On Tue, Mar 16, 2021 at 4:46 PM Christoph Hellwig hch@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