[Mesa-dev] GBM and the Device Memory Allocator Proposals

James Jones jajones at nvidia.com
Wed Dec 6 06:27:41 UTC 2017

On 11/30/2017 12:06 PM, Lyude Paul wrote:
> On Thu, 2017-11-30 at 13:20 -0500, Rob Clark wrote:
>> On Thu, Nov 30, 2017 at 12:59 AM, James Jones <jajones at nvidia.com> wrote:
>>> On 11/29/2017 04:09 PM, Miguel Angel Vico wrote:
>>>> On Wed, 29 Nov 2017 16:28:15 -0500
>>>> Rob Clark <robdclark at gmail.com> wrote:
>>>>> Do we need to define both in-place and copy transitions?  Ie. what if
>>>>> GPU is still reading a tiled or compressed texture (ie. sampling from
>>>>> previous frame for some reason), but we need to untile/uncompress for
>>>>> display.. of maybe there are some other cases like that we should
>>>>> think about..
>>>>> Maybe you already have some thoughts about that?
>>>> This is the next thing I'll be working on. I haven't given it much
>>>> thought myself so far, but I think James might have had some insights.
>>>> I'll read through some of his notes to double-check.
>>> A couple of notes on usage transitions:
>>> While chatting about transitions, a few assertions were made by others
>>> that
>>> I've come to accept, despite the fact that they reduce the generality of
>>> the
>>> allocator mechanisms:
>>> -GPUs are the only things that actually need usage transitions as far as I
>>> know thus far.  Other engines either share the GPU representations of
>>> data,
>>> or use more limited representations; the latter being the reason non-GPU
>>> usage transitions are a useful thing.
>>> -It's reasonable to assume that a GPU is required to perform a usage
>>> transition.  This follows from the above postulate.  If only GPUs are
>>> using
>>> more advanced representations, you don't need any transitions unless you
>>> have a GPU available.
>> This seems reasonable.  I can't think of any non-gpu related case
>> where you would need a transition, other than perhaps cache flush/inv.
>>>  From that, I derived the rough API proposal for transitions presented on
>>> my
>>> XDC 2017 slides.  Transition "metadata" is queried from the allocator
>>> given
>>> a pair of usages (which may refer to more than one device), but the
>>> realization of the transition is left to existing GPU APIs.  I think I put
>>> Vulkan-like pseudo-code in the slides, but the GL external objects
>>> extensions (GL_EXT_memory_object and GL_EXT_semaphore) would work as well.
>> I haven't quite wrapped my head around how this would work in the
>> cross-device case.. I mean from the API standpoint for the user, it
>> seems straightforward enough.  Just not sure how to implement that and
>> what the driver interface would look like.
>> I guess we need a capability-conversion (?).. I mean take for example
>> the the fb compression capability from your slide #12[1].  If we knew
>> there was an available transition to go from "Dev2 FB compression" to
>> "normal", then we could have allowed the "Dev2 FB compression" valid
>> set?
>> [1] https://www.x.org/wiki/Events/XDC2017/jones_allocator.pdf
>>> Regarding in-place Vs. copy: To me a transition is something that happens
>>> in-place, at least semantically.  If you need to make copies, that's a
>>> format conversion blit not a transition, and graphics APIs are already
>>> capable of expressing that without any special transitions or help from
>>> the
>>> allocator.  However, I understand some chipsets perform transitions using
>>> something that looks kind of like a blit using on-chip caches and
>>> constrained usage semantics.  There's probably some work to do to see
>>> whether those need to be accommodated as conversion blits or usgae
>>> transitions.
>> I guess part of what I was thinking of, is what happens if the
>> producing device is still reading from the buffer.  For example,
>> viddec -> gpu use case, where the video decoder is also still hanging
>> on to the frame to use as a reference frame to decode future frames?
>> I guess if transition from devA -> devB can be done in parallel with
>> devA still reading the buffer, it isn't a problem.  I guess that
>> limits (non-blit) transitions to decompression and cache op's?  Maybe
>> that is ok..

I don't know of a real case it would be a problem.  Note you can 
transition to multiple usages in the proposed API, so for the video 
decoder example, you would transition from [video decode target] to 
[video decode target, GPU sampler source] for simultaneous texturing and 
reference frame usage.

>>> For our hardware's purposes, transitions are just various levels of
>>> decompression or compression reconfiguration and potentially cache
>>> flushing/invalidation, so our transition metadata will just be some bits
>>> signaling which compression operation is needed, if any.  That's the sort
>>> of
>>> operation I modeled the API around, so if things are much more exotic than
>>> that for others, it will probably require some adjustments.
>> [snip]
>>> Gralloc-on-$new_thing, as well as hwcomposer-on-$new_thing is one of my
>>> primary goals.  However, it's a pretty heavy thing to prototype.  If
>>> someone
>>> has the time though, I think it would be a great experiment.  It would
>>> help
>>> flesh out the paltry list of usages, constraints, and capabilities in the
>>> existing prototype codebase.  The kmscube example really should have added
>>> at least a "render" usage, but I got lazy and just re-used texture for
>>> now.
>>> That won't actually work on our HW in all cases, but it's good enough for
>>> kmscube.
>> btw, I did start looking at it.. I guess this gets a bit into the
>> other side of this thread (ie. where/if GBM fits in).  So far I don't
>> think mesa has EGL_EXT_device_base, but I'm guessing that is part of
> There is wip from ajax to add support for this actually, although it didn't do
> much correctly the last time I played with it:
> https://cgit.freedesktop.org/~ajax/mesa/log/?h=egl-ext-device
> I was also hoping to write a simple egl device testing extension that lists
> devices and that sort of stuff, as well made an entire seperate repo to start
> holding glxinfo, eglinfo, and group said tool in with that. Haven't actually
> written any code for this yet though

Yes, or there's also this:


Which combined with:


provides a semantic alternative method to instantiate a platform-less 
EGLDisplay on an EGLDevice.  It's functionally equivalent to 
EGL_EXT_platform_device, but some people find it more palatable.  I'm 
roughly indifferent between the two at this point, but I slightly prefer 
EGL_EXT_platform_device just because we already have it implemented and 
have a bunch of internal code and external customers using it, so we 
have to maintain it anyway.

And just a reminder to avoid opening old wounds: EGLDevice is in no way 
tied to EGLStreams.  EGLDevice is basically just a slightly more 
detailed version of EGL_MESA_platform_surfaceless.


>> what you had in mind as alternative to GBM ;-)
>> BR,
>> -R

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