linux-kernel - Re: [linux-pm] Bug in PCI core

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Date:	Fri, 13 Oct 2006 10:29:27 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Adam Belay <abelay@....EDU>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Greg KH <greg@...ah.com>, <linux-pci@...ey.karlin.mff.cuni.cz>,
	Linux-pm mailing list <linux-pm@...ts.osdl.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] Bug in PCI core

On Fri, 13 Oct 2006, Adam Belay wrote:

> Personally, I don't think exposing a cached version of the PCI config
> space when direct device access is prohibited is the right approach
> here.  We really shouldn't be lying about the internal state of PCI
> devices (the cached version could be quite inaccurate).  After all, if
> the device is in D3cold, then the spec claims it's perfectly valid for
> it to not respond to PCI configuration access.
> 
> I can only assume this hack was done to satisfy some terribly broken
> userspace app.  It's not surprising that even reading PCI config can
> easily crash systems.  However, it's the responsibility of those apps
> with permission to access the PCI sysfs interface, not the kernel, to be
> aware of these constraints.

According to the comment in the code, access is blocked only during the
time that the device is making the transition between different power
states.

> The PCI configuration space cache was originally introduced to support
> power management.  However, it's mostly incorrect, as it unnecessarily
> stores the values of read only registers (and even BIST which is
> potentially dangerous).  A while back I posted a series of patches that
> address this issue, and the net result was that the config cache stays
> around wasting memory because of the pci_block_user_cfg_access()
> dependency despite being useless to PCI PM.
> 
> I'd like to propose that we have the pci config sysfs interface return
> -EIO  when it's blocked (e.g. active BIST or D3cold).  This accurately
> reflects the state of the device to userspace, reduces complexity, and
> could potentially save some memory per PCI device instance.

Could you resubmit your old patches and include a corresponding fix for 
this access problem?

BTW, is it possible for userspace to try accessing a device in D3cold?  
Doesn't the fact that it is D3cold rather than D3hot mean the computer is 
turned off?  Or have I been missing out on new developments?

Alan Stern

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