[PATCH] ALSA: hda/hdmi: Add Intel silent stream support

[Takashi Iwai]

On Wed, 24 Jun 2020 19:05:14 +0200,
Pierre-Louis Bossart wrote:
> 
> 
> 
> On 6/24/20 11:43 AM, Takashi Iwai wrote:
> > On Wed, 24 Jun 2020 17:33:45 +0200,
> > Pierre-Louis Bossart wrote:
> >>
> >>
> >>>>> The silent stream is enabled with a Kconfig option, as well as a kernel
> >>>>> parameter should there be a need to override the build time default.
> >>>>
> >>>> I'm not sure whether the module option is the best interface.
> >>>> An alternative is a mixer element that controls dynamically.  Then
> >>>> it'll be per card unlike the module option.
> >>>
> >>> +1, kcontrol seems the appropriate way to control this.
> >>
> >> It was my suggestion to use Kconfig+kernel parameter for
> >> simplicity/overrides.
> >>
> >> The kcontrol is a nice idea, but in practice we typically only have
> >> one card dealing with HDMI.
> >
> > Not really.  There are systems with two HDMI outputs from both
> > integrated and discrete GPUs.  Most modern systems are only with
> > hybrid graphics, though.
> 
> Ok, maybe I am mistaken, in most of the HDMI issues we've seen only
> one HDMI source.
> 
> But it's a good point that this is only supposed to be used for Intel
> whether it's a kernel parameter or a kcontrol shouldn't this be
> dependent on a PCI ID being detected and a SKYLAKE flag being set?
> it's my understanding that this applies from Skylake to TigerLake, not
> before.

I guess we can check it from the codec ID?  Change the probe function
for Skylake+ codecs to patch_i915_skl_hdmi and co, and set the flag
there.

> >> It also doesn't have a UCM representation
> >> so would force the use of amixer and manual configs, or the UCM file
> >> would always set the mode.
> >
> > But people usually use the distro kernels, so the situation is more or
> > less equivalent; you'd have to adjust a module option manually if you
> > want a different one from the default, and you'd have to be root to
> > change it.
> >
> > So, rather the question is how we should provide the setup of such
> > parameter.  It's supposed to be a part of power management stuff that
> > should be touched by either a smart PM tool or a manual override such
> > as runtime PM setup?  Or can it be seen as a more casual tuning?
> 
> I am not aware of such tools. The only thing I know is that some of
> the HDaudio power settings are already controlled by kernel
> parameters, e.g.
> 
> /etc/modprobe.d/audio_powersave.conf
> options snd_hda_intel power_save=1

Yes, it's been the primary knob for years to turn on/off the runtime
PM for HD-audio and other legacy drivers.  This was used by powertop
or some other power-aware daemons and tools, to be toggled dynamically
per the power cable state or such.

And, how the silent stream feature should be seen?
Should it be a system-wide root-only setup or adjustable per user?
Would it be changed often?  Such questions and answers will lead us to
the right direction, I hope.


Takashi