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

[Daniel Vetter]

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 think the only problem we have is that the first import for ttm
simply pulled out the struct page and ignored the sgtable otherwise,
then that copypasted to places and we're still have some of that left.
Although it's a lot better. So largely the problem is importers being
a bit silly.

I also think I should change the defaulty y to default y if
DMA_API_DEBUG or something like that, to make sure it's actually
enabled often enough.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch