Re: [Ksummit-discuss] [TECH TOPIC] System-wide interface to specify the level of PM tuning

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Jul 16, 2015 at 03:44:57PM +0000, Kristen Accardi wrote:
> On Thu, Jul 16, 2015 at 2:19 AM David Woodhouse <dwmw2@infradead.org> wrote:
> 
> > On Wed, 2015-07-15 at 18:10 -0700, Josh Triplett wrote:
> > >
> > > > The point here is whether or not we want to have a way to make all of
> > them be
> > > > enabled by default instead and see what happens, for example.
> > >
> > > To some degree that seems like an admission of defeat: we can't possibly
> > > do the right thing by default, so we give up and add a way for the user
> > > to configure it.
> > >
> > > We should be selecting the most sensible combination of power and
> > > performance by default; we should not punt that question to the average
> > > user, *or* to the distros.
> >
> > Not to the user, perhaps. But it's not *so* unreasonable to let each of
> > the distros tune things for *their* class of users. We might want base
> > profiles for 'server', 'desktop' and 'laptop on battery' that distros
> > work from.
> >
> >
> We need to be able to give even the distros more help than they have now.
> The problem is that the distros simply don't have the knowledge to tune
> *all* the knobs appropriately.  If we gave them one knob as you suggested
> (and what Rafael is suggesting) set to a sane default determined by the
> kernel - they could then use whatever userspace tool to change it based on
> whatever parameters they want.

Even if there's a single knob, that *still* doesn't let us turn on power
management features that break with certain devices.

> But although that's complex, it's not the real problem.
> >
> > The real problem, as others have said, is when we have power management
> > features which work fine for 99% of the population, but fail
> > occasionally on broken hardware.
> >
> > We can't easily blacklist the known-broken devices or whitelist the
> > good ones, and we end up having to turn the feature off by default.
> >
> > Sure, a DMI match on "HP" and "TO BE FILLED BY OEM" would often go a
> > long way for a certain class of problem, but even that's not sufficient
> > :)
> >
> 
> driver maintainers don't want to maintain blacklist/whitelist because it's
> hard.  It'd be nice to be able to load a black list or white list of
> devices from the filesystem like we do with firmware.

If the devices are identifiable from userspace, udev or similar could
set these settings from a rules file.

But the kernel already has quirk tables for various hardware, and that
seems appropriate to continue putting in the kernel.

- Josh Triplett