linux-kernel - Re: question regarding cacheline size

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Date:	Thu, 07 Sep 2006 15:19:04 +0200
From:	Tejun Heo <htejun@...il.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Arjan van de Ven <arjan@...radead.org>,
	linux-pci@...ey.karlin.mff.cuni.cz, Greg KH <greg@...ah.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: question regarding cacheline size

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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