From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752662AbbBKLNS (ORCPT ); Wed, 11 Feb 2015 06:13:18 -0500 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:34027 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751418AbbBKLNP (ORCPT ); Wed, 11 Feb 2015 06:13:15 -0500 Date: Wed, 11 Feb 2015 11:12:58 +0000 From: Russell King - ARM Linux To: Marek Szyprowski Cc: Sumit Semwal , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, robin.murphy@arm.com, robdclark@gmail.com, linaro-kernel@lists.linaro.org, stanislawski.tomasz@googlemail.com, daniel@ffwll.ch Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms Message-ID: <20150211111258.GP8656@n2100.arm.linux.org.uk> References: <1422347154-15258-1-git-send-email-sumit.semwal@linaro.org> <1422347154-15258-2-git-send-email-sumit.semwal@linaro.org> <54DB12B5.4080000@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54DB12B5.4080000@samsung.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 11, 2015 at 09:28:37AM +0100, Marek Szyprowski wrote: > Hello, > > On 2015-01-27 09:25, Sumit Semwal wrote: > >Add some helpers to share the constraints of devices while attaching > >to the dmabuf buffer. > > > >At each attach, the constraints are calculated based on the following: > >- max_segment_size, max_segment_count, segment_boundary_mask from > > device_dma_parameters. > > > >In case the attaching device's constraints don't match up, attach() fails. > > > >At detach, the constraints are recalculated based on the remaining > >attached devices. > > > >Two helpers are added: > >- dma_buf_get_constraints - which gives the current constraints as calculated > > during each attach on the buffer till the time, > >- dma_buf_recalc_constraints - which recalculates the constraints for all > > currently attached devices for the 'paranoid' ones amongst us. > > > >The idea of this patch is largely taken from Rob Clark's RFC at > >https://lkml.org/lkml/2012/7/19/285, and the comments received on it. > > > >Cc: Rob Clark > >Signed-off-by: Sumit Semwal > > The code looks okay, although it will probably will work well only with > typical cases like 'contiguous memory needed' or 'no constraints at all' > (iommu). Which is a damn good reason to NAK it - by that admission, it's a half-baked idea. If all we want to know is whether the importer can accept only contiguous memory or not, make a flag to do that, and allow the exporter to test this flag. Don't over-engineer this to make it _seem_ like it can do something that it actually totally fails with. As I've already pointed out, there's a major problem if you have already had a less restrictive attachment which has an active mapping, and a new more restrictive attachment comes along later. It seems from Rob's descriptions that we also need another flag in the importer to indicate whether it wants to have a valid struct page in the scatter list, or whether it (correctly) uses the DMA accessors on the scatter list - so that exporters can reject importers which are buggy. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.