linux-kernel - Re: SCSI breakage on non-cache coherent architectures

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Date:	Tue, 20 Nov 2007 12:05:17 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	rmk@....linux.org.uk
Subject: Re: SCSI breakage on non-cache coherent architectures

 > Actually, we already established on IRC that the lasi700 driver doesn't
 > need this, principally because the parisc architecture doesn't do an
 > invalidate for DMA_FROM_DEVICE but a flush and invalidate
 > (architecturally, if you read our manuals, even pdc is entitled to write
 > back dirty lines, so it's not clear there's actually an invalidate
 > instruction we can use).   This is also one possible temporary fix for
 > the other architectures if we can't get a different method to work
 > nicely.

I think doing a writeback and invalidate is a very fragile way to deal
with DMA into the middle of a data structure.  It may work OK for now,
but you have to make sure forever into the future that no codepath
anywhere else ever touches the cacheline that you're DMAing into while
the DMA is pending.  It just leaves a hidden trap that is too easy to
step on, because the architectures that get pretty much all testing
all have cache-coherent DMA.

Reviving my ancient __dma_buffer patch seems far preferable to me.

 - R.
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