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

[Boris Brezillon]

Hi Russel,

On Thu, 18 Jun 2015 10:04:00 +0100
Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote:

> 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.

Yes, I realized that, and I never used the value returned by
dma_map_sg() to walk the scatterlist anyway: I was using the sg_next()
and sg->length value (which I replaced by sg_dma_len() in v7 as
suggested by Herbert).
So the ->src_nents assignment to dma_map_sg() return value was just a
silly mistake caused by an uncareful read of the DMA-API-HOWTO.

Am I missing something else ?

Best Regards,

Boris


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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