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

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Date:	Thu, 17 Jul 2014 10:27:44 -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 Thu, 17 Jul 2014, Rafael J. Wysocki wrote:

> On Wednesday, July 16, 2014 04:03:45 PM Greg Kroah-Hartman wrote:
> > On Thu, Jul 17, 2014 at 12:40:23AM +0200, Rafael J. Wysocki wrote:
> > > On Wednesday, July 02, 2014 10:27:06 AM Alan Stern wrote:
> > > > 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.

My thought exactly.

> > > Greg, what's your plan toward this?
> > 
> > If I need to revert something that you all find was wrong, I'll be glad
> > to do so, sorry for merging something too early.
> 
> Alan, what do you think?

I'm in favor of reverting that commit and putting something else in its 
place.  But first we have to figure out an approach that will satisfy 
everyone.

I've been hoping that Dan would contribute to this thread.  He has
spear-headed this whole line of development, and he wrote the commit in
question.

Maybe he would be happy with the simplest approach: If the device
doesn't support runtime PM, write a message in the kernel log and make
->runtime_suspend() always return -EBUSY.

> I think we're still unsure if the approach taken by that commit is correct,
> but then I suppose we don't need to revert it at this point and we can fix
> it later.  Is that correct, or would fixing it be difficult for some reason?

As far as I can see, it would be okay to wait a while before making any 
changes.

Alan Stern

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