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

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

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(+)

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@bootlin.com>
+ */
+
+#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..6ba1c1854aee
--- /dev/null
+++ b/drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso
@@ -0,0 +1,30 @@
+// 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@bootlin.com>
+ */
+
+/dts-v1/;
+/plugin/;
+
+&{/} {
+	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>;
+	};
+};

-- 
2.53.0