linux-kernel - Re: [RESEND][PATCH 1/2] lib/scatterlist: Make ARCH_HAS_SG

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Date:	Sat, 22 Mar 2014 15:37:40 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Laura Abbott <lauraa@...eaurora.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-scsi@...r.kernel.org, linux-s390@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	x86@...nel.org, Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, linux390@...ibm.com,
	sparclinux@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-ia64@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	linuxppc-dev@...ts.ozlabs.org,
	"David S. Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RESEND][PATCH 1/2] lib/scatterlist: Make ARCH_HAS_SG_CHAIN an
 actual Kconfig

On Sat, 2014-03-22 at 22:23 +0000, Russell King - ARM Linux wrote:
> On Sat, Mar 22, 2014 at 02:31:21PM -0700, James Bottomley wrote:
> > Perhaps now might be the time to ask which are the remaining
> > architectures that cannot do SG chaining and then we can fix them and
> > pull the whole thing out.
> 
> Not quite.  You're making the assumption that we can be sure that all
> the scatterlist users on an architecture have been converted - that's
> simply not true on ARM.

No I'm not, I said "now might be the time to ask which are the remaining
architectures that cannot do SG chaining" I think it's time to list them
so we know what work remains.  I know we've got a bunch in parisc (all
of our iommu code in driver/parisc - about 5 different ones - are
unconverted).  However, the conversion is pretty simple; it's mostly
replacing sglist++ with sglist=sg_next(sglist)

>   We have some which have, and some which still
> have not been audited.
> 
> The cases that get us here would be old platform DMA code which walks
> scatterlists handed to it from drivers - stuff like
> arch/arm/mach-rpc/dma.c (which probably can cope), and drivers/scsi/arm/*
> (which definitely can't because of their SCSI pointers save/restore
> handling message.)  I know that's one case where SG_CHAIN definitely
> isn't supported on ARM.
> 
> So, we had decided not to enable it, but this means that new stuff
> isn't benefitting from this.  I've recently asked arm-soc to enable
> it for the modern multi-platform builds, because modern stuff really
> be written with correct SG chaining in mind.

OK, so lets see what the actual effort is.

James



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