driver core: Add fw_devlink kernel commandline option

fwnode_operations.add_links allows creating device links from
information provided by firmware.

fwnode_operations.add_links is currently implemented only by
OF/devicetree code and a specific case of efi. However, there's nothing
preventing ACPI or other firmware types from implementing it.

The OF implementation is currently controlled by a kernel commandline
parameter called of_devlink.

Since this feature is generic isn't limited to OF, add a generic
fw_devlink kernel commandline parameter to control this feature across
firmware types.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20200222014038.180923-3-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -1350,6 +1350,24 @@
 			can be changed at run time by the max_graph_depth file
 			in the tracefs tracing directory. default: 0 (no limit)
 
+	fw_devlink=	[KNL] Create device links between consumer and supplier
+			devices by scanning the firmware to infer the
+			consumer/supplier relationships. This feature is
+			especially useful when drivers are loaded as modules as
+			it ensures proper ordering of tasks like device probing
+			(suppliers first, then consumers), supplier boot state
+			clean up (only after all consumers have probed),
+			suspend/resume & runtime PM (consumers first, then
+			suppliers).
+			Format: { off | permissive | on | rpm }
+			off --	Don't create device links from firmware info.
+			permissive -- Create device links from firmware info
+				but use it only for ordering boot state clean
+				up (sync_state() calls).
+			on -- 	Create device links from firmware info and use it
+				to enforce probe and suspend/resume ordering.
+			rpm --	Like "on", but also use to order runtime PM.
+
 	gamecon.map[2|3]=
 			[HW,JOY] Multisystem joystick and NES/SNES/PSX pad
 			support via parallel port (up to 5 devices per port)

drivers/base/core.c

@@ -2332,6 +2332,31 @@ static int device_private_init(struct device *dev)
 	return 0;
 }
 
+u32 fw_devlink_flags;
+static int __init fw_devlink_setup(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strcmp(arg, "off") == 0) {
+		fw_devlink_flags = 0;
+	} else if (strcmp(arg, "permissive") == 0) {
+		fw_devlink_flags = DL_FLAG_SYNC_STATE_ONLY;
+	} else if (strcmp(arg, "on") == 0) {
+		fw_devlink_flags = DL_FLAG_AUTOPROBE_CONSUMER;
+	} else if (strcmp(arg, "rpm") == 0) {
+		fw_devlink_flags = DL_FLAG_AUTOPROBE_CONSUMER |
+				   DL_FLAG_PM_RUNTIME;
+	}
+	return 0;
+}
+early_param("fw_devlink", fw_devlink_setup);
+
+u32 fw_devlink_get_flags(void)
+{
+	return fw_devlink_flags;
+}
+
 /**
  * device_add - add device to device hierarchy.
  * @dev: device.
@@ -2480,7 +2505,7 @@ int device_add(struct device *dev)
 	 */
 	device_link_add_missing_supplier_links();
 
-	if (fwnode_has_op(dev->fwnode, add_links)) {
+	if (fw_devlink_flags && fwnode_has_op(dev->fwnode, add_links)) {
 		fw_ret = fwnode_call_int_op(dev->fwnode, add_links, dev);
 		if (fw_ret == -ENODEV)
 			device_link_wait_for_mandatory_supplier(dev);

include/linux/fwnode.h

@@ -170,4 +170,6 @@ struct fwnode_operations {
 	} while (false)
 #define get_dev_from_fwnode(fwnode)	get_device((fwnode)->dev)
 
+extern u32 fw_devlink_get_flags(void);
+
 #endif