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

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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@xxxxxxxxxxx>
> Cc: Alexandre Torgue <alexandre.torgue@xxxxxxxxxxx>
> Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> Cc: Alison Wang <alison.wang@xxxxxxx>
> Cc: Alyssa Rosenzweig <alyssa.rosenzweig@xxxxxxxxxxxxx>
> Cc: Andrew Jeffery <andrew@xxxxxxxx>
> Cc: Andrzej Hajda <a.hajda@xxxxxxxxxxx>
> Cc: Anitha Chrisanthus <anitha.chrisanthus@xxxxxxxxx>
> Cc: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxx>
> Cc: Ben Skeggs <bskeggs@xxxxxxxxxx>
> Cc: Boris Brezillon <bbrezillon@xxxxxxxxxx>
> Cc: Brian Starkey <brian.starkey@xxxxxxx>
> Cc: Chen Feng <puck.chen@xxxxxxxxxxxxx>
> Cc: Chen-Yu Tsai <wens@xxxxxxxx>
> Cc: Christian Gmeiner <christian.gmeiner@xxxxxxxxx>
> Cc: "Christian König" <christian.koenig@xxxxxxx>
> Cc: Chun-Kuang Hu <chunkuang.hu@xxxxxxxxxx>
> Cc: Edmund Dea <edmund.j.dea@xxxxxxxxx>
> Cc: Eric Anholt <eric@xxxxxxxxxx>
> Cc: Fabio Estevam <festevam@xxxxxxxxx>
> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx>
> Cc: Hans de Goede <hdegoede@xxxxxxxxxx>
> Cc: "Heiko Stübner" <heiko@xxxxxxxxx>
> Cc: Huang Rui <ray.huang@xxxxxxx>
> Cc: Hyun Kwon <hyun.kwon@xxxxxxxxxx>
> Cc: Inki Dae <inki.dae@xxxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Jernej Skrabec <jernej.skrabec@xxxxxxxx>
> Cc: Jerome Brunet <jbrunet@xxxxxxxxxxxx>
> Cc: Joel Stanley <joel@xxxxxxxxx>
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Jonas Karlman <jonas@xxxxxxxxx>
> Cc: Jonathan Hunter <jonathanh@xxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Joonyoung Shim <jy0922.shim@xxxxxxxxxxx>
> Cc: Jyri Sarha <jyri.sarha@xxxxxx>
> Cc: Kevin Hilman <khilman@xxxxxxxxxxxx>
> Cc: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxx>
> Cc: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
> Cc: Laurent Pinchart <Laurent.pinchart@xxxxxxxxxxxxxxxx>
> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
> Cc: Liviu Dudau <liviu.dudau@xxxxxxx>
> Cc: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> Cc: Ludovic Desroches <ludovic.desroches@xxxxxxxxxxxxx>
> Cc: Marek Vasut <marex@xxxxxxx>
> Cc: Martin Blumenstingl <martin.blumenstingl@xxxxxxxxxxxxxx>
> Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> Cc: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Melissa Wen <melissa.srw@xxxxxxxxx>
> Cc: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
> Cc: Nicolas Ferre <nicolas.ferre@xxxxxxxxxxxxx>
> Cc: "Noralf Trønnes" <noralf@xxxxxxxxxxx>
> Cc: NXP Linux Team <linux-imx@xxxxxxx>
> Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> Cc: Patrik Jakobsson <patrik.r.jakobsson@xxxxxxxxx>
> Cc: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> Cc: Pengutronix Kernel Team <kernel@xxxxxxxxxxxxxx>
> Cc: Philippe Cornu <philippe.cornu@xxxxxxxxxxx>
> Cc: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> Cc: Qiang Yu <yuq825@xxxxxxxxx>
> Cc: Rob Clark <robdclark@xxxxxxxxx>
> Cc: Robert Foss <robert.foss@xxxxxxxxxx>
> Cc: Rob Herring <robh@xxxxxxxxxx>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Roland Scheidegger <sroland@xxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Cc: Sandy Huang <hjc@xxxxxxxxxxxxxx>
> Cc: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
> Cc: Sean Paul <sean@xxxxxxxxxx>
> Cc: Seung-Woo Kim <sw0312.kim@xxxxxxxxxxx>
> Cc: Shawn Guo <shawnguo@xxxxxxxxxx>
> Cc: Stefan Agner <stefan@xxxxxxxx>
> Cc: Steven Price <steven.price@xxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: Thierry Reding <thierry.reding@xxxxxxxxx>
> Cc: Tian Tao <tiantao6@xxxxxxxxxxxxx>
> Cc: Tomeu Vizoso <tomeu.vizoso@xxxxxxxxxxxxx>
> Cc: Tomi Valkeinen <tomba@xxxxxxxxxx>
> Cc: VMware Graphics <linux-graphics-maintainer@xxxxxxxxxx>
> Cc: Xinliang Liu <xinliang.liu@xxxxxxxxxx>
> Cc: Xinwei Kong <kong.kongxinwei@xxxxxxxxxxxxx>
> Cc: Yannick Fertre <yannick.fertre@xxxxxxxxxxx>
> Cc: Zack Rusin <zackr@xxxxxxxxxx>
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Signed-off-by: Maxime Ripard <maxime@xxxxxxxxxx>
> 
> ---
> 
> 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



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