linux-kernel - Re: [RFC] Add "rpm_not

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Date:	Wed, 2 Jul 2014 10:27:06 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Allen Yu <alleny@...dia.com>, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Linux-pm mailing list <linux-pm@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Add "rpm_not_supported" flag

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.

> 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.

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.

Do you have any suggestions?

Alan Stern

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