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Re: _clear_page semantics

To: carlson@sibyte.com
Subject: Re: _clear_page semantics
From: Carsten Langgaard <carstenl@mips.com>
Date: Wed, 10 Jan 2001 09:08:08 +0100
Cc: linux-mips@oss.sgi.com
References: <01010917590106.07691@plugh.sibyte.com>
Sender: owner-linux-mips@oss.sgi.com
Justin Carlson wrote:

> Looking at the existing clear_page implementations for r4xx0, rm7k, and mips32
> in the mips/ tree, I see everyone issuing cache op 0xd for the address range 
> of
> the page being cleared.
>
> I'm wondering what the purpose is of these cache flushes...given a physically
> tagged dcache, my understanding of the semantics of clear_page are that it 
> just
> zeros the page, in which case the cache ops are pointless overhead.
>
> Especially in the mips32 case, which uses cache op 0xd, which is undefined
> implementation dependent according to my mips32 spec.

You are absolutely right, it is implementation dependent.
I just tend to use the mips32 implementation for my R4000s as well, and here as
Ralf mention it is performance improving.
Actually we have included a CPU option flag (MIPS_CPU_CACHE_CDEX), what tells us
if the CPU has the Create_Dirty_Exclusive CACHE operation available.
So we should probably use it, now it is here :-)

Thanks.

>
> Am I missing something here?
>
> Thanks,
> Justin

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