Automatically switching snd_hda_intel.power_save value when switching from battery to ac ?

[Takashi Iwai]

On Fri, 06 Mar 2020 09:25:49 +0100,
Hans de Goede wrote:
> 
> Hi,
> 
> On 3/6/20 8:59 AM, Takashi Iwai wrote:
> > On Thu, 05 Mar 2020 15:33:04 +0100,
> > Hans de Goede wrote:
> >>
> >> Hi,
> >>
> >> Because of a bug-report about power-saving related plops/clocks on a
> >> Lenovo T470s, I've asked inside Red Hat if people with a T470s and
> >> running a recent kernel were also experiencing this.
> >>
> >> Most people are happy with the audio, but I did get a few bug reports
> >> about plops on the headphones-jack.
> >>
> >> One of the suggestions which I got from 2 different users is to
> >> disable power-saving for the HDA driver when on AC, esp. since most
> >> headphones-jack use (esp. with an external amplifier which amplifies
> >> the problem) is done while the laptop is sitting on a desk and thus
> >> typically is connected to a charger.
> >>
> >> I'm personally not necessarily a fan of changing settings based
> >> on being connected to ac or not, but I guess that in this case
> >> it might not be such a bad idea ?
> >
> > Actually the power-saving-toggle-on-demand used to be the standard
> > behavior by some power management tools including some
> > thinkpad-specific one, IIRC.  So the behavior itself isn't too bad if
> > the pop noise can't be fully eliminated.
> 
> Right, atleast TLP does something like this, and maybe also some of
> the Ubuntu pm scripts. But if we are going to do this I would like to
> come up with some upstream, non 3th party (from a std gnu/linux
> perspective), solution.
> 
> Note upstream does not necessarily mean inside the kernel, it could be
> e.g. part of systemd, e.g. a udev rule (not sure if that is possible)
> or done by some pm suspend/resume hooks which are installed by default
> (not sure if systemd's suspend code supports hooks though).
> 
> If there is consensus that this is something which is a good idea /
> a reasonable thing to do; and also consensus that this should be done
> outside the kernel I can take a look at putting together a systemd
> pull-req for this.

I have a mixed feeling about introducing it in standard.
The power-saving itself is always good no matter whether it's cabled
or not, it's merely a workaround for the problem that couldn't be
fully fixed in kernel side.  So it was provided as an option for users
who really need it.  But it's certainly a feasible option.

OTOH, I still wonder about the cause of the noise on Thinkpad.  In
general, if that comes from the codec power down (the Realtek one),
it's usually fixable in a certain level.  Or if it's at the power down
of the link (power_save_controller option), it's often hard and we
keep the blacklist.


thanks,

Takashi