[PATCH v3 04/12] of: overlays: Introduce dwc3 flattening overlay

[Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>]

The Synopsys DWC3 core is often found together with vendor glue logic.
While being a single piece of hardware this has been expressed as two
independent (although with parent/child relationship) nodes in
DeviceTree - but they are not separate components, and the separation
prevents implementation of certain features (such as role switching,
when this involved both parts).

The newly introduced qcom,snps-dwc3 binding changes this representation
of the Qualcomm implementation to a single node, and in an upcoming
change the implementation follows suite - combining the two separate
drivers into a single device instance.

In order to avoid two separate implementations of the Qualcomm DWC3 glue
driver, and/or continue to live with the documented race conditions in
the driver, the driver will be changed to only operate on the new -
flattened - DeviceTree binding.

As both the Qualcomm glue driver and the dwc3 core driver is parsing
DeviceTree, the only sensible way to handle this - while maintaining
backwards compatibility with exiting DeviceTree blobs, is to convert the
representation at runtime.

The conversion between qcom,dwc3 and qcom,snps-dwc3 is performed here in
the form of an independent overlay-based mechanism, to avoid sprinkling
DeviceTree-translation code into the glue driver, which over time is
expected to allow hiding some internals of the OF-code. But this should
also make it suitable for other (than Qualcomm) vendors to reuse the
translation logic as they flatten their glue/dwc3 implementations.

The migration is implemented using two steps:
1) SoC/board integration is migrated using embedded overlays, which are
   applied based on machine compatible matching. This handles the
   complex cases such as merging "reg" and "interrupt" properties.
2) Standard snps properties, which might be board-specific, are migrated
   using of_changeset logic. Notably the of_graph is migrated this way,
   to avoid having to provide overlays for every single board dtb out
   there.

The migration code can only be enabled once the dwc3 glue driver
supports the new binding, but in order to avoid having to support both
bindings in the dwc3 glue a kill-switch is left in place, to be removed
at the instant the driver is converted.

The newly introduced Kconfig option is defaulted to follow USB_DWC3_QCOM
in order to maximize the chances of people not losing USB functionality
in defconfig or distro builds. Over time this can probably be phased
out, followed by the overlay solution itself.

Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
---
 drivers/of/Kconfig                                 |   2 +
 drivers/of/Makefile                                |   2 +
 drivers/of/overlays/Kconfig                        |  15 ++
 drivers/of/overlays/Makefile                       |   3 +
 drivers/of/overlays/dwc3-flattening/Makefile       |   4 +
 .../of/overlays/dwc3-flattening/dwc3-flattening.c  | 160 +++++++++++++++++++++
 .../of/overlays/dwc3-flattening/dwc3-flattening.h  |   7 +
 7 files changed, 193 insertions(+)

diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index 50697cc3b07e..b5f3cd69bad9 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -126,4 +126,6 @@ config OF_OVERLAY_KUNIT_TEST
 config OF_NUMA
 	bool
 
+source "drivers/of/overlays/Kconfig"
+
 endif # OF
diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index 379a0afcbdc0..1ff9d0befb38 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -25,3 +25,5 @@ obj-$(CONFIG_OF_OVERLAY_KUNIT_TEST) += overlay-test.o
 overlay-test-y := overlay_test.o kunit_overlay_test.dtbo.o
 
 obj-$(CONFIG_OF_UNITTEST) += unittest-data/
+
+obj-y += overlays/
diff --git a/drivers/of/overlays/Kconfig b/drivers/of/overlays/Kconfig
new file mode 100644
index 000000000000..8f07e6db3dc3
--- /dev/null
+++ b/drivers/of/overlays/Kconfig
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config OF_OVERLAYS_DWC3_FLATTENING
+	bool "DeviceTree overlay for migrating DWC3 glue bindings"
+	depends on OF
+	select OF_DYNAMIC
+	select OF_OVERLAY
+	default USB_DWC3_QCOM
+	help
+	  This option enables the migration of the loaded DeviceTree from the
+	  binding that splits DWC3 representation in glue and core nodes (such
+	  as "qcom,dwc3"), to the unified binding ("qcom,snps-dwc3").
+
+	  Enable this if you intend to boot the Linux kernel on a system with a
+	  DeviceTree blob using the non-flattened binding.
diff --git a/drivers/of/overlays/Makefile b/drivers/of/overlays/Makefile
new file mode 100644
index 000000000000..44dd5c09ac8d
--- /dev/null
+++ b/drivers/of/overlays/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0
+
+obj-$(CONFIG_OF_OVERLAYS_DWC3_FLATTENING) += dwc3-flattening/
diff --git a/drivers/of/overlays/dwc3-flattening/Makefile b/drivers/of/overlays/dwc3-flattening/Makefile
new file mode 100644
index 000000000000..78ed59517887
--- /dev/null
+++ b/drivers/of/overlays/dwc3-flattening/Makefile
@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
+
+obj-$(CONFIG_OF_OVERLAYS_DWC3_FLATTENING) += dwc3-flattening-overlay.o
+dwc3-flattening-overlay-y += dwc3-flattening.o
diff --git a/drivers/of/overlays/dwc3-flattening/dwc3-flattening.c b/drivers/of/overlays/dwc3-flattening/dwc3-flattening.c
new file mode 100644
index 000000000000..fe8e42627fe3
--- /dev/null
+++ b/drivers/of/overlays/dwc3-flattening/dwc3-flattening.c
@@ -0,0 +1,160 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2025, Qualcomm Innovation Center, Inc. All rights reserved.
+ */
+
+#define pr_fmt(fmt) "dwc3-flattening: " fmt
+
+#include <linux/kernel.h>
+#include <linux/of.h>
+#include <linux/slab.h>
+#include "dwc3-flattening.h"
+
+struct dwc3_overlay_symbol {
+	const char *symbol;
+	const char *path;
+};
+
+struct dwc3_overlay_data {
+	const void *fdt;
+	const void *end;
+	const char *migrate_match;
+};
+
+static const struct of_device_id dwc3_flatten_of_match[] = {
+	{}
+};
+
+static int dwc3_flattening_copy_snps_properties(struct of_changeset *ocs,
+						struct device_node *np,
+						struct device_node *dwc3)
+{
+	struct property *prop;
+	int ret = 0;
+
+	for_each_property_of_node(dwc3, prop) {
+		if (strncmp(prop->name, "snps,", 5) &&
+		    strcmp(prop->name, "usb-role-switch") &&
+		    strcmp(prop->name, "dr_mode") &&
+		    strcmp(prop->name, "tx-fifo-resize") &&
+		    strcmp(prop->name, "maximum-speed"))
+			continue;
+
+		ret = of_changeset_add_prop_copy(ocs, np, prop);
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+
+static int dwc3_flattening_copy_ports_tree(struct of_changeset *ocs,
+					   struct device_node *new_parent,
+					   struct device_node *old_node)
+{
+	struct device_node *new_node;
+	struct property *prop;
+	int ret;
+
+	new_node = of_changeset_create_node(ocs, new_parent, old_node->full_name);
+	if (!new_node)
+		return -ENOMEM;
+
+	for_each_property_of_node(old_node, prop) {
+		of_changeset_add_prop_copy(ocs, new_node, prop);
+	}
+
+	for_each_child_of_node_scoped(old_node, child) {
+		ret = dwc3_flattening_copy_ports_tree(ocs, new_node, child);
+		if (ret)
+			return ret;
+	}
+
+	return of_changeset_detach_node(ocs, old_node);
+}
+
+static int dwc3_flattening_migrate(struct of_changeset *ocs,
+				   struct device_node *np)
+{
+	struct device_node *ports;
+	struct device_node *dwc3;
+	int ret;
+
+	dwc3 = of_get_compatible_child(np, "snps,dwc3");
+	if (!dwc3)
+		return 0;
+
+	ret = dwc3_flattening_copy_snps_properties(ocs, np, dwc3);
+	if (ret) {
+		pr_err("failed to copy properties of %pOF", dwc3);
+		goto out;
+	}
+
+	ports = of_get_child_by_name(dwc3, "ports");
+	if (ports) {
+		ret = dwc3_flattening_copy_ports_tree(ocs, np, ports);
+		of_node_put(ports);
+		if (ret) {
+			pr_err("failed to clone ports child of %pOF", dwc3);
+			goto out;
+		}
+	}
+
+	ret = of_changeset_detach_node(ocs, dwc3);
+
+out:
+	of_node_put(dwc3);
+
+	return ret;
+}
+
+static int dwc3_flattening_init(void)
+{
+	const struct dwc3_overlay_data *data;
+	const struct of_device_id *match;
+	struct of_changeset migrate_ocs;
+	struct device_node *np;
+	int overlay_ovcs;
+	int ret;
+
+	/* TODO: Remove kill-switch as dwc3-qcom is migrated to qcom,snps-dwc */
+	return 0;
+
+	match = of_match_node(dwc3_flatten_of_match, of_root);
+	if (!match)
+		return 0;
+
+	data = match->data;
+
+	np = of_find_compatible_node(NULL, NULL, data->migrate_match);
+	if (!np) {
+		pr_debug("already applied\n");
+		return 0;
+	}
+	of_node_put(np);
+
+	of_changeset_init(&migrate_ocs);
+	for_each_compatible_node(np, NULL, data->migrate_match) {
+		ret = dwc3_flattening_migrate(&migrate_ocs, np);
+		if (ret < 0) {
+			of_node_put(np);
+			goto out_migrate_destroy;
+		}
+	}
+
+	ret = of_changeset_apply(&migrate_ocs);
+	if (ret < 0)
+		goto out_migrate_destroy;
+
+	ret = of_overlay_fdt_apply(data->fdt, data->end - data->fdt, &overlay_ovcs, NULL);
+	if (ret < 0) {
+		of_overlay_remove(&overlay_ovcs);
+		of_changeset_revert(&migrate_ocs);
+	}
+
+out_migrate_destroy:
+	of_changeset_destroy(&migrate_ocs);
+
+	return ret;
+}
+postcore_initcall(dwc3_flattening_init);
diff --git a/drivers/of/overlays/dwc3-flattening/dwc3-flattening.h b/drivers/of/overlays/dwc3-flattening/dwc3-flattening.h
new file mode 100644
index 000000000000..6147376d3c92
--- /dev/null
+++ b/drivers/of/overlays/dwc3-flattening/dwc3-flattening.h
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __DWC3_FLATTENING_H__
+#define __DWC3_FLATTENING_H__
+
+#include <linux/kernel.h>
+
+#endif

-- 
2.45.2