drm/bridge: imx8mp-hdmi-tx-connector-fixup: add an hdmi-connector when missing using a DT overlay at boot time

The imx8mp-hdmi-tx is one of many drivers based on dw-hdmi. dw-hdmi in turn
can operate in two different modes, depending on the platform data as set
by the driver:

 A. hdmi->plat_data->output_port = 0:
    the HDMI output (port@1) in device tree is not used [0]

 B. hdmi->plat_data->output_port = 1:
    the HDMI output (port@1) is parsed to find the next bridge

The imx8mp-hdmi-tx driver falls in case A. This implies next_bridge will
always be NULL, and so dw_hdmi_bridge_attach() [1] will always fail if
called with the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag.

In fact case A assumes that DRM_BRIDGE_ATTACH_NO_CONNECTOR is not set and
in that case it adds an HDMI Type A connector programmatically at bridge
attach time.

Support for DRM_BRIDGE_ATTACH_NO_CONNECTOR is implemented by dw-hdmi.c in
case B. However switching to base B requires that port@1 is connected to a
"next bridge" DT node, typically the HDMI connector, because dw-hdmi won't
add the connector when using DRM_BRIDGE_ATTACH_NO_CONNECTOR.

Many dts files for imx8mp-based boards in the kernel have such a connector
described and linked to port@1, so the pipeline will be fully attached up
to a display-connector and a drm_connector added by the
bridge-connector. Sadly some of those dts files don't have the connector
described. Adding it would solve the problem easily, but this would break
existing devices which do not update the dtb when upgrading to a newer
kernel.

In preparation for switching to case B while preserving backward
compatibility for such devices, introduce a module adding the
hdmi-connector node to the live device tree at init time. This will allow
the dw-hdmi code to find the next bridge (the one wrapping the
hdmi-connector) and let the pipeline work as before.

The module is inserted only if there is no endpoint in port@1. So boards
whose device tree describe the connector will not have the device tre
modified, and will start isntantiating the correct HDMI connector type as
described in the device tree.

For boards lacking a connector description in DT the overlay will be added,
abd the HDMI connector will be Type A, which is a reasonable fallback and
is what the driver is currently doing.

[0] https://elixir.bootlin.com/linux/v7.0-rc1/source/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c#L3310
[1] https://elixir.bootlin.com/linux/v7.0-rc1/source/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c#L2907

Reviewed-by: Liu Ying <victor.liu@nxp.com>
Link: https://patch.msgid.link/20260407-drm-lcdif-dbanc-v4-8-247a16e61ef9@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

0 files changed, 0 insertions, 0 deletions