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Date: Fri, 10 May 2024 11:44:49 -0500
From: Rob Herring <robh@kernel.org>
To: Luca Ceresoli <luca.ceresoli@bootlin.com>
Cc: Krzysztof Kozlowski <krzk+dt@kernel.org>,
	Conor Dooley <conor+dt@kernel.org>,
	Andrzej Hajda <andrzej.hajda@intel.com>,
	Neil Armstrong <neil.armstrong@linaro.org>,
	Robert Foss <rfoss@kernel.org>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Derek Kiernan <derek.kiernan@amd.com>,
	Dragan Cvetic <dragan.cvetic@amd.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Saravana Kannan <saravanak@google.com>,
	Paul Kocialkowski <contact@paulk.fr>,
	=?iso-8859-1?Q?Herv=E9?= Codina <herve.codina@bootlin.com>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org,
	Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Subject: Re: [PATCH v2 0/5] Add support for GE SUNH hot-pluggable connector
 (was: "drm: add support for hot-pluggable bridges")
Message-ID: <20240510164449.GB336987-robh@kernel.org>
References: <20240510-hotplug-drm-bridge-v2-0-ec32f2c66d56@bootlin.com>
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In-Reply-To: <20240510-hotplug-drm-bridge-v2-0-ec32f2c66d56@bootlin.com>

On Fri, May 10, 2024 at 09:10:36AM +0200, Luca Ceresoli wrote:
> Hello,
> 
> this series aims at supporting a Linux device with a connector to
> physically add and remove an add-on to/from the main device to augment its
> features at runtime, using device tree overlays.
> 
> This is the v2 of "drm: add support for hot-pluggable bridges" [0] which
> was however more limited in scope, covering only the DRM aspects. This new
> series also takes a different approach to the DRM bridge instantiation.
> 
> [0] https://lore.kernel.org/all/20240326-hotplug-drm-bridge-v1-0-4b51b5eb75d5@bootlin.com/
> 
> Use case
> ========
> 
> This series targets a professional product (GE SUNH) that is composed of a
> "main" part running on battery, with the main SoC and able to work
> autonomously with limited features, and an optional "add-on" that enables
> more features by adding more hardware peripherals, some of which are on
> non-discoverable busses such as I2C and MIPI DSI.
> 
> The add-on can be connected and disconnected at runtime at any moment by
> the end user, and add-on features need to be enabled and disabled
> automatically at runtime.
> 
> The add-on has status pins that are connected to GPIOs on the main board,
> allowing the CPU to detect add-on insertion and removal. It also has a
> reset GPIO allowign to reset all peripherals on the add-on at once.
> 
> The features provided by the add-on include a display and a battery charger
> to recharge the battery of the main part. The display on the add-on has an
> LVDS input but the connector between the base and the add-on has a MIPI DSI
> bus, so a DSI-to-LVDS bridge is present on the add-on.
> 
> Different add-on models can be connected to the main part, and for this a
> model ID is stored in the add-on itself so the software running on the CPU
> on the main part knows which non-discoverable hardware to probe.
> 
> Overall approach
> ================
> 
> Device tree overlays appear as the most natural solution to support the
> addition and removal of devices from a running system.
> 
> Several features are missing from the mainline Linux kernel in order to
> support this use case:
> 
>  1. runtime (un)loading of device tree overlays is not supported

Not true. Device specific applying of overlays has been supported 
since we merged DT overlay support. What's not supported is a general 
purpose interface to userspace to change any part of the DT at any point 
in time.

>  2. if enabled, overlay (un)loading exposes several bugs

Hence why there is no general purpose interface.

>  3. the DRM subsystem assumes video bridges are non-removable

Rob