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

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Message-Id: <1195502068.6970.3.camel@pasglop>
Date:	Tue, 20 Nov 2007 06:54:28 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Russell King <rmk@....linux.org.uk>
Subject: Re: SCSI breakage on non-cache coherent architectures


On Mon, 2007-11-19 at 09:09 -0600, James Bottomley wrote:
> > What other drivers do is DMA to their own allocation and then memcpy to
> > the sense buffer.
> > 
> > There is a movement to allocate the sense data as its own sg list, but
> > I don't think that patch has even been posted yet.
> 
> I'd like to be rid of it inside the command for various reasons:  every
> command has one of these, and they're expensive in the allocation (at 96
> bytes).  There's no reason we have to allocate and free that amount of
> space with every command.  In theory, the number of these is bounded at
> the queue depth, in practice, there's usually only one, and this DMA
> alignment issue does requires most drivers to double copy.

And most drivers don't and break. Take USB storage, I -think- (code path
aren't trivial to follow) it just gets the sglist cooked up by the code
in scsi_error.c no ? That just points to the buffer in scsi_cmnd. It
then pass that for DMA to the USB stack.

Ben.


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