From 0d42204f374380b6334de7dd2fe7e7d795250290 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Tue, 23 Aug 2016 14:54:48 +0200 Subject: [PATCH] drm/doc: Document uapi requirements in DRM MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Cc: Oded Gabbay Cc: Russell King Cc: Tomi Valkeinen Cc: Eric Anholt Cc: Thomas Hellstrom Cc: Sinclair Yeh Cc: Lucas Stach Cc: Benjamin Gaignard Cc: Mark Yao Cc: Laurent Pinchart Cc: Ben Skeggs Cc: Rob Clark Cc: CK Hu Cc: Xinliang Liu Cc: Philipp Zabel Cc: Stefan Agner Cc: Inki Dae Cc: Maxime Ripard Cc: Boris Brezillon Cc: Jani Nikula Cc: Daniel Vetter Cc: Thierry Reding Cc: Christian König Cc: Alex Deucher Cc: Gerd Hoffmann Cc: Brian Starkey Cc: Liviu Dudau Cc: Alexey Brodkin Acked-by: Dave Airlie Reviewed-by: Rob Clark Reviewed-by: Christian König Reviewed-by: Eric Anholt Reviewed-by: Jani Nikula Signed-off-by: Daniel Vetter Signed-off-by: Dave Airlie --- 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 9487693..12b47c3 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 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 ============ -- 2.7.4