Re: [RFC] Add "rpm_not_supported" flag

Rafael J. Wysocki
2014-07-16T22:22:12Z

That took me much more time than I had hoped, sorry about that.

On Wednesday, July 02, 2014 10:27:06 AM Alan Stern wrote:
> On Wed, 2 Jul 2014, Rafael J. Wysocki wrote:
>
> > > Then you have no other objections to the patch?
> >
> > My concern still is that it will be confusing, because people won't read the
> > documentation carefully enough and will confuse "runtime PM never used" with
> > "hardware can't do PM". I'm not sure how to make that more clear, though.
>
> I could emphasize that distinction a little more strongly in the
> documentation.

So it looks like we'd like to cover the "enabled but always active" case for
runtime PM of a device?

> > Also we have the no_callbacks flag and I wonder if/how it is related to the
> > new one. Do we still need both?
>
> They mean different things. The no_callbacks flag is used when we want
> the PM core to think the device can be in RPM_SUSPENDED at times (it is
> "logically suspended"). rpm_not_supported is used when we want the PM
> core to think the device must always be in RPM_ACTIVE.
>
> > In addition to that, I think that "hardware can't do PM" should apply to the
> > handling of system suspend resume too.
>
> Maybe. For the use case Dan Williams and I are working on, it doesn't
> matter; for other cases it might matter. That's why I named the flag
> "rpm_not_supported" -- it applies specifically to runtime PM, not
> system PM.

Maybe it'd be better to call the flag always_active and make rpm_suspend()
and rpm_resume() react to it accordingly?

So before enabling runtime PM the driver will set always_active and that will
mean the device cannot be suspended. And we could add a separate sysfs
attribute for that instead of re-purposing the status thing, if needed
(I'm not sure about that, however).

> Here's a brief summary of the story behind this patch...
>
> At one point, I suggested to Dan that instead of doing something
> special for these devices, we could simply have the runtime_suspend()
> routine always return -EBUSY. He didn't like that idea because then
> the user would see the device was never powering down but would have no
> idea why. The rpm_not_supported flag provides this information to the
> user by causing the power/runtime_status attribute to say "not
> supported". (Although to be entirely fair, we could just put a message
> in the kernel log during probe if the hardware doesn't support runtime
> suspend.)
>
> Instead, Dan introduced a messy PM QoS mechanism in commit
> e3d105055525. I didn't like that approach, but Greg merged it before I
> objected.

That really looks a bit like a hack to me to be honest.

Greg, what's your plan toward this?

> Do you have any suggestions?

We could use an "always_active" type of flag for that, but I'm not sure how
much value to the user that will bring.

Rafael