On 06.12.2017 14:25, Rob Clark wrote:
On Wed, Dec 6, 2017 at 2:07 AM, James Jones <[email protected]> wrote:
Note I also posed the question of whether things like cached (and similarly
compression, since I view compression as roughly an equivalent mechanism to
a cache) in one of the open issues on my XDC 2017 slides because of this
very problem of over-pruning it causes. It's on slide 15, as "No
device-local capabilities". You'll have to listen to my coverage of it in
the recorded presentation for that slide to make any sense, but it's the
same thing Nicolai has laid out here.
As I continued working through our prototype driver support, I found I
didn't actually need to include cached or compressed as capabilities: The
GPU just applies them as needed and the usage transitions make it
transparent to the non-GPU engines. That does mean the GPU driver currently
needs to be the one to realize the allocation from the capability set to get
optimal behavior. We could fix that by reworking our driver though. At
this point, not including device-local properties like on-device caching in
capabilities seems like the right solution to me. I'm curious whether this
applies universally though, or if other hardware doesn't fit the
"compression and stuff all behaves like a cache" idiom.
Possibly a SoC(ish) type device which has a "system" cache that some
but not all devices fall into. I *think* the intel chips w/ EDRAM
might fall into this category. I know the idea has come up elsewhere,
although not sure if anything like that ended up in production. It
seems like something we'd at least want to have an idea how to deal
with, even if it isn't used for device internal caches.
Not sure if similar situation could come up w/ discrete GPU and video
decode/encode engines on the same die?
It definitely could. Our GPUs currently don't have shared caches between
gfx and video engines, but moving more and more clients under a shared
L2 cache has been a theme over the last few generations. I doubt that's
going to happen for the video engines any time soon, but you never know.
I don't think we really need caches as a capability for our current
GPUs, but it may change, and in any case, we do want compression as a
capability.
[snip]
I think I like the idea of having transitions being part of the
per-device/engine cap sets, so that such information can be used upon
merging to know which capabilities may remain or have to be dropped.
I think James's proposal for usage transitions was intended to work
with flows like:
1. App gets GPU caps for RENDER usage
2. App allocates GPU memory using a layout from (1)
3. App now decides it wants use the buffer for SCANOUT
4. App queries usage transition metadata from RENDER to SCANOUT,
given the current memory layout.
5. Do the transition and hand the buffer off to display
No, all usages the app intends to transition to must be specified up front
when initially querying caps in the model I assumed. The app then specifies
some subset (up to the full set) of the specified usages as a src and dst
when querying transition metadata.
The problem I see with this is that it isn't guaranteed that there will
be a chain of transitions for the buffer to be usable by display.
hmm, I guess if a buffer *can* be shared across all uses, there by
definition has to be a chain of transitions to go from any
usage+device to any other usage+device.
Possibly a separate step to query transitions avoids solving for every
possible transition when merging the caps set.. although until you do
that query I don't think you know the resulting merged caps set is
valid.
Maybe in practice for every cap FOO there exists a FOO->null (or
FOO->generic if you prefer) transition, ie. compressed->uncompressed,
cached->clean, etc. I suppose that makes the problem easier to solve.
It really would, to the extent that I would prefer if we could bake it
into the system as an assumption.
I have my doubts about how to manage calculating transitions cleanly at
all without it. The metadata stuff is very vague to me.
I hadn't thought hard about it, but my initial thoughts were that it would
be required that the driver support transitioning to any single usage given
the capabilities returned. However, transitioning to multiple usages (E.g.,
to simultaneously rendering and scanning out) could fail to produce a valid
transition, in which case the app would have to fall back to a copy in that
case, or avoid that simultaneous usage combination in some other way.
Adding transition metadata to the original capability sets, and using
that information when merging could give us a compatible memory layout
that would be usable by both GPU and display.
I'll look into extending the current merging logic to also take into
account transitions.
Yes, it'll be good to see whether this can be made to work. I agree Rob's
example outcomes above are ideal, but it's not clear to me how to code up
such an algorithm. This also all seems unnecessary if "device local"
capabilities aren't needed, as posited above.
Probably things like device private caches, and transitions between
usages on the same device(+driver?[1]) could be left out. For the
cache case, if you have a cache shared between some but not all
devices, that problem looks to me to be basically the same problem as
compressed buffers when some but not all devices support a particular
compression scheme.
[1] can we assume magic under the hood for vk and gl interop with
drivers from same vendor on same device?
In my book, the fewer assumptions we have to make for that, the better.
Cheers,
Nicolai
--
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.
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