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

Quoting Daniel Vetter (2021-01-14 09:47:40)

On Thu, Jan 14, 2021 at 09:45:37AM +0000, Chris Wilson wrote:

quoted

Quoting Daniel Vetter (2021-01-14 09:30:32)

quoted

On Thu, Jan 14, 2021 at 10:23 AM Chris Wilson [off-list ref] wrote:

quoted

The only other problem I see with the implementation is that there's
nothing that says that each dmabuf->ops->map_dma_buf() returns a new
sg_table, so we may end up undoing the xor. Or should each dma-buf
return a fresh dma-mapping for iommu isolation?

Maybe I screwed it up, but that's why I extracted the little helpers:
We scramble when we get the sgtable from exporter, and unscramble
before we pass it back. dma-buf.c does some caching and will hand back
the same sgtable, but for that case we don't re-scramble.

The attachment is only mapped once, but there can be more than one
attachment, and the backend could return the same sg_table for each
mapping. Conceivably, it could return its own private sg_table where it
wants to maintain the struct page. Seems like just adding a sentence to
@map_dma_buf to clarify that each call should return a new sg_table will
suffice.

Ah yes good point, will augment (once CI stops being angry at me).

Fwiw, with a quick explanation of "don't do this" in the docs,
Reviewed-by: Chris Wilson <redacted>
-Chris