Subject: Re: [PATCHSET] block: fix PIO cache coherency bug
From: James Bottomley <James.Bottomley@SteelEye.com>
To: Russell King <rmk+lkml@arm.linux.org.uk>
Cc: Tejun Heo <htejun@gmail.com>, Dave Miller <davem@redhat.com>,
       axboe@suse.de, bzolnier@gmail.com, james.steward@dynamicratings.com,
       jgarzik@pobox.com, linux-kernel@vger.kernel.org, mattjreimer@gmail.com
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Date: Thu, 02 Mar 2006 14:43:59 -0600
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On Thu, 2006-03-02 at 20:30 +0000, Russell King wrote:
> Your understanding of the problem on ARM remains fundamentally flawed.
> I see no way to resolve this since you don't seem to listen or accept
> my reasoning.

You have advanced no reason for using flush_dcache_page() instead of
flush_kernel_dcache_page() except this:

>    ISTR davem recommended flush_dcache_page() be used for this.

and this:


> 2. (this is my own)  The cachetlb document specifies quite clearly
> what
>    is required whenever a page cache page is written to - that is
>    flush_dcache_page() is called.  The situation when a driver uses
> PIO
>    quote clearly violates the requirements set out in that document.
> 

Your problem, as you state:


>    However, in the PIO case, there is the possibility that the data
> read
>    from the device into the kernel mapping results in cache lines
>    associated with the page.  Moreover, if the cache is
> write-allocate,
>    you _will_ have cache lines.
> 
>    Therefore, you have two completely differing system states
> depending
>    on how the driver decided to transfer data from the device to the
> page
>    cache.

It is my contention that flush_kernel_dcache_page() ejects the cache
lines that may be dirty in the kernel mapping and makes the underlying
memory coherent again.  This is the same net effect as a DMA transfer
(data in memory but not in kernel cache).

> Therefore, message I'm getting from you is that we are not allowed to
> have an ARM system which can possibly work correctly with PIO.
> 
> As a result, I have no further interest in trying to resolve this issue,
> period.  ARM people will just have to accept that PIO mode IDE drivers
> just will not be an option.

Could you actually address the argument instead of getting all huffy?

James


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