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

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Date:	Fri, 06 Feb 2009 08:05:44 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Hugh Dickins <hugh@...itas.com>
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

> 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.)

Well, first X should certainly not -fail- to launch if it fails to map
legacy memory, which is generally not useful anyway. That's where the
bug is. Jesse, did you have a chance to fix that yet or should I give it
a go ?

The second problem is that if I just don't expose the legacy_mem file,
then X has no way to know whether the kernel doesn't support the
interface or whether the HW doesn't support legacy memory access. So X
will fallback to whacking /dev/mem which is even more bogus. At least
that's what I remember from last I looked at that part of X code.

It should be a trivial fix on X side tho.

Cheers,
Ben.

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