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. 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. This would be however quite large task, especially taking into account all current users of DMA-buf framework...
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 -- Marek Szyprowski, PhD Samsung R&D Institute Poland
