On Tue, Mar 16, 2021 at 4:46 PM Christoph Hellwig [off-list ref] wrote:

On Tue, Mar 16, 2021 at 04:33:02PM +0100, Daniel Vetter wrote:

quoted

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

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