drm/doc: Document uapi requirements in DRM

Everyone knows them, except all the new folks joining from the ARM
side haven't lived through all the pain of the past years and are
entirely surprised when I raise this. Definitely time to document
this.

Last time this was a big discussion was about 6 years ago, when qcom
tried to land a kernel driver without userspace. Dave Airlie made the
rules really clear:

http://airlied.livejournal.com/73115.html

This write-up here is essentially what I've put into a presentation a
while ago, which was also reviewed by Dave:

http://blog.ffwll.ch/2015/05/gfx-kernel-upstreaming-requirements.html

v2: Fix typos Eric&Rob spotted.

v3: Nitpick from Jani.

Cc: Dave Airlie <[e-mail]>
Cc: Oded Gabbay <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: Eric Anholt <[e-mail]>
Cc: Thomas Hellstrom <[e-mail]>
Cc: Sinclair Yeh <[e-mail]>
Cc: Lucas Stach <[e-mail]>
Cc: Benjamin Gaignard <[e-mail]>
Cc: Mark Yao <[e-mail]>
Cc: Laurent Pinchart <[e-mail]>
Cc: Ben Skeggs <[e-mail]>
Cc: Rob Clark <[e-mail]>
Cc: CK Hu <[e-mail]>
Cc: Xinliang Liu <[e-mail]>
Cc: Philipp Zabel <[e-mail]>
Cc: Stefan Agner <[e-mail]>
Cc: Inki Dae <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Boris Brezillon <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: Gerd Hoffmann <[e-mail]>
Cc: Brian Starkey <[e-mail]>
Cc: Liviu Dudau <[e-mail]>
Cc: Alexey Brodkin <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
Reviewed-by: Rob Clark <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: Eric Anholt <[e-mail]>
Reviewed-by: Jani Nikula <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

Documentation/gpu/drm-uapi.rst

···
 Open-Source Userspace Requirements
 ==================================
 
+The DRM subsystem has stricter requirements than most other kernel subsystems on
+what the userspace side for new uAPI needs to look like. This section here
+explains what exactly those requirements are, and why they exist.
+
+The short summary is that any addition of DRM uAPI requires corresponding
+open-sourced userspace patches, and those patches must be reviewed and ready for
+merging into a suitable and canonical upstream project.
+
+GFX devices (both display and render/GPU side) are really complex bits of
+hardware, with userspace and kernel by necessity having to work together really
+closely.  The interfaces, for rendering and modesetting, must be extremely wide
+and flexible, and therefore it is almost always impossible to precisely define
+them for every possible corner case. This in turn makes it really practically
+infeasible to differentiate between behaviour that's required by userspace, and
+which must not be changed to avoid regressions, and behaviour which is only an
+accidental artifact of the current implementation.
+
+Without access to the full source code of all userspace users that means it
+becomes impossible to change the implementation details, since userspace could
+depend upon the accidental behaviour of the current implementation in minute
+details. And debugging such regressions without access to source code is pretty
+much impossible. As a consequence this means:
+
+- The Linux kernel's "no regression" policy holds in practice only for
+  open-source userspace of the DRM subsystem. DRM developers are perfectly fine
+  if closed-source blob drivers in userspace use the same uAPI as the open
+  drivers, but they must do so in the exact same way as the open drivers.
+  Creative (ab)use of the interfaces will, and in the past routinely has, lead
+  to breakage.
+
+- Any new userspace interface must have an open-source implementation as
+  demonstration vehicle.
+
+The other reason for requiring open-source userspace is uAPI review. Since the
+kernel and userspace parts of a GFX stack must work together so closely, code
+review can only assess whether a new interface achieves its goals by looking at
+both sides. Making sure that the interface indeed covers the use-case fully
+leads to a few additional requirements:
+
+- The open-source userspace must not be a toy/test application, but the real
+  thing. Specifically it needs to handle all the usual error and corner cases.
+  These are often the places where new uAPI falls apart and hence essential to
+  assess the fitness of a proposed interface.
+
+- The userspace side must be fully reviewed and tested to the standards of that
+  userspace project. For e.g. mesa this means piglit testcases and review on the
+  mailing list. This is again to ensure that the new interface actually gets the
+  job done.
+
+- The userspace patches must be against the canonical upstream, not some vendor
+  fork. This is to make sure that no one cheats on the review and testing
+  requirements by doing a quick fork.
+
+- The kernel patch can only be merged after all the above requirements are met,
+  but it **must** be merged **before** the userspace patches land. uAPI always flows
+  from the kernel, doing things the other way round risks divergence of the uAPI
+  definitions and header files.
+
+These are fairly steep requirements, but have grown out from years of shared
+pain and experience with uAPI added hastily, and almost always regretted about
+just as fast. GFX devices change really fast, requiring a paradigm shift and
+entire new set of uAPI interfaces every few years at least. Together with the
+Linux kernel's guarantee to keep existing userspace running for 10+ years this
+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.
+
 Render nodes
 ============