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Re: how i can know the linux-mips implememt cache strategy?

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: how i can know the linux-mips implememt cache strategy?
From: Shane McDonald <mcdonald.shane@gmail.com>
Date: Sat, 21 Nov 2009 09:18:26 -0600
Cc: figo zhang <figo1802@gmail.com>, linux-mips@linux-mips.org
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On Wed, Nov 18, 2009 at 5:44 AM, Ralf Baechle <ralf@linux-mips.org> wrote:
>
> The kernel will always use cache stategy 3 for non-coherent systems and
> caching strategy 5 for cache coherent systems.  These two select the most
> aggressive caching strategy on all processors and that's what gives the
> best performance.

OK, dumb question -- how is this implemented?  Poking through the code,
it looks to me that the cache strategy used comes from the K0 field of the
coprocessor 0 Config register, which I think is whatever gets set up by
the bootloader, or if that wasn't done, the default value of that
field for the processor.
See function coherency_setup() in arch/mips/mm/c-r4k.c:

        if (cca < 0 || cca > 7)
                cca = read_c0_config() & CONF_CM_CMASK;
        _page_cachable_default = cca << _CACHE_SHIFT;

This can be overridden on the kernel command line with the "cca" parameter,
but as Ralf said in
http://www.linux-mips.org/archives/linux-mips/2008-06/msg00186.html,
"passing a CCA value on the command line is nothing a user should
ever, ever have to do".

I can see how this was implemented in 2.6.25, but commit 3513369
[MIPS] Allow setting of the cache attribute at run time, seems to have changed
from the behaviour Ralf described.

Shane

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