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Date: Wed, 14 Aug 2002 10:16:54 +0200
From: Ralf Baechle <ralf@linux-mips.org>
To: Imran Badr <imran.badr@cavium.com>
Cc: "'Alan Cox'" <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
Subject: Re: Cache coherency and snooping
Message-ID: <20020814101654.B14197@linux-mips.org>
References: <20020814022958.B11645@linux-mips.org> <0aa401c2432e$04a95cc0$9e10a8c0@IMRANPC>
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In-Reply-To: <0aa401c2432e$04a95cc0$9e10a8c0@IMRANPC>; from imran.badr@cavium.com on Tue, Aug 13, 2002 at 06:00:43PM -0700
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On Tue, Aug 13, 2002 at 06:00:43PM -0700, Imran Badr wrote:

> Please advise if following sequence of operations are going to help:
> 
> alloc memory
> reserve the page
> flush every cache

That's an extremly expensive operation on some platforms and there's no
portable kernel API to do it.

Note that the flush_cache_*() functions are not suitable for I/O as you
want to do it.  Many platforms such as i386 implement these functions
as empty functions.

> call ioremap_nocache

You said you intend to remap memory, that is RAM.  The ioremap*() functions
will refuse to remap RAM; they're only suitable for other types of memory
such as PCI boards.  If the latter was your intension then ioremap_nocache()
will work as intended.

Again, unless your hardware is fucked beyond recognition it'll do a better
job at keeping cache coherence than software.

  Ralf
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