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

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Message-Id: <200902051433.23294.jbarnes@virtuousgeek.org>
Date:	Thu, 5 Feb 2009 14:33:22 -0800
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
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>
Subject: Re: [Bug #12608] 2.6.29-rc powerpc G5 Xorg legacy_mem regression

On Thursday, February 5, 2009 1:05 pm Benjamin Herrenschmidt 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 ?

No, sorry, but I just took a look and as long as the various callers can 
handle it (haven't checked), this patch would work.

> 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.

One option there would be to provide the file but just use anonymous memory to 
back it.  X will happily think it's messing with legacy VGA memory, but it 
shouldn't matter that it's not actually affecting hw.

diff --git a/hw/xfree86/os-support/bus/linuxPci.c 
b/hw/xfree86/os-support/bus/li
index 263fd8f..5d2da32 100644
--- a/hw/xfree86/os-support/bus/linuxPci.c
+++ b/hw/xfree86/os-support/bus/linuxPci.c
@@ -484,8 +484,9 @@ xf86MapDomainMemory(int ScreenNum, int Flags, struct 
pci_dev
     if (fd >= 0)
        close(fd);
     if (addr == NULL || addr == MAP_FAILED) {
-       perror("mmap failure");
-       FatalError("xf86MapDomainMem():  mmap() failure\n");
+       xf86Msg(X_WARNING, "xf86MapDomainMem():  mmap() failure: %s\n",
+               strerror(errno));
+       return NULL;
     }
     return addr;
 }


-- 
Jesse Barnes, Intel Open Source Technology Center
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