Re: [PATCH v2] Documentation: gpu: Mention the requirements for new properties

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Maxime,

Thank you for the patch.

On Thu, May 20, 2021 at 04:24:35PM +0200, Maxime Ripard wrote:
> New KMS properties come with a bunch of requirements to avoid each
> driver from running their own, inconsistent, set of properties,
> eventually leading to issues like property conflicts, inconsistencies
> between drivers and semantics, etc.
> 
> Let's document what we expect.
> 
> Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
> Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: Alison Wang <alison.wang@nxp.com>
> Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> Cc: Andrew Jeffery <andrew@aj.id.au>
> Cc: Andrzej Hajda <a.hajda@samsung.com>
> Cc: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
> Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
> Cc: Ben Skeggs <bskeggs@redhat.com>
> Cc: Boris Brezillon <bbrezillon@kernel.org>
> Cc: Brian Starkey <brian.starkey@arm.com>
> Cc: Chen Feng <puck.chen@hisilicon.com>
> Cc: Chen-Yu Tsai <wens@csie.org>
> Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
> Cc: Edmund Dea <edmund.j.dea@intel.com>
> Cc: Eric Anholt <eric@anholt.net>
> Cc: Fabio Estevam <festevam@gmail.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
> Cc: Hans de Goede <hdegoede@redhat.com>
> Cc: "Heiko Stübner" <heiko@sntech.de>
> Cc: Huang Rui <ray.huang@amd.com>
> Cc: Hyun Kwon <hyun.kwon@xilinx.com>
> Cc: Inki Dae <inki.dae@samsung.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Jernej Skrabec <jernej.skrabec@siol.net>
> Cc: Jerome Brunet <jbrunet@baylibre.com>
> Cc: Joel Stanley <joel@jms.id.au>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Jonas Karlman <jonas@kwiboo.se>
> Cc: Jonathan Hunter <jonathanh@nvidia.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Joonyoung Shim <jy0922.shim@samsung.com>
> Cc: Jyri Sarha <jyri.sarha@iki.fi>
> Cc: Kevin Hilman <khilman@baylibre.com>
> Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
> Cc: Kyungmin Park <kyungmin.park@samsung.com>
> Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
> Cc: Linus Walleij <linus.walleij@linaro.org>
> Cc: Liviu Dudau <liviu.dudau@arm.com>
> Cc: Lucas Stach <l.stach@pengutronix.de>
> Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
> Cc: Marek Vasut <marex@denx.de>
> Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Melissa Wen <melissa.srw@gmail.com>
> Cc: Neil Armstrong <narmstrong@baylibre.com>
> Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
> Cc: "Noralf Trønnes" <noralf@tronnes.org>
> Cc: NXP Linux Team <linux-imx@nxp.com>
> Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
> Cc: Paul Cercueil <paul@crapouillou.net>
> Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
> Cc: Philippe Cornu <philippe.cornu@foss.st.com>
> Cc: Philipp Zabel <p.zabel@pengutronix.de>
> Cc: Qiang Yu <yuq825@gmail.com>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Robert Foss <robert.foss@linaro.org>
> Cc: Rob Herring <robh@kernel.org>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Roland Scheidegger <sroland@vmware.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Sam Ravnborg <sam@ravnborg.org>
> Cc: Sandy Huang <hjc@rock-chips.com>
> Cc: Sascha Hauer <s.hauer@pengutronix.de>
> Cc: Sean Paul <sean@poorly.run>
> Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
> Cc: Shawn Guo <shawnguo@kernel.org>
> Cc: Stefan Agner <stefan@agner.ch>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Thierry Reding <thierry.reding@gmail.com>
> Cc: Tian Tao <tiantao6@hisilicon.com>
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Tomi Valkeinen <tomba@kernel.org>
> Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
> Cc: Xinliang Liu <xinliang.liu@linaro.org>
> Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
> Cc: Yannick Fertre <yannick.fertre@foss.st.com>
> Cc: Zack Rusin <zackr@vmware.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> 
> ---
> 
> Changes from v2:
>   - Typos and wording reported by Daniel and Alex
> ---
>  Documentation/gpu/drm-kms.rst | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> index 87e5023e3f55..c28b464dd397 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -463,6 +463,25 @@ KMS Properties
>  This section of the documentation is primarily aimed at user-space developers.
>  For the driver APIs, see the other sections.
>  
> +Requirements
> +------------
> +
> +KMS drivers might need to add extra properties to support new features.
> +Each new property introduced in a driver need to meet a few

s/need/needs/

> +requirements, in addition to the one mentioned above.:

s/above./above/

> +
> +- It must be standardized, with some documentation to describe how the
> +  property can be used.
> +
> +- It must provide a generic helper in the core code to register that
> +  property on the object it attaches to.
> +
> +- Its content must be decoded by the core and provided in the object's
> +  associated state structure. That includes anything drivers might want to
> +  precompute, like :c:type:`struct drm_clip_rect <drm_clip_rect>` for planes.

Does this effectively mean that we completely forbid driver-specific
properties ? While I agree that we should strive for standardization,
there are two issues that worry me. The first one is simple, we may need
to control features that would be very device-specific, and
standardizing properties doesn't seem to make much sense in that case.

The second issue relates to properties that could be applicable to
multiple devices, but for which we have a single driver. Designing a
standard with a single data point usually leads to a bad design. I'm not
sure how to handle this correctly though, as we certainly don't want
this to be taken as an excuse to create driver-specific properties when
generic properties would make sense.

> +- An IGT test must be submitted where reasonable.
> +
>  Property Types and Blob Property Support
>  ----------------------------------------
>  

-- 
Regards,

Laurent Pinchart