[coreboot] unrv2b delay (v2)

Eric W. Biederman ebiederm at xmission.com
Wed Nov 12 03:36:16 CET 2008

Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006 at gmx.net> writes:

> On 30.10.2008 23:33, Marc Jones wrote:
>> Stefan Reinauer wrote:
>>> On 30.10.2008, at 15:03, Marc Jones <marc.jones at amd.com> wrote:
>>>> I think that this is in all K8 and fam10 disable_car code.
>>>> The copy to memory is probably good. Running from the rom and
>>>> decompressing from the rom is going to thrash. It may affect some
>>>> cpu/chipset combinations more than others.
>>>> Marc
>>> So what's the reason for that?
>> off the top of my head:
>> 1. ROM access speed: lpc and or spi may be run at different speeds.
>> (faster on some platforms)
>> 2. domain crossed - ht -> maybe pci/pcie -> pci/pcie -> lpc ( less is
>> better)
>> 3. prefetch and code alignment. Being a few bytes off can cause and
>> prefetching to thrash.
>> It shouldn't thrash as bad if the ROM is being cached but I am not
>> sure that it is or that it is setup correctly.
> A system using SPI for its ROM at the usual speed without caching has a
> per-byte latency of ~3 microseconds.
> That translates to per-instruction latencies of roughly 6 microseconds
> for simple instructions accessing data in the ROM. That's roughly 500
> times slower than an instruction in I-Cache accessing data in RAM.

Historically all of this was handled by setting up an MTRR over the
ROM chip.  I think the type was write-through, and I think the config
option was uncompressed ROM size.

So if you are seeing slow decompression performance I'm guessing you can
fix it with just a few config fiddles.  But look at the early mtrr setup
to be certain.


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