linux-kernel - Re: [PATCH] PCI / PM: Use per-device D3 delays

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Date:	Fri, 1 Jan 2010 22:42:28 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Andreas Mohr <andi@...as.de>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	"Maciej J. Woloszyk" <mat@....com.pl>
Subject: Re: [PATCH] PCI / PM: Use per-device D3 delays

On Friday 01 January 2010, Andreas Mohr wrote:
> Hi,
> 
> On Fri, Jan 01, 2010 at 07:55:27PM +0100, Rafael J. Wysocki wrote:
> > On Friday 01 January 2010, Andreas Mohr wrote:
> > > Hi,
> > > 
> > > While the bug report mentions "So it's just quirky hardware.",
> > > the implementation of your patch makes it seem like this delay attribute is
> > > totally "norm"al behaviour - I'm missing some more aggressive wording.
> > 
> > That's because it works both ways (please look at the changelog).
> 
> Ah, ok.
> 
> > I know of a few devices that don't need the PCI-prescribed 10 ms wait when
> > going from D3 to D0 and their drivers may use the d3_delay field to actually
> > set a _shorter_ delay.
> 
> Then why is the value lower-bounded by pci_pm_d3_delay
> (which, puzzlingly, was initialized to PCI_PM_D3_WAIT and thus 10
> before, which the patch now removes!),

That's because dev->d3_delay is initialized to PCI_PM_D3_WAIT for all devices.

> in pci_dev_d3_sleep()? (and pci_pm_d3_delay is being quirked in
> drivers/pci/quirks.c only, to 120)

Exactly because pci_pm_d3_delay is only necessary for some quirky chipsets
that require longer delays for _all_ devices (note that this cannot be handled
at the driver level).  So, we use dev->d3_delay (that the driver gave us),
unless the chipset is known to be quirky and requires a longer delay for
all devices (the driver has no chance to know about that).

Rafael
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