Am 21.05.21 um 14:54 schrieb Daniel Stone:
On Fri, 21 May 2021 at 13:28, Christian König <christian.koenig@xxxxxxx> wrote:
Am 21.05.21 um 14:22 schrieb Daniel Stone:
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.
Well that sounds familiar :)
I laughed when I first saw it, because it was better than crying I guess.
In Germany we say "Ich freue mich drauf wie auf Zahnschmerzen".
If you're curious, the interface definitions are in the csf/ directory
in the 'Bifrost kernel driver' r30p0 download you can get from the Arm
developer site. Unfortunately the exact semantics aren't completely
clear.
Well it is actually relatively simple. Take a look at the timeline
semaphores from Vulkan, everybody is basically implementing the same
semantics now.
When you queued up a bunch of commands on your hardware, the first one
will write value 1 to a 64bit memory location, the second one will write
value 2, the third value 3 and so on. After writing the value the
hardware raises and interrupt signal to everybody interested.
In other words pretty standard memory fence behavior.
When you now have a second queue which depends on work of the first one
you look at the memory location and do a compare. If you depend on the
third submission you just wait for the value to be >3 and are done.
Regards,
Christian.
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 ...
Yeah, going to keep you in CC when we start to code and review user fences.
Awesome, thanks Christian. Appreciate it. :)
Cheers,
Daniel
