[Mesa-dev] Allocator Nouveau driver, Mesa EXT_external_objects, and DRM metadata import interfaces

[Miguel Angel Vico]

Hi all,

As many of you already know, I've been working with James Jones on the
Generic Device Allocator project lately. He started a discussion thread
some weeks ago seeking feedback on the current prototype of the library
and advice on how to move all this forward, from a prototype stage to
production. For further reference, see:

   https://lists.freedesktop.org/archives/mesa-dev/2017-November/177632.html

From the thread above, we came up with very interesting high level
design ideas for one of the currently missing parts in the library:
Usage transitions. That's something I'll personally work on during the
following weeks.


In the meantime, I've been working on putting together an open source
implementation of the allocator mechanisms using the Nouveau driver for
all to be able to play with.

Below I'm seeking feedback on a bunch of changes I had to make to
different components of the graphics stack:

** Allocator **

  An allocator driver implementation on top of Nouveau. The current
  implementation only handles pitch linear layouts, but that's enough
  to have the kmscube port working using the allocator and Nouveau
  drivers.

  You can pull these changes from

      https://github.com/mvicomoya/allocator/tree/wip/mvicomoya/nouveau-driver

** Mesa **

  James's kmscube port to use the allocator relies on the
  EXT_external_objects extension to import allocator allocations to
  OpenGL as a texture object. However, the Nouveau implementation of
  these mechanisms is missing in Mesa, so I went ahead and added them.

  You can pull these changes from

      https://github.com/mvicomoya/mesa/tree/wip/mvicomoya/EXT_external_objects-nouveau

  Also, James's kmscube port uses the NVX_unix_allocator_import
  extension to attach allocator metadata to texture objects so the
  driver knows how to deal with the imported memory.

  Note that there isn't a formal spec for this extension yet. For now,
  it just serves as an experimental mechanism to import allocator
  memory in OpenGL, and attach metadata to texture objects.

  You can pull these changes (written on top of the above) from:

      https://github.com/mvicomoya/mesa/tree/wip/mvicomoya/NVX_unix_allocator_import

** kmscube **

  Mostly minor fixes and improvements on top of James's port to use the
  allocator. Main thing is the allocator initialization path will use
  EGL_MESA_platform_surfaceless if EGLDevice platform isn't supported
  by the underlying EGL implementation.

  You can pull these changes from:

      https://github.com/mvicomoya/kmscube/tree/wip/mvicomoya/allocator-nouveau


With all the above you should be able to get kmscube working using the
allocator on top of the Nouveau driver.


Another of the missing pieces before we can move this to production is
importing allocations to DRM FB objects. This is probably one of the
most sensitive parts of the project as it requires modification/addition
of kernel driver interfaces.

At XDC2017, James had several hallway conversations with several people
about this, all having different opinions. I'd like to take this
opportunity to also start a discussion about what's the best option to
create a path to get allocator allocations added as DRM FB objects.

These are the few options we've considered to start with:

  A) Have vendor-private ioctls to set properties on GEM objects that
     are inherited by the FB objects. This is how our (NVIDIA) desktop
     DRM driver currently works. This would require every vendor to add
     their own ioctl to process allocator metadata, but the metadata is
     actually a vendor-agnostic object more like DRM modifiers. We'd
     like to come up with a vendor-agnostic solutions that can be
     integrated to core DRM.

  B) Add a new drmModeAddFBWithMetadata() command that takes allocator
     metadata blobs for each plane of the FB. Some people in the
     community have mentioned this is their preferred design. This,
     however, means we'd have to go through the exercise of adding
     another metadata mechanism to the whole graphics stack.

  C) Shove allocator metadata into DRM by defining it to be a separate
     plane in the image, and using the existing DRM modifiers mechanism
     to indicate there is another plane for each "real" plane added. It
     isn't clear how this scales to surfaces that already need several
     planes, but there are some people that see this as the only way
     forward. Also, we would have to create a separate GEM buffer for
     the metadatada itself, which seems excessive.

We personally like option (B) better, and have already started to
prototype the new path (which is actually very similar to the
drmModeAddFB2() one). You can take a look at the new interfaces here:

    https://github.com/mvicomoya/linux/tree/wip/mvicomoya/drm_addfb_with_metadata__4.14-rc8

There may be other options that haven't been explored yet that could be
a better choice than the above, so any suggestion will be greatly
appreciated.


Thanks,

-- 
Miguel