[Mesa-dev] Linux Graphics Next: Userspace submission update

[Christian König]

Am 02.06.21 um 20:48 schrieb Daniel Vetter:
> On Wed, Jun 02, 2021 at 05:38:51AM -0400, Marek Olšák wrote:
>> On Wed, Jun 2, 2021 at 5:34 AM Marek Olšák <maraeo at gmail.com> wrote:
>>
>>> Yes, we can't break anything because we don't want to complicate things
>>> for us. It's pretty much all NAK'd already. We are trying to gather more
>>> knowledge and then make better decisions.
>>>
>>> The idea we are considering is that we'll expose memory-based sync objects
>>> to userspace for read only, and the kernel or hw will strictly control the
>>> memory writes to those sync objects. The hole in that idea is that
>>> userspace can decide not to signal a job, so even if userspace can't
>>> overwrite memory-based sync object states arbitrarily, it can still decide
>>> not to signal them, and then a future fence is born.
>>>
>> This would actually be treated as a GPU hang caused by that context, so it
>> should be fine.
> This is practically what I proposed already, except your not doing it with
> dma_fence. And on the memory fence side this also doesn't actually give
> what you want for that compute model.
>
> This seems like a bit a worst of both worlds approach to me? Tons of work
> in the kernel to hide these not-dma_fence-but-almost, and still pain to
> actually drive the hardware like it should be for compute or direct
> display.
>
> Also maybe I've missed it, but I didn't see any replies to my suggestion
> how to fake the entire dma_fence stuff on top of new hw. Would be
> interesting to know what doesn't work there instead of amd folks going of
> into internal again and then coming back with another rfc from out of
> nowhere :-)

Well to be honest I would just push back on our hardware/firmware guys 
that we need to keep kernel queues forever before going down that route.

That syncfile and all that Android stuff isn't working out of the box 
with the new shiny user queue submission model (which in turn is mostly 
because of Windows) already raised some eyebrows here.

Christian.

> -Daniel