[PATCH] drm/doc: Document uapi requirements in DRM

[Daniel Vetter]

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

Cc: Dave Airlie <airlied at gmail.com>
Cc: Oded Gabbay <oded.gabbay at gmail.com>
Cc: Russell King <rmk+kernel at armlinux.org.uk>
Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
Cc: Eric Anholt <eric at anholt.net>
Cc: Thomas Hellstrom <thellstrom at vmware.com>
Cc: Sinclair Yeh <syeh at vmware.com>
Cc: Lucas Stach <l.stach at pengutronix.de>
Cc: Benjamin Gaignard <benjamin.gaignard at linaro.org>
Cc: Mark Yao <mark.yao at rock-chips.com>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: CK Hu <ck.hu at mediatek.com>
Cc: Xinliang Liu <z.liuxinliang at hisilicon.com>
Cc: Philipp Zabel <p.zabel at pengutronix.de>
Cc: Stefan Agner <stefan at agner.ch>
Cc: Inki Dae <inki.dae at samsung.com>
Cc: Maxime Ripard  <maxime.ripard at free-electrons.com>
Cc: Boris Brezillon <boris.brezillon at free-electrons.com>
Cc: Jani Nikula <jani.nikula at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at intel.com>
Cc: Thierry Reding <thierry.reding at gmail.com>
Cc: Christian König <christian.koenig at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: Gerd Hoffmann <kraxel at redhat.com>
Cc: Brian Starkey <brian.starkey at arm.com>
Cc: Liviu Dudau <liviu.dudau at arm.com>
Cc: Alexey Brodkin <abrodkin at synopsys.com>
Acked-by: Dave Airlie <airlied at gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 Documentation/gpu/drm-uapi.rst | 67 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 94876938aef3..a7e3aa27167d 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -36,6 +36,73 @@ Primary Nodes, DRM Master and Authentication
 Open-Source Userspace Requirements
 ==================================
 
+The DRM subsystem has stricter requirements 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 turns 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
+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'd add a few more complete mistakes into the
+mix every year it would be entirely unmanagable.
+
 Render nodes
 ============
 
-- 
2.8.1