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

[Maxime Ripard <maxime@cerno.tech>]

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Hi Pekka,

On Thu, Jun 17, 2021 at 11:20:36AM +0300, Pekka Paalanen wrote:
> 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.

I've taken into account your previous comments, thanks

> 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.

I'm not sure about this one: fbdev and fbcon are still optional features
of the kernel and can be disabled at the user discretion. Having any
part of the user-space rely on the fbdev behavior seems a bit broken,
especially when we have a mechanism to retrieve the state when the
application starts.

Maxime

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