[alsa-devel] [patch][saa7134] do not change mute state for capturing audio

Stas Sergeev stsp at list.ru
Tue Jul 19 20:38:45 CEST 2011

19.07.2011 22:06, Mauro Carvalho Chehab wrote:
>> Unless I am mistaken, this control is usually called a
>> "Master Playback Switch" in the alsa world.
> No, you're mistaken: on most boards, you have only one volume control/switch,
> for capture. So, it would be a "master capture switch",
Well, for such a cards we don't need to export
the additional element, they are fine already.
We can rename it to "Master Capture Switch",
or may not.

>  but I don't think
> that there's such alsa "generic" volume control. Even in the case where
> you have a volume control for the LINE OUT pin[1], in general, you also need to
> unmute the capture, so, it would be a "master capture and LINE OUT switch",
> and, for sure alsa currently not provide anything like that.
I think you can still call it a "Master Capture Switch",
if it enables everything.

>> So, am I right that the only problem is that it is not
>> exported to the user by some drivers right now?
> No, you're mistaken again. Such "master capture and LINE OUT switch" type of control
> _is_exported_ via the V4L2 API as V4L2_CID_AUDIO_MUTE. 
Sorry, I meant the _alsa_ drivers here.
So, to rephrase:

So, am I right that the only problem is that it is not
exported to the user by some _alsa_ drivers right now?

> Some applications like mplayer don't use V4L2_CID_AUDIO_MUTE to unmute a video
> device. They assume the current behavior that starting video also unmutes audio.
> (mplayer is not symmetric with regard to the usage of this control, as it uses
> V4L2_CID_AUDIO_MUTE to mute the device after the end of a capture).
> So, changing the logic at the drivers will break existing applications.
I do not propose changing any V4L2 ioctls, my
change concerns only the alsa driver.

> It is probably doable to split the mute control for the LINE OUT pin from the
> mute control of the PCM capture. Such patch would make sense, as the alsa
> capture doesn't need to touch at the line out pin, but the patch should
> let V4L2_CID_AUDIO_MUTE control to affect both LINE OUT and PCM capture
> mutes, otherwise applications will break.
That's exactly what I was talking about from the very
beginning, saying that the single control currently controls
way too much, and providing an examples about 2 separate
controls. But... I haven't found the way to implement that,
not sure of this is possible at all. :(

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