[Intel-gfx] [PATCH] drm-buf: Add debug option

[John Stultz]

On Thu, Jan 14, 2021 at 1:03 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> On Wed, Jan 13, 2021 at 10:08 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > Quoting Daniel Vetter (2021-01-13 20:50:11)
> > > On Wed, Jan 13, 2021 at 4:43 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > >
> > > > Quoting Daniel Vetter (2021-01-13 14:06:04)
> > > > > We have too many people abusing the struct page they can get at but
> > > > > really shouldn't in importers. Aside from that the backing page might
> > > > > simply not exist (for dynamic p2p mappings) looking at it and using it
> > > > > e.g. for mmap can also wreak the page handling of the exporter
> > > > > completely. Importers really must go through the proper interface like
> > > > > dma_buf_mmap for everything.
> > > >
> > > > If the exporter doesn't want to expose the struct page, why are they
> > > > setting it in the exported sg_table?
> > >
> > > You need to store it somewhere, otherwise the dma-api doesn't work.
> > > Essentially this achieves clearing/resetting the struct page pointer,
> > > without additional allocations somewhere, or tons of driver changes
> > > (since presumably the driver does keep track of the struct page
> > > somewhere too).
> >
> > Only for mapping, and that's before the export -- if there's even a
> > struct page to begin with.
> >
> > > Also as long as we have random importers looking at struct page we
> > > can't just remove it, or crashes everywhere. So it has to be some
> > > debug option you can disable.
> >
> > Totally agreed that nothing generic can rely on pages being transported
> > via dma-buf, and memfd is there if you do want a suitable transport. The
> > one I don't know about is dma-buf heap, do both parties there consent to
> > transport pages via the dma-buf? i.e. do they have special cases for
> > import/export between heaps?
>
> heaps shouldn't be any different wrt the interface exposed to
> importers. Adding John just in case I missed something.

I'm not aware of how this would be an issue right off for dma-buf
heaps. Obviously there may be some corner cases with things like
secure heaps, but I've not gotten to work on any of those yet and
there's none in-tree.  I did test out the patch on HiKey960 (using the
cma and system heap for display and gpu buffers - admittedly not
particularly complex) and didn't see any issues with it enabled.

I've added Suren and Hridya for more input but don't have any
objections right off.

thanks
-john