On 12/17/18 20:42, Liam Mark wrote: > On Sun, 16 Dec 2018, Alexey Skidanov wrote: > >> >> >> On 12/16/18 7:20 AM, Liam Mark wrote: >>> On Tue, 6 Feb 2018, Alexey Skidanov wrote: >>> >>>> >>>> >>>> On 02/07/2018 01:56 AM, Laura Abbott wrote: >>>>> On 01/31/2018 10:10 PM, Alexey Skidanov wrote: >>>>>> >>>>>> On 01/31/2018 03:00 PM, Greg KH wrote: >>>>>>> On Wed, Jan 31, 2018 at 02:03:42PM +0200, Alexey Skidanov wrote: >>>>>>>> Any driver may access shared buffers, created by ion, using >>>>>>>> dma_buf_vmap and >>>>>>>> dma_buf_vunmap dma-buf API that maps/unmaps previosuly allocated >>>>>>>> buffers into >>>>>>>> the kernel virtual address space. The implementation of these API is >>>>>>>> missing in >>>>>>>> the current ion implementation. >>>>>>>> >>>>>>>> Signed-off-by: Alexey Skidanov <alexey.skidanov@xxxxxxxxx> >>>>>>>> --- >>>>>>> >>>>>>> No review from any other Intel developers? :( >>>>>> Will add. >>>>>>> >>>>>>> Anyway, what in-tree driver needs access to these functions? >>>>>> I'm not sure that there are the in-tree drivers using these functions >>>>>> and ion as> buffer exporter because they are not implemented in ion :) >>>>>> But there are some in-tre> drivers using these APIs (gpu drivers) with >>>>>> other buffer exporters. >>>>> >>>>> It's still not clear why you need to implement these APIs. >>>> How the importing kernel module may access the content of the buffer? :) >>>> With the current ion implementation it's only possible by dma_buf_kmap, >>>> mapping one page at a time. For pretty large buffers, it might have some >>>> performance impact. >>>> (Probably, the page by page mapping is the only way to access large >>>> buffers on 32 bit systems, where the vmalloc range is very small. By the >>>> way, the current ion dma_map_kmap doesn't really map only 1 page at a >>>> time - it uses the result of vmap() that might fail on 32 bit systems.) >>>> >>>>> Are you planning to use Ion with GPU drivers? I'm especially >>>>> interested in this if you have a non-Android use case. >>>> Yes, my use case is the non-Android one. But not with GPU drivers. >>>>> >>>>> Thanks, >>>>> Laura >>>> >>>> Thanks, >>>> Alexey >>> >>> 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. Alexey _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
