Re: [PATCH v4 08/11] drm/bridge: imx8mp-hdmi-tx-connector-fixup: add an hdmi-connector when missing using a DT overlay at boot time

[Liu Ying <victor.liu@nxp.com>]

On Tue, Apr 07, 2026 at 02:24:22PM +0200, Luca Ceresoli wrote:
> 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
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> ---
> 
> Changes in v4:
> - fixed warning in overlay build
> 
> Changes in v3:
> - removed unused variable
> - simplified overlay using full path to port@1
> - handle of_overlay_fdt_apply() errors
> - use of_graph_get_endpoint_by_regs() for more robust endpoint lookup
> - improve subject line
> 
> Changes in v2:
> - don't apply the overlay if the SoC is not i.MX8MP
> - build unconditionally, but depend on DRM_IMX_LCDIF
> - remove useless error check
> - add missing cleanup.h and init.h includes, remove unneeded includes
> - avoid dtc warnings on overlay
> - fix typo in Kconfig help text
> - not added the Tested-bys because the code has changed
> - split the 'plat_data->output_port = 1' line to a separate patch
> - improve commit message
> 
> This patch is inspired by commit 0ff223d99147 ("drm/tilcdc: Convert legacy
> panel binding via DT overlay at boot time")
> ---
>  drivers/gpu/drm/bridge/imx/Kconfig                 | 18 +++++++
>  drivers/gpu/drm/bridge/imx/Makefile                |  2 +
>  .../bridge/imx/imx8mp-hdmi-tx-connector-fixup.c    | 58 ++++++++++++++++++++++
>  .../bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso | 30 +++++++++++
>  4 files changed, 108 insertions(+)

Reviewed-by: Liu Ying <victor.liu@nxp.com>
Thanks!

-- 
Regards,
Liu Ying