[PATCH] gpu/docs: Clarify what userspace means for gl

[Daniel Vetter]

Clear rules avoid arguing.

Note that this just aims to document current expectations. If that
shifts (e.g. because gl isn't the main api anymore, replaced by vk),
then we need to update this text.

I think it'd be good to have an equally solid list on the kms side.
But kms is much more meant to be a standard, and the list of userspace
projects we've accepted in the past is constantly shifting and
adjusting. So I figured I'll leave that as an exercise for later on.

v2: Try to clarify that we don't want a mesa driver just for mesa's
sake, and more clearly exclude anything that just doesn't make sense
technically.  Example would be a compute driver that makes sense to be
merged into drm (for kernel side code-sharing), but where the intended
use is some single-source CUDA-style compute without ever bothering
about any of the 3D/rendering side baggage that comes with gl/vk.

v3: Drop vulkan for now, the situation there isn't as obviously
clear-cut as on the gl side, and I don't want to tank this idea on a
hot discussion about vk and mesa. Plus I think once we have 1-2 more
vk drivers in mesa the situation on the vk side is clear-cut too, and
we can do a follow-up patch to add vk to the list where we expect the
userspace to be in upstream mesa. That's would give nice precedence to
make it clear that this isn't cast in stone, but meant to reflect
reality and should be adjusted as needed.

v4: Fix typo.

v5: Add a note to the commit message that this text needs to be
updated when the situation changes.

v6: Add a sentence why mesa will give the most meaningful review on gl
stuff - it's a very active project with lots of developers.

Acked-by: Dave Airlie <airlied at gmail.com> (v4)
Acked-by: Eric Anholt <eric at anholt.net> (v4)
Acked-by: Alex Deucher <alexdeucher at gmail.com> (v5)
Acked-by: Sean Paul <sean at poorly.run> (v5)
Acked-by: Kenneth Graunke <kenneth at whitecape.org> (v5)
Acked-by: Karol Herbst <karolherbst at gmail.com> (v5)
Acked-by: Rob Clark <robdclark at gmail.com>
Acked-by: Jérôme Glisse <jglisse at redhat.com>
Acked-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
Acked-by: Ben Skeggs <skeggs at redhat.com>
Cc: Dave Airlie <airlied at gmail.com>
Cc: Eric Anholt <eric at anholt.net>
Cc: Alex Deucher <alexdeucher at gmail.com>
Cc: Sean Paul <sean at poorly.run>
Cc: Kenneth Graunke <kenneth at whitecape.org>
Cc: Karol Herbst <karolherbst at gmail.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: Jérôme Glisse <jglisse at redhat.com>
Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
Cc: Ben Skeggs <skeggs at redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
I chatted with a pile of people in private, and there's clearly some
solid support for this. But there's also some big concerns brought up
by other people. The main one summed up is "what if everyone just
ships vk, with a generic gl-on-vk like ANGLE?", but there's other
concerns too.

So all together I think this doesn't clear the bar of (almost)
unanimous support which we need to make documentation actually help
with clarifying what's expected. And if/when someone comes up with a
more creative userspace approach for gl/vk we'll need to figure this
all out with the time honored tradition of having a few massive
threads on dri-devel :-)

Hence this is more fyi as a guidance I guess, not a strict&hard rule.
And I don't plan on merging this.

Cheers, Daniel
---
 Documentation/gpu/drm-uapi.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index c9fd23efd957..0f767cfd5db6 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -105,6 +105,31 @@ is already rather painful for the DRM subsystem, with multiple different uAPIs
 for the same thing co-existing. If we add a few more complete mistakes into the
 mix every year it would be entirely unmanageable.
 
+Below some clarifications what this means for specific areas in DRM.
+
+Compute&Rendering Userspace
+---------------------------
+
+Userspace API for enabling compute and rendering blocks which are capable of at
+least supporting one of the OpenGL or OpenGL ES standards from Khronos need to
+be enabled in the upstream `Mesa3D project<https://www.mesa3d.org/>`.
+
+Mesa3D is the canonical upstream for these areas because it is a fully
+compliant, performant and cross-vendor implementation that supports all kernel
+drivers in DRM. It is also an active project with plenty of developers who
+can perform meaningful review. It is therefore the best platform to validate
+userspace API and especially make sure that cross-vendor interoperation is
+assured.
+
+Other userspace is only admissible if exposing a given feature through OpenGL or
+OpenGL ES would result in a technically unsound design, incomplete driver or
+an implementation which isn't useful in real world usage.
+
+Other areas, like media codec, which Mesa3D supports for just some drivers, but
+isn't the clear universal choice, are excluded from this policy. Driver teams
+are still encourage to aim for shared, cross-vendor infrastructure in userspace
+as much as possible.
+
 .. _drm_render_node:
 
 Render nodes
-- 
2.20.1