From: Shuah Khan <firstname.lastname@example.org> To: Russell King - ARM Linux <email@example.com>, Marek Szyprowski <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, Robin.Murphy@arm.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, Shuah Khan <email@example.com> Subject: Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page Date: Wed, 19 Apr 2017 17:38:10 -0600 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <20170414094643.GG17774@n2100.armlinux.org.uk> Hi Russell, and Marek, On 04/14/2017 03:46 AM, Russell King - ARM Linux wrote: > On Fri, Apr 14, 2017 at 09:56:07AM +0200, Marek Szyprowski wrote: >>>> This would be however quite large task, especially taking into account >>>> all current users of DMA-buf framework... >>> Yeah it will be a large task. >> >> Maybe once scatterlist are switched to pfns, changing dmabuf internal >> memory representation to pfn array might be much easier. > > Switching to a PFN array won't work either as we have no cross-arch > way to translate PFNs to a DMA address and vice versa. Yes, we have > them in ARM, but they are an _implementation detail_ of ARM's > DMA API support, they are not for use by drivers. > > So, the very first problem that needs solving is this: > > How do we go from a coherent DMA allocation for device X to a set > of DMA addresses for device Y. > > Essentially, we need a way of remapping the DMA buffer for use with > another device, and returning a DMA address suitable for that device. > This could well mean that we need to deal with setting up an IOMMU > mapping. My guess is that this needs to happen at the DMA coherent > API level - the DMA coherent API needs to be augmented with support > for this. I'll call this "DMA coherent remap". > > We then need to think about how to pass this through the dma-buf API. > dma_map_sg() is done by the exporter, who should know what kind of > memory is being exported. The exporter can avoid calling dma_map_sg() > if it knows in advance that it is exporting DMA coherent memory. > Instead, the exporter can simply create a scatterlist with the DMA > address and DMA length prepopulated with the results of the DMA > coherent remap operation above. As Russell pointed to armama-drm case, I looked at that closely. armada-drm is creating sg_table and populating it with DMA-address in its map_dma_buf ops and unmap_dma_buf ops handles the special case and doesn't call dma_unmap_sg(). In the case of drm, gem_prime_map_dma_buf interfaces and the common drm_gem_map_dma_buf() will need modification to not do dma_map_sg() and create scatterlist with the DMA address and DMA length instead. We have to get drm_gem_map_dma_buf() info. to have it not do dma_map_sg() and create scatterlist. Focusing on drm for now, looks like there are probably about 15 or so map_dma_buf interfaces will need to handle coherent memory case. > > What the scatterlist can't carry in this case is a set of valid > struct page pointers, and an importer must not walk the scatterlist > expecting to get at the virtual address parameters or struct page > pointers. Right - importers need handling to not walk the sg_list and handle it differently. Is there a good example drm you can point me to for this? aramda-drm seems to special case this in armada_gem_map_import() if I am not mistaken. > > On the mmap() side of things, remember that DMA coherent allocations > may require special mapping into userspace, and which can only be > mapped by the DMA coherent mmap support. kmap etc will also need to > be different. So it probably makes sense for DMA coherent dma-buf > exports to use a completely separate set of dma_buf_ops from the > streaming version. > I agree. It would make is easier and also limits the scope of changes. > I think this is the easiest approach to solving the problem without > needing massive driver changes all over the kernel. > Anyway this is a quick note to say that I am looking into this and haven't drooped it :) thanks, -- Shuah
prev parent reply other threads:[~2017-04-19 23:38 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <CGME20170405160251epcas4p14cc5d5f6064c84b133b9e280ac987a93@epcas4p1.samsung.com> 2017-04-05 16:02 ` Shuah Khan 2017-04-05 23:14 ` Russell King - ARM Linux 2017-04-10 14:52 ` Shuah Khan 2017-04-06 4:11 ` kbuild test robot 2017-04-06 12:01 ` Marek Szyprowski 2017-04-10 22:50 ` Shuah Khan 2017-04-14 7:56 ` Marek Szyprowski 2017-04-14 9:46 ` Russell King - ARM Linux 2017-04-17 1:10 ` Shuah Khan 2017-04-17 10:22 ` Russell King - ARM Linux 2017-04-19 23:38 ` Shuah Khan [this message]
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --email@example.com \ --firstname.lastname@example.org \ --cc=Robin.Murphy@arm.com \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --subject='Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).