Re: [PATCH v3 08/11] drm/bridge: imx8mp-hdmi-tx-connector-fixup: add an hdmi-connector when missing using a DT overlay at boot time

[Liu Ying]

Hi Luca,

On Thu, Apr 02, 2026 at 11:26:03AM +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@xxxxxxxxxxx>
> 
> ---
> 
> 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 | 33 ++++++++++++
>  4 files changed, 111 insertions(+)
> 
> diff --git a/drivers/gpu/drm/bridge/imx/Kconfig b/drivers/gpu/drm/bridge/imx/Kconfig
> index b9028a5e5a06..49f074559b00 100644
> --- a/drivers/gpu/drm/bridge/imx/Kconfig
> +++ b/drivers/gpu/drm/bridge/imx/Kconfig
> @@ -18,6 +18,8 @@ config DRM_IMX8MP_DW_HDMI_BRIDGE
>  	depends on OF
>  	depends on COMMON_CLK
>  	select DRM_DW_HDMI
> +	select OF_OVERLAY
> +	select DRM_DISPLAY_CONNECTOR
>  	imply DRM_IMX8MP_HDMI_PAI
>  	imply DRM_IMX8MP_HDMI_PVI
>  	imply PHY_FSL_SAMSUNG_HDMI_PHY
> @@ -25,6 +27,22 @@ config DRM_IMX8MP_DW_HDMI_BRIDGE
>  	  Choose this to enable support for the internal HDMI encoder found
>  	  on the i.MX8MP SoC.
>  
> +config DRM_IMX8MP_DW_HDMI_BRIDGE_CONNECTOR_FIXUP
> +	bool
> +	default y
> +	depends on DRM_IMX_LCDIF
> +	depends on DRM_IMX8MP_DW_HDMI_BRIDGE
> +	depends on OF
> +	help
> +	  Modifies at early boot the live device tree of boards using the
> +	  i.MX8MP fsl,imx8mp-hdmi-tx adding a hdmi-connector node linked to
> +	  the hdmi-tx. This is needed to support bridge-connector usage in
> +	  the i.MX8MP LCDIF driver.
> +
> +	  You need this if you use the i.MX8MP HDMI output and your board
> +	  device tree file does not have an hdmi-connector node connected
> +	  to it.
> +
>  config DRM_IMX8MP_HDMI_PAI
>  	tristate "Freescale i.MX8MP HDMI PAI bridge support"
>  	depends on OF
> diff --git a/drivers/gpu/drm/bridge/imx/Makefile b/drivers/gpu/drm/bridge/imx/Makefile
> index 8d01fda25451..84499fe2e444 100644
> --- a/drivers/gpu/drm/bridge/imx/Makefile
> +++ b/drivers/gpu/drm/bridge/imx/Makefile
> @@ -1,6 +1,8 @@
>  obj-$(CONFIG_DRM_IMX_LDB_HELPER) += imx-ldb-helper.o
>  obj-$(CONFIG_DRM_IMX_LEGACY_BRIDGE) += imx-legacy-bridge.o
>  obj-$(CONFIG_DRM_IMX8MP_DW_HDMI_BRIDGE) += imx8mp-hdmi-tx.o
> +obj-$(CONFIG_DRM_IMX8MP_DW_HDMI_BRIDGE_CONNECTOR_FIXUP) += imx8mp-hdmi-tx-connector-fixup.o \
> +							   imx8mp-hdmi-tx-connector-fixup.dtbo.o
>  obj-$(CONFIG_DRM_IMX8MP_HDMI_PAI) += imx8mp-hdmi-pai.o
>  obj-$(CONFIG_DRM_IMX8MP_HDMI_PVI) += imx8mp-hdmi-pvi.o
>  obj-$(CONFIG_DRM_IMX8QM_LDB) += imx8qm-ldb.o
> diff --git a/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.c b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.c
> new file mode 100644
> index 000000000000..dc1736bfc3ac
> --- /dev/null
> +++ b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.c
> @@ -0,0 +1,58 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Add an hdmi-connector node to boards using the imx8mp hdmi_tx which
> + * don't have one. This is needed for the i.MX LCDIF to work with
> + * DRM_BRIDGE_ATTACH_NO_CONNECTOR.
> + *
> + * Copyright (C) 2026 GE HealthCare
> + * Author: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> + */
> +
> +#include <linux/cleanup.h>
> +#include <linux/init.h>
> +#include <linux/of.h>
> +#include <linux/of_graph.h>
> +
> +/* Embedded dtbo symbols created by cmd_wrap_S_dtb in scripts/Makefile.dtbs */
> +extern char __dtbo_imx8mp_hdmi_tx_connector_fixup_begin[];
> +extern char __dtbo_imx8mp_hdmi_tx_connector_fixup_end[];
> +
> +static int __init imx8mp_hdmi_tx_connector_fixup_init(void)
> +{
> +	struct device_node *soc      __free(device_node) = NULL;
> +	struct device_node *hdmi_tx  __free(device_node) = NULL;
> +	struct device_node *endpoint __free(device_node) = NULL;
> +	void *dtbo_start;
> +	u32 dtbo_size;
> +	int ovcs_id;
> +	int err;
> +
> +	soc = of_find_node_by_path("/soc@0");
> +	if (!soc)
> +		return 0;
> +
> +	/* This applies to i.MX8MP only, do nothing on other systems */
> +	if (!of_device_is_compatible(soc, "fsl,imx8mp-soc"))
> +		return 0;
> +
> +	hdmi_tx = of_find_node_by_path("/soc@0/bus@32c00000/hdmi@32fd8000");
> +	if (!of_device_is_available(hdmi_tx))
> +		return 0;
> +
> +	/* If endpoint exists, assume an hdmi-connector exists already */
> +	endpoint = of_graph_get_endpoint_by_regs(hdmi_tx, 1, -1);
> +	if (endpoint)
> +		return 0;
> +
> +	dtbo_start = __dtbo_imx8mp_hdmi_tx_connector_fixup_begin;
> +	dtbo_size = __dtbo_imx8mp_hdmi_tx_connector_fixup_end -
> +		    __dtbo_imx8mp_hdmi_tx_connector_fixup_begin;
> +
> +	err = of_overlay_fdt_apply(dtbo_start, dtbo_size, &ovcs_id, NULL);
> +	if (err)
> +		err = of_overlay_remove(&ovcs_id);
> +
> +	return err;
> +}
> +
> +subsys_initcall(imx8mp_hdmi_tx_connector_fixup_init);
> diff --git a/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso
> new file mode 100644
> index 000000000000..070be24fed3e
> --- /dev/null
> +++ b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso
> @@ -0,0 +1,33 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * DTS overlay adding an hdmi-connector node to boards using the imx8mp hdmi_tx
> + *
> + * Copyright (C) 2026 GE HealthCare
> + * Author: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> + */
> +
> +/dts-v1/;
> +/plugin/;
> +
> +&{/} {
> +	#address-cells = <2>;
> +	#size-cells = <2>;
> +
> +	fixup-hdmi-connector {
> +		compatible = "hdmi-connector";
> +		label = "HDMI";
> +		type = "a";
> +
> +		port {
> +			fixup_hdmi_connector_in: endpoint {
> +				remote-endpoint = <&hdmi_tx_out>;
> +			};
> +		};
> +	};
> +};
> +
> +&{/soc@0/bus@32c00000/hdmi@32fd8000/ports/port@1} {
> +	hdmi_tx_out: endpoint {
> +		remote-endpoint = <&fixup_hdmi_connector_in>;
> +	};
> +};
> 

There is a build warning(W=1):

  DTC     drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtbo
drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso:12.6-27.3: Warning (avoid_unnecessary_addr_size): /fragment@0/__overlay__: unnecessary #address-cells/#size-cells without "ranges", "dma-ranges" or child "reg" or "ranges" property

-- 
Regards,
Liu Ying