On Tue, Aug 23, 2016 at 02:54:48PM +0200, Daniel Vetter 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.

Thanks for re-iterating this. Although some of us from ARM are new to the KMS world, there have been previous engagements with the community from others in the company.

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:

I'm guessing the topic was mainly about GPUs at that time. ARM's current upstreaming efforts are limited to display engines and supporting the open-source userspace with the mainline driver.

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

Reviewed-by: Liviu Dudau Liviu.Dudau@arm.com

Best regards, Liviu

Cc: Alexey Brodkin abrodkin@synopsys.com Acked-by: Dave Airlie airlied@gmail.com Reviewed-by: Rob Clark robdclark@gmail.com Reviewed-by: Christian König christian.koenig@amd.com Reviewed-by: Eric Anholt eric@anholt.net Reviewed-by: Jani Nikula jani.nikula@intel.com Signed-off-by: Daniel Vetter daniel.vetter@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..12b47c30fe2e 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

-- 2.9.3