[coreboot] AMD cache setup is broken

Marc Jones marcj303 at gmail.com
Wed Sep 8 20:38:16 CEST 2010

On Mon, Sep 6, 2010 at 6:40 PM, Scott Duplichan <scott at notabs.org> wrote:
> Stefan Reinauer <stefan.reinauer at coresystems.de> writes:
>>> Can you see if the patches posted in
>>> http://article.gmane.org/gmane.linux.bios/57707 make any difference for
>>> you?
>> Did we ever figure out what is causing this?
> The last time I really dug into this, it was fairly obvious that it was
> caused by instruction fetch thrashing towards the ROM.  I tried to amend
> this with MTRR settings, but I was unable to make that work correctly.
> For some reason it seemed like the HT requests were sublty changed when
> the MTRR was applied, and didn't hit the legacy southbridge properly.
>> The patch would require 4KB more stack on all supported systems, so if
>> we can we should do things differently.
> It doesn't have to be stack, but it is nice to have it memory mananged
> in some way.  The unrv2b patch I posted addressing the same problem was
> even more kludgy.
>> Also, it's not really guaranteed that the code works from the new
>> location since we don't compile coreboot with -fPIC (and as far as I
>> understand the GCC guys, even that would not help), so I am a bit
>> hesitant to check this in.
> Agreed, it is a bit icky.  Not sure what the best way to handle that is,
> though.  On the pro side, I assume breakage here is going to be obvious,
> and (supposing these patches actually help Nick) this is an issue people
> are running into with some regularity.
> --
>                                                        Arne.
> --
> coreboot mailing list: coreboot at coreboot.org
> http://www.coreboot.org/mailman/listinfo/coreboot
> One necessary condition for caching MMIO such as the flash chip on
> AMD family 10h processors is not well known:
> If the processor has an L3 cache, then bit 15 of msr C001_102A
> (ClLinesToNbDis) must be set. This bit needs to eventually be cleared
> in order for the OS to use the L3 cache. But BIOS must not clear this
> bit until cacheable accesses to the flash chip are no longer needed.
> This situation applies only to family 10h processors that have L3 cache.
> Often BIOS clears this bit too early and slow execution results.As an
> experiment, you could add code to set this bit before the slow function
> and see what happens.
> Last night I tried to debug this code on simnow. An HT modeling problem
> kept me from getting past HT init. I may try it again today.
> The recommended cacheability setting for MMIO is WP. At the point the
> simnow model hangs in HT init, the setting is WB. While this should
> be OK for family 10h, it will be important to use WP for families
> 14h and 15. ClLinesToNbDis is properly set for MMIO caching at this point
> (HT init):
> ------------Effective memory type and destination by address------------
>             NORMAL    NORMAL    NORMAL     SMM       SMM       SMM
>             READ      WRITE     EXECUTE    READ      WRITE     EXECUTE
> 00000-C3FFF  UC MMIO....................    UC MMIO....................
> C4000-CFFFF  WB DRAM....................    WB DRAM....................
> D0000-FFFFF  UC MMIO....................    UC MMIO....................
> 00100000-00FFFFFF  UC DRAM
> 01000000-FFEFFFFF  UC MMIO
> FFF00000-FFF7FFFF  WB MMIO   <=== really should be WP
> -msr c001102a
> 00000040_010080C8 <=== good at this point
> Thanks,
> Scott
> Simnow testing with Tilapia confirms that the coreboot AMD family 10h
> code _does_ have the problem of clearing ClLinesToNbDis too early. To
> confirm this problem, someone testing on real AMD family 10h hardware
> should remove the msr C001_001a write from STOP_CAR_AND_CPU() and
> from mct_ClrClToNB_D(). An AMD F10h system running an optimized legacy
> bios can boot to a DOS ptompt in less than one second. There is no
> reason coreboot should be any slower.
> Thanks,
> Scott

Ah, this makes sense now. Is c001_001a a shared msr? Is it ok for the
APs to be disabled and just leave it enabled on the BSP until the
ramstage is decompressed? I should have a patch ready this afternoon.



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