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 would suggest to fix the contract and to keep the dma_buf_kmap() support in ION. > > Based on discussions at LPC here is what was proposed: > - #1 Add support to ION for dma_buf_vmap and dma_buf_vunmap > - #2 Move any existing ION clients over from using dma_buf_kmap to > dma_buf_vmap > - #3 Deprecate support in ION for dma_buf_kmap? > - #4 Make the above performance optimization to > ion_dma_buf_begin_cpu_access to remove the creation of a kernel mapping. > > Thoughts? > > Liam > > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > a Linux Foundation Collaborative Project > Thanks, Alexey _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
