On Fri, 4 Jan 2019, Skidanov, Alexey wrote: > > > > -----Original Message----- > > From: Liam Mark [mailto:lmark@xxxxxxxxxxxxxx] > > Sent: Friday, January 04, 2019 19:42 > > To: Skidanov, Alexey <alexey.skidanov@xxxxxxxxx> > > Cc: Laura Abbott <labbott@xxxxxxxxxx>; Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>; > > devel@xxxxxxxxxxxxxxxxxxxx; tkjos@xxxxxxxxxxx; rve@xxxxxxxxxxx; linux- > > kernel@xxxxxxxxxxxxxxx; maco@xxxxxxxxxxx; sumit.semwal@xxxxxxxxxx > > Subject: Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and > > dma_buf_vunmap > > > > 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 > > I'm a little bit confused. Why making the buffer accessible by CPU (mapping the buffer) > and making the content of the buffer valid (coherent) are so tightly coupled in DMA-BUF? > Hi Sumit, Hope you are feeling better. I was wondering if you would be open to changes to to the DMA-BUF contract so that we can remove the creation of kernel mappings in begin_cpu_access. This would have the benefit of improving the performance of begin_cpu_access and removing the ability for userspace to add and remove kernel mappings. 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
