On Wed, Oct 7, 2020 at 12:47 PM Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote: > > Hi Daniel, > > On 03.10.2020 11:40, Daniel Vetter wrote: > >> After he three places above should use pin_user_pages_fast(), then > >> this whole broken API should be moved into videobuf2-memops.c and a > >> big fat "THIS DOESN'T WORK" stuck on it. > >> > >> videobuf2 should probably use P2P DMA buf for this instead. > > Yup this should be done with dma_buf instead, and v4l has that. > > Yes, V4L2 has dma_buf support NOW. That days, using so called V4L2 > USERPTR method was the only way to achieve zero copy buffer sharing > between devices, so this is just a historical baggage. I've been > actively involved in implementing that. I've tried to make it secure as > much as possible assuming the limitation of that approach. With a few > assumptions it works fine. Buffers are refcounted both by the > vm_ops->open or by incrementing the refcount of the vm->file. This > basically works with any sane driver, which doesn't free the mmaped > buffer until the file is released. This is true for V4L2 and FBdev devices. I'm not seeing any of that vm->file refcounting going on, so not seeing anything that prevents the mmap area from being removed. Can you pls give me some pointers about which code you're thinking of? -Daniel > This API is considered as deprecated in V4L2 world, so I think > supporting this hack can be removed one day as nowadays userspace should > use dma buf. > > > ... > > Best regards > -- > Marek Szyprowski, PhD > Samsung R&D Institute Poland > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
