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.skida...@intel.com> > >>>> --- > >>> > >>> 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. 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
