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

[Rob Clark]

On Sat, Nov 25, 2017 at 12:46 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On November 24, 2017 09:29:43 Rob Clark <robdclark at gmail.com> wrote:
>>
>>
>> On Mon, Nov 20, 2017 at 8:11 PM, James Jones <jajones at nvidia.com> wrote:
>>>
>>> As many here know at this point, I've been working on solving issues
>>> related
>>> to DMA-capable memory allocation for various devices for some time now.
>>> I'd
>>> like to take this opportunity to apologize for the way I handled the EGL
>>> stream proposals.  I understand now that the development process followed
>>> there was unacceptable to the community and likely offended many great
>>> engineers.
>>>
>>> Moving forward, I attempted to reboot talks in a more constructive manner
>>> with the generic allocator library proposals & discussion forum at XDC
>>> 2016.
>>> Some great design ideas came out of that, and I've since been prototyping
>>> some code to prove them out before bringing them back as official
>>> proposals.
>>> Again, I understand some people are growing concerned that I've been
>>> doing
>>> this off on the side in a github project that has primarily NVIDIA
>>> contributors.  My goal was only to avoid wasting everyone's time with
>>> unproven ideas.  The intent was never to dump the prototype code as-is on
>>> the community and presume acceptance. It's just a public research
>>> project.
>>>
>>> Now the prototyping is nearing completion, and I'd like to renew
>>> discussion
>>> on whether and how the new mechanisms can be integrated with the Linux
>>> graphics stack.
>>>
>>> I'd be interested to know if more work is needed to demonstrate the
>>> usefulness of the new mechanisms, or whether people think they have value
>>> at
>>> this point.
>>>
>>> After talking with people on the hallway track at XDC this year, I've
>>> heard
>>> several proposals for incorporating the new mechanisms:
>>>
>>> -Include ideas from the generic allocator design into GBM.  This could
>>> take
>>> the form of designing a "GBM 2.0" API, or incrementally adding to the
>>> existing GBM API.
>>>
>>> -Develop a library to replace GBM.  The allocator prototype code could be
>>> massaged into something production worthy to jump start this process.
>>>
>>> -Develop a library that sits beside or on top of GBM, using GBM for
>>> low-level graphics buffer allocation, while supporting non-graphics
>>> kernel
>>> APIs directly.  The additional cross-device negotiation and sorting of
>>> capabilities would be handled in this slightly higher-level API before
>>> handing off to GBM and other APIs for actual allocation somehow.
>>
>>
>> tbh, I kinda see GBM and $new_thing sitting side by side.. GBM is
>> still the "winsys" for running on "bare metal" (ie. kms).  And we
>> don't want to saddle $new_thing with aspects of that, but rather have
>> it focus on being the thing that in multiple-"device"[1] scenarious
>> figures out what sort of buffer can be allocated by who for sharing.
>> Ie $new_thing should really not care about winsys level things like
>> cursors or surfaces.. only buffers.
>>
>> The mesa implementation of $new_thing could sit on top of GBM,
>> although it could also just sit on top of the same internal APIs that
>> GBM sits on top of.  That is an implementation detail.  It could be
>> that GBM grows an API to return an instance of $new_thing for
>> use-cases that involve sharing a buffer with the GPU.  Or perhaps that
>> is exposed via some sort of EGL extension.  (We probably also need a
>> way to get an instance from libdrm (?) for display-only KMS drivers,
>> to cover cases like etnaviv sharing a buffer with a separate display
>> driver.)
>>
>> [1] where "devices" could be multiple GPUs or multiple APIs for one or
>> more GPUs, but also includes non-GPU devices like camera, video
>> decoder, "image processor" (which may or may not be part of camera),
>> etc, etc
>
>
> I'm not quite some sure what I think about this.  I think I would like to
> see $new_thing at least replace the guts of GBM. Whether GBM becomes a
> wrapper around $new_thing or $new_thing implements the GBM API, I'm not
> sure.  What I don't think I want is to see GBM development continuing on
> it's own so we have two competing solutions.

I don't really view them as competing.. there is *some* overlap, ie.
allocating a buffer.. but even if you are using GBM w/out $new_thing
you could allocate a buffer externally and import it.  I don't see
$new_thing as that much different from GBM PoV.

But things like surfaces (aka swap chains) seem a bit out of place
when you are thinking about implementing $new_thing for non-gpu
devices.  Plus EGL<->GBM tie-ins that seem out of place when talking
about a (for ex.) camera.  I kinda don't want to throw out the baby
with the bathwater here.

*maybe* GBM could be partially implemented on top of $new_thing.  I
don't quite see how that would work.  Possibly we could deprecate
parts of GBM that are no longer needed?  idk..  Either way, I fully
expect that GBM and mesa's implementation of $new_thing could perhaps
sit on to of some of the same set of internal APIs.  The public
interface can be decoupled from the internal implementation.

> I *think* I like the idea of having $new_thing implement GBM as a deprecated
> legacy API.  Whether that means we start by pulling GBM out into it's own
> project or we start over, I don't know.  My feeling is that the current
> dri_interface is *not* what we want which is why starting with GBM makes me
> nervous.

/me expects if we pull GBM out of mesa, the interface between GBM and
mesa (or other GL drivers) is 'struct gbm_device'.. so "GBM the
project" is just a thin shim plus some 'struct gbm_device' versioning.

BR,
-R

> I need to go read through your code before I can provide a stronger or more
> nuanced opinion.  That's not going to happen before the end of the year.
>
>>> -I have also heard some general comments that regardless of the
>>> relationship
>>> between GBM and the new allocator mechanisms, it might be time to move
>>> GBM
>>> out of Mesa so it can be developed as a stand-alone project.  I'd be
>>> interested what others think about that, as it would be something worth
>>> coordinating with any other new development based on or inside of GBM.
>>
>>
>> +1
>>
>> We already have at least a couple different non-mesa implementations
>> of GBM (which afaict tend to lag behind mesa's GBM and cause
>> headaches).
>>
>> The extracted part probably isn't much more than a header and shim.
>> But probably does need to grow some versioning for the backend to know
>> if, for example, gbm->bo_map() is supported.. at least it could
>> provide stubs that return an error, rather than having link-time fail
>> if building something w/ $vendor's old gbm implementation.
>>
>>> And of course I'm open to any other ideas for integration.  Beyond just
>>> where this code would live, there is much to debate about the mechanisms
>>> themselves and all the implementation details.  I was just hoping to kick
>>> things off with something high level to start.
>>
>>
>> My $0.02, is that the place where devel happens and place to go for
>> releases could be different.  Either way, I would like to see git tree
>> for tagged release versions live on fd.o and use the common release
>> process[2] for generating/uploading release tarballs that distros can
>> use.
>
>
> Agreed.  I think fd.o is the right place for such a project to live.  We can
> have mirrors on GitHub and other places but fd.o is where Linux graphics
> stack development currently happens.
>
>> [2] https://cgit.freedesktop.org/xorg/util/modular/tree/release.sh
>>
>>> For reference, the code Miguel and I have been developing for the
>>> prototype
>>> is here:
>>>
>>>    https://github.com/cubanismo/allocator
>>>
>>> And we've posted a port of kmscube that uses the new interfaces as a
>>> demonstration here:
>>>
>>>    https://github.com/cubanismo/kmscube
>>>
>>> There are still some proposed mechanisms (usage transitions mainly) that
>>> aren't prototyped, but I think it makes sense to start discussing
>>> integration while prototyping continues.
>>
>>
>> btw, I think a nice end goal would be a gralloc implementation using
>> this new API for sharing buffers in various use-cases.  That could
>> mean converting gbm-gralloc, or perhaps it means something new.
>>
>> AOSP has support for mesa + upstream kernel for some devices which
>> also have upstream camera and/or video decoder in addition to just
>> GPU.. and this is where you start hitting the limits of a GBM based
>> gralloc.  In a lot of way, I view $new_thing as what gralloc *should*
>> have been, but at least it provides a way to implement a generic
>> gralloc.
>
>
> +100
>
>
>> Maybe that is getting a step ahead, there is a lot we can prototype
>> with kmscube.  But gralloc gets us into interesting real-world
>> use-cases that involve more than just GPUs.  Possibly this would be
>> something that linaro might be interested in getting involved with?
>>
>> BR,
>> -R
>>
>>> In addition, I'd like to note that NVIDIA is committed to providing open
>>> source driver implementations of these mechanisms for our hardware, in
>>> addition to support in our proprietary drivers.  In other words, wherever
>>> modifications to the nouveau kernel & userspace drivers are needed to
>>> implement the improved allocator mechanisms, we'll be contributing
>>> patches
>>> if no one beats us to it.
>>>
>>> Thanks in advance for any feedback!
>>>
>>> -James Jones
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
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>
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