linux-kernel - Re: [Bug #12608] 2.6.29-rc powerpc G5 Xorg legacy

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Date:	Thu, 5 Feb 2009 17:23:29 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>
Subject: Re: [Bug #12608] 2.6.29-rc powerpc G5 Xorg legacy_mem regression

On Thu, 5 Feb 2009, Benjamin Herrenschmidt wrote:
> On Wed, 2009-02-04 at 11:24 +0100, Rafael J. Wysocki wrote:
> > This message has been generated automatically as a part of a report
> > of recent regressions.
> > 
> > The following bug entry is on the current list of known regressions
> > from 2.6.28.  Please verify if it still should be listed and let me know
> > (either way).
> 
> I still don't know what's the best way to handle that one... the bug is
> in X and I don't see a way to work around it without removing support
> for legacy memory access from the kernel :-( Or doing it in a way that
> doesn't allow userspace to differenciate between the kernel not
> supporting it vs. the HW not supporting it, causing X to fallback to
> even more broken crap.

Is it a really a bug in X, or a misunderstanding between X and
the kernel as to what existence of the legacy_mem file implies?

I may have got this quite wrong, but to me it appears that X assumes
that existence of the legacy_mem file implies that it will be useful;
whereas the kernel thinks it can make the legacy_mem file available,
even if it cannot be used for mmapping mem - which is its sole purpose?

What if pci_create_legacy_files() were to call some new verification
routine, and only create the legacy_mem file if it would be usable?
(But perhaps that cannot be known at the time it needs to be created.)

Hugh

> 
> I'll try to find out the extent of the X problem and whether that's
> fixable in a way that can hit distros.
> 
> > 
> > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=12608
> > Subject		: 2.6.29-rc powerpc G5 Xorg legacy_mem regression
> > Submitter	: Hugh Dickins <hugh@...itas.com>
> > Date		: 2009-01-21 21:12 (15 days old)
> > First-Bad-Commit: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d3a54014e2a94bd37b7dee5e76e03f7bc4fab49a
> > References	: http://marc.info/?l=linux-kernel&m=123257250431870&w=4
> > Handled-By	: Benjamin Herrenschmidt <benh@...nel.crashing.org>
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