On 04/06/2017 06:01 AM, Marek Szyprowski wrote: > Hi Shuah, > > On 2017-04-05 18:02, Shuah Khan wrote: >> When coherent DMA memory without struct page is shared, importer >> fails to find the page and runs into kernel page fault when it >> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found >> in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html >> for more information on this problem. >> >> This solution allows coherent DMA memory without struct page to be >> shared by providing a way for the exporter to tag the DMA buffer as >> a special buffer without struct page association and passing the >> information in sg_table to the importer. This information is used >> in attach/map_sg to avoid cleaning D-cache and mapping. >> >> The details of the change are: >> >> Framework: >> - Add a new dma_attrs field to struct scatterlist. >> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify >> Coherent memory without struct page. >> - Add a new dma_check_dev_coherent() interface to check if memory is >> the device coherent area. There is no way to tell where the memory >> returned by dma_alloc_attrs() came from. >> >> Exporter logic: >> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set >> DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is >> done in the exporter context. >> - Add logic to arm_dma_get_sgtable() to identify memory without struct >> page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is >> set, arm_dma_get_sgtable() will set page as the cpu_addr and update >> dma_address and dma_attrs fields in struct scatterlist for this sgl. >> This is done in exporter context when buffer is exported. With this >> Note: This change is made on top of Russell King's patch that added >> !pfn_valid(pfn) check to arm_dma_get_sgtable() to error out on invalid >> pages. Coherent memory without struct page will trigger this error. >> >> Importer logic: >> - Add logic to vb2_dc_dmabuf_ops_attach() to identify memory without >> struct page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute when it copies >> the sg_table from the exporter. It will copy dma_attrs and dma_address >> fields. With this logic, dmabuf_ops_attach will no longer trip on an >> invalid page. >> - Add logic to arm_dma_map_sg() to avoid mapping the page when sg_table >> has DMA_ATTR_DEV_COHERENT_NOPAGE buffer. >> - Add logic to arm_dma_unmap_sg() to do nothing for sg entries with >> DMA_ATTR_DEV_COHERENT_NOPAGE attribute. >> >> Without this change the following use-case that runs into kernel >> pagefault when importer tries to attach the exported buffer. >> >> With this change it works: (what a relief after watching pagefaults for >> weeks!!) >> >> gst-launch-1.0 filesrc location=~/GH3_MOV_HD.mp4 ! qtdemux ! h264parse ! v4l2video4dec capture-io-mode=dmabuf ! v4l2video7convert output-io-mode=dmabuf-import ! kmssink force-modesetting=true >> >> I am sending RFC patch to get feedback on the approach and see if I missed >> anything. > > Frankly, once You decided to hack around dma-buf and issues with coherent, > carved out memory, it might be a bit better to find the ultimate solution > instead of the another hack. Please note that it will still not allow to > share a buffer allocated from carved-out memory and a device, which is > behind IOMMU. With your patch s5p-mfc patch series does address the problem for this use-case for 4.12 onwards. However I am still concerned about prior release and this pagefault is bad. Invalid page test partially solves the problem. Would it helpful to at least prevent the pagfault with a definitive test. Please see my response to Russell. Let me know your thoughts on that. > > I thought a bit about this and the current shape of dma-buf code. > > IMHO the proper way of solving all those issues would be to replace > dma-buf internal representation of the memory from struct scatter_list > to pfn array. This would really solve the problem of buffers which > cannot be properly represented by scatter lists/struct pages and would > even allow sharing buffers between all kinds of devices. Scatter-lists > are also quite over-engineered structures to represent a single buffer > (pfn array is a bit more compact representation). Also there is a lots > of buggy code which use scatter-list in a bit creative way (like > assuming that each page maps to a single scatter list entry for > example). The only missing piece, required for such change would be > extending DMA-mapping with dma_map_pfn() interface. I agree with you on scatterlists being clumsy. Changing over to pfn array could simplify things. I am exploring a slightly different option that might not require too many changes. I will respond with concrete ideas later on this week. > > 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. thanks, -- Shuah > >> Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx> >> --- >> arch/arm/mm/dma-mapping.c | 34 ++++++++++++++++++++++---- >> drivers/base/dma-coherent.c | 25 +++++++++++++++++++ >> drivers/media/v4l2-core/videobuf2-dma-contig.c | 6 +++++ >> include/linux/dma-mapping.h | 8 ++++++ >> include/linux/scatterlist.h | 1 + >> 5 files changed, 69 insertions(+), 5 deletions(-) >> [...] > > Best regards
