linux-kernel - Re: [RFC PATCH] PCIe: Add PCIe runtime D3cold support

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Date:	Wed, 18 Apr 2012 21:51:33 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	huang ying <huang.ying.caritas@...il.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>, bhelgaas@...gle.com,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-pm@...r.kernel.org, Lin Ming <ming.m.lin@...el.com>,
	Zhang Rui <rui.zhang@...el.com>,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>
Subject: Re: [RFC PATCH] PCIe: Add PCIe runtime D3cold support

On Wednesday, April 18, 2012, huang ying wrote:
> On Wed, Apr 18, 2012 at 4:20 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Tuesday, April 17, 2012, huang ying wrote:
> >> On Tue, Apr 17, 2012 at 1:07 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> >> > Moreover, I really don't think it's a good idea to put PCI Express ports into
> >> > low-power states in general.  It might work on your platform, whatever it is,
> >> > but that doesn't mean it's going to work on every PCI Express system out
> >> > there.  I actually know of a number of such systems where it surely won't
> >> > work at all.
> >>
> >> The runtime PM support for PCIe port is not turned on by default, we need
> >>
> >> # echo auto > /sys/devices/pcixxxx:xx/<pci id>/power/control
> >>
> >> to turn on it.  And we will turn on it on system it works.  Is this sufficient?
> >
> > No, it is not.  In some cases it shouldn't be enabled at all for PCI Express
> > ports, as far as I can say, so to be on the safe side we should only enable it
> > on platforms where PCI Express ports are known to work correctly with runtime PM.
> 
> Sorry, I did not catch your idea exactly.  I think disable it by
> default and enable it by sysfs interface is what you suggested, isn't
> it?

No, it isn't.  User space shouldn't be able to enable functionality that's
going to break on a subset of systems.

> Or you suggest something like "white list"?

Yes, something like this.

Thanks,
Rafael
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