[RFC PATCH 2/2] vc4: introduce DMA-BUF heap

[Simon Ser]

On Thursday, November 9th, 2023 at 20:17, Maxime Ripard <mripard at kernel.org> wrote:

> > Can we add another function pointer to the struct drm_driver (or
> > similar) to do the allocation, and move the actual dmabuf handling
> > code into the core?
> 
> Yeah, I agree here, it just seems easier to provide a global hook and a
> somewhat sane default to cover all drivers in one go (potentially with
> additional restrictions, like only for modeset-only drivers or
> something).

First off not all drivers are using the GEM DMA helpers (e.g. vc4 with
!vc5 does not). The heap code in this patch only works with drivers
leveraging GEM DMA helpers.

Then maybe it's somewhat simple to cover typical display devices found
on split render/display SoCs, however for the rest it's going to be much
more complicated. For x86 typically there are multiple buffer placements
supported by the GPU and we need to have one heap per possible placement.
And then figuring out all of the rules, priority and compatibility stuff
is a whole other task in and of itself.

In short: I think it's unreasonable to expose a heap without good
knowledge of the specific hardware. In other words, unreasonable to
expose a heap for all drivers at once.