> Yes, no PA modifications are required with my mechanism. The PA will just see
> >> the new user space control - mute switch - created in alsa-lib - which will be
> >> synced the internal PA path mute state like for the hardware mute
> >> switch.
> > 
> > OK, but how would we create and manage the user control element?  And
> > why it has to be user control?
> 
> The softvol or alsactl can create the user control element. The alsa-lib
> softvol plugin and PA can silence stream according the state.

And that's tricky if it's only with PA, as PA won't open a softvol PCM
stream...

> I see your point to create this control in the kernel space, but any other
> name than "Mic Capture Switch" (in the ACP case) will be misleading for users,
> because the user-space does the appropriate real silencing job instead of hw.
> 
> And if we create "Mic Capture Switch" in the kernel space, it may be
> misleading for applications (they can think that there's hardware mute control).
> 
> Perhaps, we can create "Mic Phantom Capture Switch" in kernel which may
> resolve both problems (no hw mute information + no user confusion).

Yes, something like that would work.
The advantage of in-kernel implementation is that it's self-contained,
so just deploying the new kernel makes everything working.

thanks,

Takashi