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Message-ID: <45001C48.6050803@gmail.com>
Date: Thu, 07 Sep 2006 15:19:04 +0200
From: Tejun Heo <htejun@gmail.com>
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To: Matthew Wilcox <matthew@wil.cx>
CC: Arjan van de Ven <arjan@infradead.org>, linux-pci@atrey.karlin.mff.cuni.cz,
       Greg KH <greg@kroah.com>, lkml <linux-kernel@vger.kernel.org>
Subject: Re: question regarding cacheline size
References: <44FFD8C6.8080802@gmail.com> <20060907111120.GL2558@parisc-linux.org> <45000076.4070005@gmail.com> <20060907120756.GA29532@flint.arm.linux.org.uk> <20060907122311.GM2558@parisc-linux.org> <1157632405.14882.27.camel@laptopd505.fenrus.org> <20060907124026.GN2558@parisc-linux.org> <45001665.9050509@gmail.com> <20060907130401.GO2558@parisc-linux.org>
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Matthew Wilcox wrote:
> On Thu, Sep 07, 2006 at 02:53:57PM +0200, Tejun Heo wrote:
>> The spec says that devices can put additional restriction on supported 
>> cacheline size (IIRC, the example was something like power of two >= or 
>> <= certain size) and should ignore (treat as zero) if unsupported value 
>> is written.  So, there might be need for more low level driver 
>> involvement which knows device restrictions, but I don't know whether 
>> such devices exist.
> 
> That's nothing we can do anything about.  The system cacheline size is
> what it is.  If the device doesn't support it, we can't fall back to a
> different size, it'll cause data corruption.  So we'll just continue on,
> and devices which live up to the spec will act as if we hadn't
> programmed a cache size.  For devices that don't, we'll have the quirk.

For MWI, it will cause data corruption.  For READ LINE and MULTIPLE, I 
think it would be okay.  The memory is prefetchable after all.  Anyways, 
this shouldn't be of too much problem and probably can be handled by 
quirks if ever needed.

> Arguably devices which don't support the real system cacheline size
> would only get data corruption if they used MWI, so we only have to
> prevent them from using MWI; they could use a different cacheline size
> for MRM and MRL without causing data corruption.  But I don't think we
> want to go down that route; do you?

Oh yeah, that's what I was trying to say, and I don't want to go down 
that route.  So, I guess this one is settled.

Thanks.

-- 
tejun
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