Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Daniel Vetter]

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