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@xxxxxxxxx> Cc: Oded Gabbay <oded.gabbay@xxxxxxxxx> Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxx> Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxx> Cc: Eric Anholt <eric@xxxxxxxxxx> Cc: Thomas Hellstrom <thellstrom@xxxxxxxxxx> Cc: Sinclair Yeh <syeh@xxxxxxxxxx> Cc: Lucas Stach <l.stach@xxxxxxxxxxxxxx> Cc: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxx> Cc: Mark Yao <mark.yao@xxxxxxxxxxxxxx> Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> Cc: Ben Skeggs <bskeggs@xxxxxxxxxx> Cc: Rob Clark <robdclark@xxxxxxxxx> Cc: CK Hu <ck.hu@xxxxxxxxxxxx> Cc: Xinliang Liu <z.liuxinliang@xxxxxxxxxxxxx> Cc: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx> Cc: Stefan Agner <stefan@xxxxxxxx> Cc: Inki Dae <inki.dae@xxxxxxxxxxx> Cc: Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx> Cc: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx> Cc: Thierry Reding <thierry.reding@xxxxxxxxx> Cc: Christian König <christian.koenig@xxxxxxx> Cc: Alex Deucher <alexander.deucher@xxxxxxx> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx> Cc: Brian Starkey <brian.starkey@xxxxxxx> Cc: Liviu Dudau <liviu.dudau@xxxxxxx> Cc: Alexey Brodkin <abrodkin@xxxxxxxxxxxx> Acked-by: Dave Airlie <airlied@xxxxxxxxx> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> --- 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
