linux-kernel - Re: PCI PM: Restore standard config registers of all devices early

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Date:	Mon, 2 Feb 2009 22:32:33 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	Andreas Schwab <schwab@...e.de>
Subject: Re: PCI PM: Restore standard config registers of all devices early

On Monday 02 February 2009, Benjamin Herrenschmidt wrote:
> On Mon, 2009-02-02 at 12:41 -0800, Linus Torvalds wrote:
> > 
> > On Tue, 3 Feb 2009, Benjamin Herrenschmidt wrote:
> > > 
> > > I'd rather hook it up inside pci_set_power_state()...
> > 
> > Umm. But you already _have_ that. Look at platform_pci_set_power_state(). 
> > It gets called before turning the device on (pci_raw_set_power_state(D0)) 
> > and after turning the device off (pci_raw_set_power_state(D1+))
> > 
> > Maybe you missed it? It's how ACPI does things.
> 
> I may well have, it didn't exist when my stuff was written.
> 
> > However, the issue you see is that pci_restore_standard_config() doesn't 
> > call it, because at least with ACPI, the ACPI code simply isn't ready to 
> > be called with interrupts off. So it looks like you may be looking at the 
> > wrong thing, hmm?
> 
> Well.. yes and no... I still don't see how it can be sane to whack config
> space back before the device has been turned back on. I wouldn't be surprised
> if some devices don't grok well their BARs being written while not in D0.
> 
> I suppose I can always add another hook inside pci_restore_blah for my specific
> case but I can also very easily see that can of worms hitting even more badly
> in tight embedded environments such as handhelds, who use things like fine clock
> control a lot more extensively.
> 
> In fact, even on x86, I'm not sure it's kosher to restore the config space
> before you called ACPI...

It need not be in theory, but I haven't seen any real life example of that.

I think devices without the PM capability are suspicious, so to speak.

Thanks,
Rafael
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