linux-kernel - Re: [PATCH v6 05/14] crypto: marvell/CESA: add TDMA support

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Date:	Thu, 18 Jun 2015 10:04:00 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Lior Amsalem <alior@...vell.com>, Andrew Lunn <andrew@...n.ch>,
	Nadav Haklai <nadavh@...vell.com>,
	Mark Rutland <mark.rutland@....com>,
	Tawfik Bayouk <tawfik@...vell.com>,
	Eran Ben-Avi <benavi@...vell.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	devicetree@...r.kernel.org, Jason Cooper <jason@...edaemon.net>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Arnaud Ebalard <arno@...isbad.org>,
	Rob Herring <robh+dt@...nel.org>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	Kumar Gala <galak@...eaurora.org>,
	Imre Kaloz <kaloz@...nwrt.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v6 05/14] crypto: marvell/CESA: add TDMA support

On Wed, Jun 17, 2015 at 05:50:01PM +0800, Herbert Xu wrote:
> On Wed, Jun 17, 2015 at 09:45:33AM +0200, Boris Brezillon wrote:
> >
> > +	ret = dma_map_sg(cesa_dev->dev, req->src, creq->src_nents,
> > +			 DMA_TO_DEVICE);
> > +	if (!ret)
> > +		return -ENOMEM;
> > +
> > +	creq->src_nents = ret;
> 
> DMA-API-HOWTO says that you must retain the original nents and
> use it when you call dma_unmap_sg.  So I'm afraid one more repost
> is needed :)

It's worse than that...  You're right on that point, but there's an
additional point.

If dma_map_sg() coalesces scatterlist entries, then ret will be smaller
than src_nents, and ret indicates how many scatterlist entries to be
walked during DMA - you should not use src_nents for that.  I couldn't
see where the driver used that information.  In fact, the driver seems
to be capable of walking more than src_nents/ret numbers of scatterlist
entries: it just keeps going with sg_next() until it hits the end of
the allocated scatterlist.

-- 
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