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 _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
