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

From: Christoph Hellwig
Date: Tue Mar 16 2021 - 11:46:54 EST

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?