[alsa-devel] Why analog loopback of sigmatel devices got removed?

[Maxim Levitsky]

On Fri, 2009-06-19 at 08:32 +0200, Takashi Iwai wrote:
> At Fri, 19 Jun 2009 02:50:35 +0300,
> Maxim Levitsky wrote:
> > 
> > Hi,
> > 
> > Due to lack of time, I didn't update kernel on my desktop system.
> > Now I notice that analog loopback go removed. Why?
> 
> Because it harms than helps more often.  I've got tons of bug reports
> and been upset just because users blindly set "Analog Loopback" mixer
> switch and lost their sound output.  This is no mixer element that
> behaves intuitively -- it shuts out PCM output instead of adding the
> analog loopback like other hardwares.

Here it doesn't shut PCM.
I just hear input in headphones, mixed with output.
I suspect that this feature might not work correctly on newer devices
that ones I have added.

> 
> The feature is still available, though.  You just need to give the
> driver an instruction via sysfs.  Namely, write to sysfs files
> corresponding to the IDT codec like
> 	# echo "loopback yes" > /sys/class/sound/hwC0D0/hints
> 	# echo 1 > /sys/class/sound/hwC0D0/reconfig
> Then the driver will reconfigure with the loopback mixer element.
> 	
> I have a patch series to add this with a "patch" firmware file to
> make it easier, which I'm going to send an RFC to this ML soon later.
> Take a look at topic/hda-patch branch on sound-unstable tree.
> 
> 
> thanks,
> 
> Takashi