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@xxxxxxxxxxx/ > > 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
