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

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Message-ID: <21d7e9970902051345h76fb26c1hc0397f6262f70eae@mail.gmail.com>
Date:	Fri, 6 Feb 2009 07:45:05 +1000
From:	Dave Airlie <airlied@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Hugh Dickins <hugh@...itas.com>, "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 Fri, Feb 6, 2009 at 7:05 AM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
>
>> 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.

I think the correct answer is the ugly one, try again.

Add a new legacy_mem interface that works cleanly, update X to use it,
leave the old
broken one broken as it for older X to use.

Dave.

>
> Cheers,
> Ben.
>
>
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