linux-kernel - [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Sat, 10 Oct 2009 03:22:36 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	linux-kernel@...r.kernel.org
Cc:	Thomas Hellstrom <thellstrom@...are.com>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: [RFC Patch] use MTRR for write combining if PAT is not available

Hi,

I've found a problem with X.org not setting up MTRR for the framebuffer 
memory. After I investigated I think this is not a X.org problem, but a kernel 
issue.

X.org uses libpciaccess to map the framebuffer memory. This library opens 
/sys/bus/pci/devices/*/resource0_wc and mmaps the memory. Unfortunately, the 
kernel only enables write combining if PAT is enabled, if it is not, the 
memory is mmapped uncached. But Xorg (respectively libpciaccess) thinks it was 
successfully mapped with write combining enabled and thus does not 
additionally set up MTRR entries.

The corresponding libpciaccess code can be found here:
  http://cgit.freedesktop.org/xorg/lib/libpciaccess/tree/src/linux_sysfs.c#n501

If the kernel behavior is intentional and X.org should always set up MTRR 
entries, why should it use /sys/.../resource0_wc at all? I think there are 2 
possibilities to make the kernel behavior consistent:

 1. The mmap_wc should fail if PAT is not enabled.
    (libpciaccess will then map the framebuffer uncached and set up
     MTRR entries)
 2. Use MTRR to enable write combining if PAT is not available.

In an earlier thread about ioremap_wc, Arjan van de Ven wrote that option 2 is 
preferred over option 1:

    http://lkml.indiana.edu/hypermail/linux/kernel/0805.3/2925.html

So, I've created the attached patch implementing option 2. For me this solves 
the problem with the slow Video playback due to not correctly set up MTRR 
entries.

Kind regards,
Thomas Schlichter

View attachment "0001-Use-MTRR-for-write-combining-mmap-ioremap-if-PAT-is-.patch" of type "text/x-patch" (3240 bytes)