Buggy MTRR configuration set by the BIOS without an easy fix
From: Laurent Pinchart
Date: Thu Aug 19 2010 - 07:35:20 EST
Hi everybody,
I've been struggling with my Dell E6500 MTRR setup to configure the video RAM
in write-combining mode.
The system memory map is
00000000-00000fff : reserved
00001000-0009bbff : System RAM
0009bc00-0009ffff : reserved
000a0000-000bffff : PCI Bus 0000:00
000c0000-000d7fff : pnp 00:0c
000d8000-000dffff : PCI Bus 0000:00
000e0000-000fffff : pnp 00:0c
00100000-7904d3ff : System RAM
01000000-01521c44 : Kernel code
01521c45-017a7cef : Kernel data
01821000-018be5e3 : Kernel bss
7904d400-7904f3ff : ACPI Non-volatile Storage
7904f400-7bffffff : reserved
79b00000-79bfffff : pnp 00:0c
7c000000-f7ffffff : PCI Bus 0000:00
7c000000-7fffffff : PCI Bus 0000:03
7c000000-7fffffff : PCI CardBus 0000:04
80000000-801fffff : PCI Bus 0000:0b
80200000-803fffff : PCI Bus 0000:0b
80400000-805fffff : PCI Bus 0000:0c
80600000-807fffff : PCI Bus 0000:0d
80800000-809fffff : PCI Bus 0000:0d
80a00000-80a00fff : Intel Flush Page
84000000-87ffffff : PCI CardBus 0000:04
e0000000-efffffff : 0000:00:02.0
f0000000-f01fffff : PCI Bus 0000:0e
f6500000-f65fffff : PCI Bus 0000:03
f6500000-f6500fff : 0000:03:01.0
f6500000-f6500fff : yenta_socket
f65ff600-f65ff6ff : 0000:03:01.2
f65ff600-f65ff6ff : mmc0
f65ff800-f65fffff : 0000:03:01.1
f65ff800-f65fffff : ohci1394
f6600000-f68fffff : PCI Bus 0000:0e
f6900000-f69fffff : PCI Bus 0000:0c
f69fe000-f69fffff : 0000:0c:00.0
f69fe000-f69fffff : iwlagn
f6ad9ef0-f6ad9eff : 0000:00:03.0
f6ad9f00-f6ad9fff : 0000:00:1f.3
f6ada000-f6adafff : 0000:00:03.3
f6adb000-f6adbfff : 0000:00:19.0
f6adb000-f6adbfff : e1000e
f6adc000-f6adffff : 0000:00:1b.0
f6adc000-f6adffff : ICH HD audio
f6ae0000-f6afffff : 0000:00:19.0
f6ae0000-f6afffff : e1000e
f6b00000-f6bfffff : 0000:00:02.1
f6c00000-f6ffffff : 0000:00:02.0
f8000000-fbffffff : PCI MMCONFIG 0000 [bus 00-3f]
f8000000-fbffffff : reserved
f8000000-fbffffff : pnp 00:0c
fc000000-febfffff : PCI Bus 0000:00
fec00000-fec0ffff : reserved
fec00000-fec003ff : IOAPIC 0
fec10000-fecfffff : PCI Bus 0000:00
fed00000-fed003ff : HPET 0
fed00000-fed003ff : pnp 00:08
fed18000-fed1bfff : reserved
fed18000-fed1bfff : pnp 00:0c
fed1c000-fed1ffff : PCI Bus 0000:00
fed1c000-fed1c3ff : 0000:00:1d.7
fed1c000-fed1c3ff : ehci_hcd
fed1c400-fed1c7ff : 0000:00:1a.7
fed1c400-fed1c7ff : ehci_hcd
fed1c800-fed1cfff : 0000:00:1f.2
fed1c800-fed1cfff : pnp 00:0c
fed1c800-fed1cfff : ahci
fed20000-fed8ffff : reserved
fed20000-fed3ffff : pnp 00:0c
fed40000-fed44fff : PCI Bus 0000:00
fed40000-fed44fff : pnp 00:09
fed45000-fed8ffff : pnp 00:0c
fed90000-fed9ffff : PCI Bus 0000:00
feda0000-feda5fff : reserved
feda0000-feda3fff : pnp 00:0c
feda4000-feda4fff : pnp 00:0c
feda5000-feda5fff : pnp 00:0c
feda6000-feda6fff : pnp 00:0c
feda7000-fedfffff : PCI Bus 0000:00
fee00000-fee0ffff : reserved
fee00000-fee0ffff : pnp 00:0c
fee00000-fee00fff : Local APIC
fee10000-ff9fffff : PCI Bus 0000:00
ffa00000-ffbfffff : pnp 00:0c
ffc00000-ffdfffff : PCI Bus 0000:00
ffe60000-ffffffff : reserved
My CPU has 7 MTRRs setup by the BIOS the following way:
MTRR default type: uncachable
MTRR fixed ranges enabled:
00000-9FFFF write-back
A0000-BFFFF uncachable
C0000-D7FFF write-protect
D8000-EFFFF uncachable
F0000-FFFFF write-protect
MTRR variable ranges enabled:
0 base 000000000 mask 800000000 write-back
1 base 0E0000000 mask FE0000000 uncachable
2 base 079C00000 mask FFFC00000 uncachable
3 base 07A000000 mask FFE000000 uncachable
4 base 07C000000 mask FFC000000 uncachable
5 disabled
6 disabled
The resulting memory configuration is
0000 0000 - 000f ffff Covered by fixed-range MTRRs
0010 0000 - 79bf ffff Write-back (MTRR 0)
79c0 0000 - 7fff ffff Uncachable (MTRRS 2, 3 and 4)
8000 0000 - dfff ffff Write-back (MTRR 0)
e000 0000 - ffff ffff Uncachable (MTRR 4)
I'm a bit suprised that the 8000 0000 - 87ff ffff range is configured as
write-back, as it covers a PCI memory range. Maybe that's a BIOS bug that just
goes unnoticed because no peripheral uses that range.
To configure video RAM in write-combining mode, I need to remove the write-
back mapping covering all system memory (as a write-combining mapping can't
overlap with any other mapping) and reprogram the MTRRs accordingly.
If I keep the default uncached memory type, the MTRR setup closest to my
memory map that I've been able to find is
0000 0000 - 000f ffff Covered by fixed-range MTRRs
0010 0000 - 400f ffff (1024 MB) Write-back (MTRR 0)
4010 0000 - 600f ffff (512 MB) Write-back (MTRR 1)
6010 0000 - 700f ffff (256 MB) Write-back (MTRR 2)
7010 0000 - 780f ffff (128 MB) Write-back (MTRR 3)
7810 0000 - 790f ffff (16 MB) Write-back (MTRR 4)
7910 0000 - 7fff ffff Uncachable (default)
8000 0000 - 87ff ffff (128 MB) Write-back (MTRR 5)
8800 0000 - dfff ffff Uncachable (default)
e000 0000 - efff ffff (256 MB) Write-combining (MTRR 6)
f000 0000 - ffff ffff Uncachable (default)
Compared to what the BIOS configures, this will modify the 7910 0000 - 79bf
ffff and 8800 0000 - dfff ffff ranges from write-back to uncachable. While the
second range doesn't worry me as it's not used in the memory map (but maybe I
should still worry :-)), the first one is reserved and used for an unknown (to
me) purpose, so it could have a negative impact on performances.
Another solution would be to turn the default memory type from uncachable to
write-back. The memory map would then become
0000 0000 - 000f ffff Covered by fixed-range MTRRs
0010 0000 - 79bf ffff Write-back (default)
79c0 0000 - 79ff ffff (4 MB) Uncachable (MTRR 0)
7a00 0000 - 7bff ffff (32 MB) Uncachable (MTRR 1)
7c00 0000 - 7fff ffff (64 MB) Uncachable (MTRR 2)
8000 0000 - dfff ffff Write-back (default)
e000 0000 - efff ffff (256 MB) Write-combining (MTRR 3)
f000 0000 - ffff ffff (256 MB) Uncachable (MTRR 4)
The memory map will then be identical to what the BIOS configured (except for
video RAM of course). The problem here is that /proc/mtrr doesn't support
changing the default memory type, so I can't easily apply this configuration.
I'd appreciate if someone could help me to fix my MTRRs. Are my understanding
of the problem and the two proposed solutions correct ? Should I use one of
those two, or is there another easier/more efficient solution (maybe moving
some PCI memory ranges around - if at all possible - to make the memory map
simpler) ?
Please CC me on answers.
--
Regards,
Laurent Pinchart
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