[RFC 0/1] ALSA: hda: Add a power_save blacklist

[Hans de Goede <hdegoede@redhat.com>]

Hi All,

On some boards setting power_save to a non 0 value leads to clicking /
popping sounds when ever we enter/leave powersaving mode. Ideally we would
figure out how to avoid these sounds, but that is not always feasible.

This commit adds a blacklist for devices where powersaving is known to
cause problems and disables it on these devices.

Note this patch ATM is a RFC because I still need to get test-feedback
that it actually works on affected boards.

I tried to put this blacklist in userspace first:
https://github.com/systemd/systemd/pull/8128

But the systemd maintainers rightfully pointed out that it would be
impossible to then later remove entries once we actually find a way to
make power-saving work on listed boards without issues. Having this list
in the kernel will allow removal of the blacklist entry in the same commit
which fixes the clicks / plops.

I've chosen to also apply the blacklist to changes made through
/sys/module/snd_hda_intel/parameters/power_save, since tools such as
powertop and TLP do this automatically.

About the choice to also apply this to changes made through
/sys/module/snd_hda_intel/parameters/power_save, one could argue that this
will get in the way of users who want the power-saving anyways and are
willing to live with the clicks/plops. An alternative approach would be to
only apply this to the value of the module-parameter at boot and then only
when it matches CONFIG_SND_HDA_POWER_SAVE_DEFAULT. I'm open to changing this.

Regards,

Hans