Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964898Ab3GRS6Z (ORCPT ); Thu, 18 Jul 2013 14:58:25 -0400 Received: from comal.ext.ti.com ([198.47.26.152]:37132 "EHLO comal.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933065Ab3GRS6X (ORCPT ); Thu, 18 Jul 2013 14:58:23 -0400 Message-ID: <51E83A9D.5020008@ti.com> Date: Thu, 18 Jul 2013 13:57:33 -0500 From: Joel Fernandes User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7 MIME-Version: 1.0 To: Russell King - ARM Linux CC: Tony Lindgren , Sekhar Nori , Matt Porter , Grant Likely , Rob Herring , Vinod Koul , Mark Brown , Benoit Cousson , Balaji TK , Gururaja Hebbar , Chris Ball , Jason Kridner , Mark Jackson , Devicetree Discuss , Linux OMAP List , Linux ARM Kernel List , Linux DaVinci Kernel List , Linux Kernel Mailing List , Linux Documentation List , Linux MMC List , Linux SPI Devel List , Arnd Bergmann , Subject: Re: [PATCH 1/3] dmaengine: add dma_get_slave_sg_limits() References: <1374166001-31340-1-git-send-email-joelf@ti.com> <1374166001-31340-2-git-send-email-joelf@ti.com> <20130718170825.GZ21614@n2100.arm.linux.org.uk> In-Reply-To: <20130718170825.GZ21614@n2100.arm.linux.org.uk> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3499 Lines: 83 On 07/18/2013 12:08 PM, Russell King - ARM Linux wrote: > On Thu, Jul 18, 2013 at 11:46:39AM -0500, Joel Fernandes wrote: >> The API is optionally implemented by dmaengine drivers and when >> unimplemented will return a NULL pointer. A client driver using >> this API provides the required dma channel, address width, and >> burst size of the transfer. dma_get_slave_sg_limits() returns an >> SG limits structure with the maximum number and size of SG segments >> that the given channel can handle. > > Please look at what's already in struct device: > > struct device { > ... > struct device_dma_parameters *dma_parms; > ... > }; > > This provides: > > struct device_dma_parameters { > /* > * a low level driver may set these to teach IOMMU code about > * sg limitations. > */ > unsigned int max_segment_size; > unsigned long segment_boundary_mask; > }; > > Now, these are helpfully accessed via: > > dma_get_max_seg_size(dev) > dma_set_max_seg_size(dev) > dma_get_seg_boundary(dev) > dma_set_seg_boundary(dev, mask) > Drivers already use these to work out how to construct the scatterlist > before passing it to the DMA API, which means that they should also be > used when creating a scatterlist for the DMA engine (think about it - > you have to use the DMA API to map the buffers for the DMA engine too.) > > So, we already have two properties defined on a per-device basis: the > maximum size of a scatterlist segment, and the boundary over which any > segment must not cross. > > The former ties up with your max_seg_len() property, though arguably it > may depend on the DMA engine access size. The problem with implementing > this new API though is that the subsystems (such as SCSI) which already > use dma_get_max_seg_size() will be at odds with what is possible via the > DMA engine. Not very clear for this particular case, are you saying the DMAEngine driver implementation should set the max_seg_size of its own struct dev, and then the drivers retrieve it from the channel they are allocated? > I strongly suggest using the infrastructure at device level and not > implementing some private DMA engine API to convey this information. Certainly see the value. OK with either approach. Can Vinod add to the discussion here, and we can decide a way forward? Is it ok to use the new CAPS API added for now so that we can keep AM33xx MMC alive? seg_size atleast is a real regression, the number of slots limit however is related more to MMC grabbing a lot of slots. Atleast for -rc cycle the seg_size and MMC fixes should go in. > As for the maximum number of scatterlist entries, really that's a bug in > the DMA engine implementations if they can't accept arbitary lengths. > I've created DMA engine drivers for implementations where you have to > program each segment individually, ones which can have the current and > next segments, as well as those which can walk a list. Provided you get > informed of a transfer being completed, there really is no reason for a > DMA engine driver to limit the number of scatterlist entries that it > will accept. Sure, that makes sense. Can you point to such a typical example implementation to get some ideas? Thanks, -Joel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/