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Re: [PATCH 3/3] ide: move dcache flushing to generic ide code

To: David Miller <davem@davemloft.net>
Subject: Re: [PATCH 3/3] ide: move dcache flushing to generic ide code
From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Date: Mon, 1 Mar 2010 20:58:58 +0100
Cc: ralf@linux-mips.org, linux-mips@linux-mips.org, linux-ide@vger.kernel.org
In-reply-to: <20100228.183417.52179576.davem@davemloft.net>
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* David Miller | 2010-02-28 18:34:17 [-0800]:

>From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
>Date: Sun, 28 Feb 2010 16:35:41 +0100
>
>> the pio callbacks are called with different kind of buffers. It could be
>> a straight kernel addr, kernel stack or a kmaped highmem page.
>> Some of this break the virt_to_page() assumptions.
>> This patch moves the dcache flush from architecture code into generic
>> ide code. ide_pio_bytes() is the only place where user pages might be
>> written as far as I can see.
>> The dcache flush is avoided in two cases:
>> - data which is written to the device (i.e. they are comming from the
>>   userland)
>
>This needs a flush on sparc, otherwise an alias now exists in the
>kernel side copy of the buffer.  The D-cache flush is intentionally
>unconditional for PIO mode.  I definitely don't want to take the same
>risks you guys seem to be willing to take for this optimization which
>is of questionable value.
It is not us guys it is just me. And if it is a stupid thing to do then
I get probably punched by Ralf as well once he gets back.
The part I don't get is why you have to flush dcache after the copy
process. I would understand a flush before reading. The dcache alias in
kernel shouldn't hurt since it is not used anymore. Unless we read twice
from the same page. Is this the trick?

>I also, intrinsically, really don't like these changes.
>
>For one thing, you're optimizing PIO mode.
>
>Secondly, IDE is in deep maintainence mode, if you want to optimize
>cache flushing do it in the ATA layer.
This patch patch was mostly driven by the fact that the buffer can be a
normal kernel mapping or a virtual address. The latter doesn't work with
virt_to_page().
Anyway I should probably spent more time getting ATA layer wokring than
on the IDE layer since it is somehow working since patch#1.

>
>Thanks.

Sebastian

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