Poor graphics performance due to wrong mtrr settings

Question

I am using the latest kernel on a Dell LAtitude E4310 with 8Gbyte RAM

2.6.38-10-generic #44-Ubuntu SMP Thu Jun 2 21:32:22 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

At boot I get the following message

[drm] MTRR allocation failed.  Graphics performance may suffer.

and indeed my /proc/mtrr looks strange

reg00: base=0x000000000 (    0MB), size= 8192MB, count=1: write-back
reg01: base=0x200000000 ( 8192MB), size=  512MB, count=1: write-back
reg02: base=0x0e0000000 ( 3584MB), size=  512MB, count=1: uncachable
reg03: base=0x0dc000000 ( 3520MB), size=   64MB, count=1: uncachable
reg04: base=0x0db400000 ( 3508MB), size=    4MB, count=1: uncachable
reg05: base=0x21c000000 ( 8640MB), size=   64MB, count=1: uncachable

I tried mtrr-uncover but it does not work:

./mtrr-uncover 
Initial MTRR configuration:
 0  0x000000000-0x1ffffffff write-back
         4  0x0db400000-0x0db7fffff uncachable
         3  0x0dc000000-0x0dfffffff uncachable
         2  0x0e0000000-0x0ffffffff uncachable
 1  0x200000000-0x21fffffff write-back
         5  0x21c000000-0x21fffffff uncachable
./mtrr-uncover: 9 MTRRs needed but only 8 in architecture.

Does anybody know how to properly set the mtrr in my architecture?

here the relevant output of lspci -v

00:02.0 VGA compatible controller: Intel Corporation Core Processor Integrated Graphics Controller (rev 02) (prog-if 00 [VGA controller])
    Subsystem: Dell Device 0410
    Flags: bus master, fast devsel, latency 0, IRQ 42
    Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
    Memory at e0000000 (64-bit, prefetchable) [size=256M]
    I/O ports at 60b0 [size=8]
    Expansion ROM at <unassigned> [disabled]
    Capabilities: <access denied>
    Kernel driver in use: i915
    Kernel modules: i915

Solution

There is a simple solution for this. Just use kernel boot parameter enable_mtrr_cleanup.

Edit /etc/default/grub config file:

$ sudo nano /etc/default/grub

find a line containing GRUB_CMDLINE_LINUX_DEFAULT and you'll probably see these default params:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

So to use enable_mtrr_cleanup you'll add it like this:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash enable_mtrr_cleanup"

Then you must update your grub configuration with this command:

$ sudo update-grub2

And reboot:

$ sudo reboot

Then the message will dissappear and you'll see the system has found proper values for MTRR.

Comments

[Andy]

I believe that the mtrr over e0000000 is the AGP gart, therefore you already have an mtrr over the region drm is trying to change. I suspect if you remove and replace that with a WC it may work. The rules for overlapping them are very complex and prone to error though.

[mulllhausen]

didn't work for me on debian wheezy