[PATCH v3 05/12] drm/ttm: Expose ttm_tt_unpopulate for driver use

[Daniel Vetter]

On Wed, Nov 25, 2020 at 5:56 PM Michel Dänzer <michel at daenzer.net> wrote:
>
> On 2020-11-25 1:57 p.m., Christian König wrote:
> >
> > Well thinking more about this, it seems to be a another really good
> > argument why mapping pages from DMA-bufs into application address space
> > directly is a very bad idea :)
>
> Apologies for going off on a tangent here...
>
> Since allowing userspace mmap with dma-buf fds seems to be a trap in
> general[0], I wonder if there's any way we could stop supporting that?
>
>
> [0] E.g. mutter had to disable handing out dma-bufs for screen capture
> by default with non-i915 for now, because in particular with discrete
> GPUs, direct CPU reads can be unusably slow (think single-digit frames
> per second), and of course there's other userspace which goes "ooh,
> dma-buf, let's map and read!".

I think a pile of applications (cros included) use it to do uploads
across process boundaries. Think locked down jpeg decoder and stuff
like that. For that use-case it seems to work ok.

But yeah don't read from dma-buf. I'm pretty sure it's dead slow on
almost everything, except integrated gpu which have A) a coherent
fabric with the gpu and B) that fabric is actually faster for
rendering in general, not just for dedicated buffers allocated for
down/upload.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch