From: Russell King - ARM Linux Subject: Re: [PATCH v6 05/14] crypto: marvell/CESA: add TDMA support Date: Thu, 18 Jun 2015 10:48:03 +0100 Message-ID: <20150618094803.GH7557@n2100.arm.linux.org.uk> References: <1434527142-3609-1-git-send-email-boris.brezillon@free-electrons.com> <1434527142-3609-6-git-send-email-boris.brezillon@free-electrons.com> <20150617095001.GA11538@gondor.apana.org.au> <20150618090400.GG7557@n2100.arm.linux.org.uk> <20150618113324.2d469103@bbrezillon> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Herbert Xu , Lior Amsalem , Andrew Lunn , Nadav Haklai , Mark Rutland , Tawfik Bayouk , Eran Ben-Avi , Jason Gunthorpe , Sebastian Hesselbarth , devicetree@vger.kernel.org, Jason Cooper , Pawel Moll , Ian Campbell , Arnaud Ebalard , Rob Herring , Gregory CLEMENT , linux-arm-kernel@lists.infradead.org, Thomas Petazzoni , linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, Kumar Gala , Imre Kaloz , "David S. Miller" Return-path: Received: from pandora.arm.linux.org.uk ([78.32.30.218]:39651 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753178AbbFRJsZ (ORCPT ); Thu, 18 Jun 2015 05:48:25 -0400 Content-Disposition: inline In-Reply-To: <20150618113324.2d469103@bbrezillon> Sender: linux-crypto-owner@vger.kernel.org List-ID: On Thu, Jun 18, 2015 at 11:33:24AM +0200, Boris Brezillon wrote: > Hi Russel, > > On Thu, 18 Jun 2015 10:04:00 +0100 > Russell King - ARM Linux 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 ? Yes. 'ret' should be used to indicate the number of scatterlist entries to walk for DMA purposes after the scatterlist has been mapped. For PIO purposes, using src_nents is still acceptable. As Herbert points out, you're stopping after the sum of transferred bytes matches, so I suppose that's fine. One other point though: you should use sg_dma_address() rather than dereferencing sg->dma_address directly. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.