[PATCH 04/11] drm/panfrost: Fix implicit sync

[Daniel Stone]

Hi,

On Fri, 21 May 2021 at 10:10, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> Currently this has no practial relevance I think because there's not
> many who can pull off a setup with panfrost and another gpu in the
> same system. But the rules are that if you're setting an exclusive
> fence, indicating a gpu write access in the implicit fencing system,
> then you need to wait for all fences, not just the previous exclusive
> fence.
>
> panfrost against itself has no problem, because it always sets the
> exclusive fence (but that's probably something that will need to be
> fixed for vulkan and/or multi-engine gpus, or you'll suffer badly).
> Also no problem with that against display.

Yeah, the 'second-generation Valhall' GPUs coming later this year /
early next year are starting to get pretty weird. Firmware-mediated
job scheduling out of multiple queues, userspace having direct access
to the queues and can do inter-queue synchronisation (at least I think
so), etc. For bonus points, synchronisation is based on $addr = $val
to signal and $addr == $val to wait, with a separate fence primitive
as well.

Obviously Arm should be part of this conversation here, but I guess
we'll have to wait for a while yet to see how everything's shaken out
with this new gen, and hope that whatever's been designed upstream in
the meantime is actually vaguely compatible ...

Cheers,
Daniel