Re: [PATCH] lan78xx: Protect runtime_auto check by #ifdef CONFIG_PM
From: Alan Stern
Date: Mon Mar 21 2016 - 15:31:02 EST
On Mon, 21 Mar 2016, Oliver Neukum wrote:
> On Mon, 2016-03-21 at 14:24 -0400, Alan Stern wrote:
> > On Mon, 21 Mar 2016, Oliver Neukum wrote:
> >
> > > On Mon, 2016-03-21 at 10:57 -0400, Alan Stern wrote:
> > >
> > > > One possible solution is to export a sysfs parameter to prevent
> > > > statistics collection (or more generally, to change the interval at
> > > > which it occurs).
> > >
> > > Surely, not performing a task can hardly be beaten in terms of power
> > > consumption. That is not meant to be flippant, but I think these
> > > issues are orthogonal. The question of how much to do doesn't
> > > solve the question of doing efficiently what we do.
> >
> > In other words, what's the best way to collect the statistics without
> > interfering with runtime PM, right?
>
> Yes.
>
> > If the device is suspended, presumably we know there's nothing to
> > collect -- especially if we already collected the statistics at the
> > time the device got suspended. Hence my suggestion to avoid querying
> > the device while it is suspended.
>
> That is perfectly alright if we just collect statistics.
> As a generic mechanism it is bad. Think about the polling
> for media detection.
True. Here I'm talking specifically about collecting statistics.
Media detection has its own requirements.
> > But this leaves open the issue that querying the device too often will
> > prevent it from going into autosuspend. It seems to me that the best
> > way to deal with this is to make sure that the autosuspend timeout is
> > shorter than the interal between queries, not to make the querying
> > conditional on !runtime_auto.
> [..]
> > > If we know when the next activity will come, why not pass this
> > > information down?
>
> We have an autosuspend timeout because we think that IO, if it will
> come at all, is likeliest to come soon. If, however, the IO is
> periodic that heuristics is false.
> To save most power the driver must either decide that the interval
> is too short or suspend immediately. So if we are lucky enough
> to have the frequency in the kernel, we should use that information.
The autosuspend timeout is set by userspace. The kernel may assign a
default value, but the user can always override it. Given this, I
don't see how the kernel can use frequency information (and I'm not
sure where that information would come from in the first place).
Alan Stern