linux-kernel - Re: [PATCH] PCI/PM: Keep runtime PM enabled for unbound PCI devices

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Message-ID: <1962888.rQgbRUetgj@vostro.rjw.lan>
Date:	Wed, 28 Nov 2012 23:43:56 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Huang Ying <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-pm@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] PCI/PM: Keep runtime PM enabled for unbound PCI devices

On Wednesday, November 28, 2012 03:25:59 PM Bjorn Helgaas wrote:
> On Tue, Nov 20, 2012 at 1:08 AM, Huang Ying <ying.huang@...el.com> wrote:
> > For unbound PCI devices, what we need is:
> >
> >  - Always in D0 state, because some devices does not work again after
> >    being put into D3 by the PCI bus.
> >
> >  - In SUSPENDED state if allowed, so that the parent devices can still
> >    be put into low power state.
> >
> > To satisfy these requirement, the runtime PM for the unbound PCI
> > devices are disabled and set to SUSPENDED state.  One issue of this
> > solution is that the PCI devices will be put into SUSPENDED state even
> > if the SUSPENDED state is forbidden via the sysfs interface
> > (.../power/control) of the device.  This is not an issue for most
> > devices, because most PCI devices are not used at all if unbounded.
> > But there are exceptions.  For example, unbound VGA card can be used
> > for display, but suspend its parents make it stop working.
> >
> > To fix the issue, we keep the runtime PM enabled when the PCI devices
> > are unbound.  But the runtime PM callbacks will do nothing if the PCI
> > devices are unbound.  This way, we can put the PCI devices into
> > SUSPENDED state without put the PCI devices into D3 state.
> 
> Does this fix a reported problem?  Is there a bug report URL?  What
> problem would a user notice?

There is a BZ: https://bugzilla.kernel.org/show_bug.cgi?id=48201

Unfortunately, the reporter hasn't confirmed that the bug is fixed,
although we're quite confident that it will be.

> This doesn't look critical enough to try to put in v3.7 (correct me if
> I'm wrong).  It's getting pretty late for the v3.8 merge window, but
> it looks like it qualifies as a bug fix that we would merge even after
> the merge window, so I'll put it in my -next branch headed for v3.8.
> Then it can be backported to v3.6 and v3.7.

I think that's OK.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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