On Mon, 22 Aug 2016, Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > 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. > > 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> > Reviewed-by: Rob Clark <robdclark@xxxxxxxxx> > Reviewed-by: Christian König <christian.koenig@xxxxxxx> > Reviewed-by: Eric Anholt <eric@xxxxxxxxxx> > 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..747b51f8c422 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. Nitpick, stricter requirements than what? I know what you mean, but the comparative begs for the thing to compare against. Otherwise, Reviewed-by: Jani Nikula <jani.nikula@xxxxxxxxx> > + > +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 > ============ -- Jani Nikula, Intel Open Source Technology Center _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
