[PATCH 2/3] media/videobuf1|2: Mark follow_pfn usage as unsafe

[Daniel Vetter]

On Tue, Mar 16, 2021 at 4:46 PM Christoph Hellwig <hch at infradead.org> wrote:
>
> On Tue, Mar 16, 2021 at 04:33:02PM +0100, Daniel Vetter wrote:
> > The media model assumes that buffers are all preallocated, so that
> > when a media pipeline is running we never miss a deadline because the
> > buffers aren't allocated or available.
> >
> > This means we cannot fix the v4l follow_pfn usage through
> > mmu_notifier, without breaking how this all works. The only real fix
> > is to deprecate userptr support for VM_IO | VM_PFNMAP mappings and
> > tell everyone to cut over to dma-buf memory sharing for zerocopy.
> >
> > userptr for normal memory will keep working as-is, this only affects
> > the zerocopy userptr usage enabled in 50ac952d2263 ("[media]
> > videobuf2-dma-sg: Support io userptr operations on io memory").
>
> Maybe I'm missing something, but wasn't the conclusion last time that
> this hackish early device to device copy support can just go away?

My understanding is mostly, but with some objections. And I kinda
don't want to let this die in a bikeshed and then not getting rid of
follow_pfn as a result. There's enough people who acked this, and the
full removal got some nack from Mauro iirc.

Maybe if no bug report ever shows up for 1-2 years we can sunset it
for real&completely.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch