On Tue, 18 Dec 2018, Alexey Skidanov wrote: > >>> I was wondering if we could re-open the discussion on adding support to > >>> ION for dma_buf_vmap. > >>> It seems like the patch was not taken as the reviewers wanted more > >>> evidence of an upstream use case. > >>> > >>> Here would be my upstream usage argument for including dma_buf_vmap > >>> support in ION. > >>> > >>> Currently all calls to ion_dma_buf_begin_cpu_access result in the creation > >>> of a kernel mapping for the buffer, unfortunately the resulting call to > >>> alloc_vmap_area can be quite expensive and this has caused a performance > >>> regression for certain clients when they have moved to the new version of > >>> ION. > >>> > >>> The kernel mapping is not actually needed in ion_dma_buf_begin_cpu_access, > >>> and generally isn't needed by clients. So if we remove the creation of the > >>> kernel mapping in ion_dma_buf_begin_cpu_access and only create it when > >>> needed we can speed up the calls to ion_dma_buf_begin_cpu_access. > >>> > >>> An additional benefit of removing the creation of kernel mappings from > >>> ion_dma_buf_begin_cpu_access is that it makes the ION code more secure. > >>> Currently a malicious client could call the DMA_BUF_IOCTL_SYNC IOCTL with > >>> flags DMA_BUF_SYNC_END multiple times to cause the ION buffer kmap_cnt to > >>> go negative which could lead to undesired behavior. > >>> > >>> One disadvantage of the above change is that a kernel mapping is not > >>> already created when a client calls dma_buf_kmap. So the following > >>> dma_buf_kmap contract can't be satisfied. > >>> > >>> /** > >>> * dma_buf_kmap - Map a page of the buffer object into kernel address > >>> space. The > >>> * same restrictions as for kmap and friends apply. > >>> * @dmabuf: [in] buffer to map page from. > >>> * @page_num: [in] page in PAGE_SIZE units to map. > >>> * > >>> * This call must always succeed, any necessary preparations that might > >>> fail > >>> * need to be done in begin_cpu_access. > >>> */ > >>> > >>> But hopefully we can work around this by moving clients to dma_buf_vmap. > >> I think the problem is with the contract. We can't ensure that the call > >> is always succeeds regardless the implementation - any mapping might > >> fail. Probably this is why *all* clients of dma_buf_kmap() check the > >> return value (so it's safe to return NULL in case of failure). > >> > > > > I think currently the call to dma_buf_kmap will always succeed since the > > DMA-Buf contract requires that the client first successfully call > > dma_buf_begin_cpu_access(), and if dma_buf_begin_cpu_access() succeeds > > then dma_buf_kmap will succeed. > > > >> I would suggest to fix the contract and to keep the dma_buf_kmap() > >> support in ION. > > > > I will leave it to the DMA-Buf maintainers as to whether they want to > > change their contract. > > > > Liam > > > > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > > a Linux Foundation Collaborative Project > > > > Ok. We need the list of the clients using the ION in the mainline tree. > Looks to me like the only functions which might be calling dma_buf_kmap/dma_buf_kunmap on ION buffers are tegra_bo_kmap/tegra_bo_kunmap, I assume Tegra is used in some Android automotive products. Looks like these functions could be moved over to using dma_buf_vmap/dma_buf_vunmap but it wouldn't be very clean and would add a performance hit. Liam Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
