Re: [PATCH v4] Documentation: gpu: Mention the requirements for new properties

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Wed, 16 Jun 2021 16:38:42 +0200
Maxime Ripard <maxime@cerno.tech> 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: Pekka Paalanen <pekka.paalanen@collabora.com>
> 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: Simon Ser <contact@emersion.fr>
> 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 v3:
>   - Roll back to the v2
>   - Add Simon and Pekka in Cc
> 
> Changes from v2:
>   - Take into account the feedback from Laurent and Lidiu to no longer
>     force generic properties, but prefix vendor-specific properties with
>     the vendor name
> 
> Changes from v1:
>   - 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
> +requirements, in addition to the one mentioned above.:
> +
> +- It must be standardized, with some documentation to describe how the
> +  property can be used.

Hi,

I might replace "some" with "full" documentation. Also not only how it
can be used but also what it does.

FYI, some common things that tend to be forgotten IME:
- Spell out exactly the name string for the property in the
  documentation so that it is unambiguous what string userspace should
  look for.
- The same for string names of enum values.
- Explicitly document what each enum value means, do not trust that the
  value name describes it well enough.
- Explain how the property interacts with other, existing properties.

Not sure if these should be written down here or anywhere though.
Interaction with other properties is kind of important.

> +
> +- 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.
> +
> +- An IGT test must be submitted where reasonable.

Would it be too much to replace "where reasonable" with "if it is at
all possible to write a test."?

> +

How about adding the following somewhere?

- The initial state of the property (set during driver initialization)
  must match how the driver+hardware behaved before introducing this
  property. It may be some fixed value or it may be inherited from e.g.
  the firmware that booted the system. How the initial state is
  determined must also be documented, that is, where does it come from.

The initial state must not be called "default", because I want to
reserve the term default for something else if possible: the phrase
"reset everything to defaults", which is a whole another discussion.

How about also saying that fbcon/fbdev must set this property when
taking over? That sounds to be like a common omission leading to funky
KMS state in fbcon. The value fbdev sets it to only needs to make
sense to fbdev, and it does not need to be ~~the initial value~~ nor the
default value. Or are we hoping to kill fbcon in favor of a userspace
kmscon soon? ;-)

Ooh, maybe the KMS property documentation should also say what value
fbdev will set the property to. That's kind of UABI, because userspace
probably implicitly relies on it in many cases. ...which means fbdev
should set the property to its initial value, otherwise userspace will
break.


Thanks,
pq

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