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List: linux-arm-kernel
Subject: Re: [CFT] Mark IO as bufferable
From: Matthias Welwarsky <mwelwarsky () web ! de>
Date: 2005-06-28 15:59:20
Message-ID: 200506281759.21080.mwelwarsky () web ! de
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On Monday 27 June 2005 18:37, Russell King - ARM Linux wrote:
> Can people please test out the effect of this change. Potentially it
> should get slightly better IO performance. Or it could prevent the
> kernel booting.
I wonder what this change does in detail. Does bufferable mean "use write-back
cache" or is it just to use the write buffer?
Either way, I think it's quite a dangerous change. Imagine an IDE driver: The
ATAPI4 spec says (not literally): "After writing to the command register,
wait for at least 400ns before reading the status register". So the command
sequence looks something like
writeb(cmd, HD_COMMAND);
nop();
nop();
...
nop();
stat = readb(HD_STATUS);
Can I be sure to not violate the protocol if the writes are buffered?
regards,
matthias
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