linux-kernel - Re: [PATCH v2 1/4] drm/bridge: Add fwnode based helpers to get the next bridge

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Message-ID: <CAA8EJpphapGxz9ztD7FUE5y_CVuuz=gySbOJquNXSw29vexKgQ@mail.gmail.com>
Date: Sat, 9 Mar 2024 15:29:29 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Sui Jingfeng <sui.jingfeng@...ux.dev>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Phong LE <ple@...libre.com>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] drm/bridge: Add fwnode based helpers to get the
 next bridge

On Sat, 9 Mar 2024 at 14:03, Sui Jingfeng <sui.jingfeng@...ux.dev> wrote:
>
> Hi,
>
>
> On 2024/3/9 18:39, Dmitry Baryshkov wrote:
> >> The code path of "creating a connector" plus the code path of "not creating a connector"
> >> forms a 'side-by-side' implementation imo.
> >>
> >> Besides, I have repeated many times: the DT already speak everything.
> >> Device drivers can completely know if there is a display connector OF device created and how many
> >> display bridges in the whole chain. If there are connector device node in the DT, then it should
> >> has a device driver bound to it(instead of create it manually) for a perfect implementation. As
> >> you told me we should not*over play*  the device-driver model, right?
> > Please, don't mix the connector node in DT and the drm_connector. If
> > you check the code, you will see that the driver for hdmi-connector,
> > dp-connector and other such devices creates a drm_bridge.
>
>
> OK, I'm not mixed them, I'm very clear from the very beginning. I have checked
> the code years ago. Let's make it clear by iterating one more time:
>
> If DT contains one or more HDMI connector node, then there will be one or
> more display connector platform devices created by OF core, Then, according to
> your "don't overplay device-driver model" criterion or modern drm bridge standard,
> we shouldn't create a display connector instance in the drm birdge driver, right?

Yeah, if the platform is updated, yes, we do. If there is an
hdmi-connector node, I can only assume that the DRM driver also has
been updated to pass the DRM_BRIDGE_ATTACH_NO_CONNECTOR. In such case
the lt9611uxc driver will not create the drm_connector and everything
works as expected. If this is one of the legacy platforms, the DRM
driver will not pass the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag, but at
the same time the DT will not have the connector node.

>
> As otherwise we will have two display connector driver (or code) for a single entity,
> right?
>
> Another side effect is that
> when you create a hdmi display connector instance manually without reference to the
> DT, then *you made an assumption!*. But there are users who have don't a different
> need(or  different typology), for example, they need to read edid directly from the
> KMS driver side. This may because the i2c bus is directly connected to the connector
> part, but the display bridge's I2C slave interface. sii9022, it66121 and tfp410 support
> this kind of usage.
>
> So the real problem is that it is a policy level code  when you creating a hdmi
> display connector instance manually without reference to the DT in a common drm bridge
> driver, not a mechanism.

Only if requested by the DRM driver itself.

--
With best wishes
Dmitry