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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jun 17, 2021 at 05:38:36PM +0200, Philippe CORNU wrote:
> 
> 
> On 6/16/21 4:38 PM, 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: 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.
> > +
> > +- 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.
> > +
> >   Property Types and Blob Property Support
> >   ----------------------------------------
> > 
> 
> Hi,
> 
> Regarding properties, we have a “case study example” related in a certain
> way to this documentation update :-)
> 
> The use case: on a front desk at an exhibition, there is a welcome screen
> you can touch for searching various information. When this welcome screen is
> in idle, a small logo is displayed at its center (around 20% of the
> fullscreen). The logo has a white background color. We want to reduce the
> ddr usage for lowering the power (the board is battery powered) so the idea
> is to use a white background color around this logo, produced by the drm
> CRTC so the image in ddr is only the size of the logo.
> 
> Reading the thread
> https://lists.freedesktop.org/archives/dri-devel/2019-October/239733.html
> dissuade us from coding a generic solution, so we started to implement a
> "STM_" private background color property, it works... but we are not at all
> convince this is the right way and we clearly prefer mainline/generic sw for
> both kernel & userland.
> 
> So now, what are our options... well, this v4 documentation update is I
> think clear enough: we have to document + provide a generic helper in the
> core code (similar to the original patch) + update IGT test, right?

Yeah, also background color has been talked about for a while (lots of hw
can do it), it's just that no one ever found a use-case to make the
background somethign else than "black". There's a pile of drivers who
could expose this, so definitely makes sense to have this generic.
-Daniel

> 
> Thanks
> Philippe :-)
> 
> Note: It is really a pleasure to read such interesting thread, exposing the
> “complexity” of our job, dealing with various hw and sw... thank you to all
> of you.
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch