regulator: core: Resolve supply name earlier to prevent double-init

Previously, an unresolved regulator supply reference upon calling
regulator_register on an always-on or boot-on regulator caused
set_machine_constraints to be called twice.

This in turn may initialize the regulator twice, leading to voltage
glitches that are timing-dependent. A simple, unrelated configuration
change may be enough to hide this problem, only to be surfaced by
chance.

One such example is the SD-Card voltage regulator in a NanoPI R4S that
would not initialize reliably unless the registration flow was just
complex enough to allow the regulator to properly reset between calls.

Fix this by re-arranging regulator_register, trying resolve the
regulator's supply early enough that set_machine_constraints does not
need to be called twice.

Signed-off-by: Christian Kohlschütter <christian@kohlschutter.com>
Link: https://lore.kernel.org/r/20220818124646.6005-1-christian@kohlschutter.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/core.c

@@ -5496,7 +5496,39 @@ regulator_register(const struct regulator_desc *regulator_desc,
 	BLOCKING_INIT_NOTIFIER_HEAD(&rdev->notifier);
 	INIT_DELAYED_WORK(&rdev->disable_work, regulator_disable_work);
 
-	/* preform any regulator specific init */
+	/* set regulator constraints */
+	if (init_data)
+		rdev->constraints = kmemdup(&init_data->constraints,
+					    sizeof(*rdev->constraints),
+					    GFP_KERNEL);
+	else
+		rdev->constraints = kzalloc(sizeof(*rdev->constraints),
+					    GFP_KERNEL);
+	if (!rdev->constraints) {
+		ret = -ENOMEM;
+		goto clean;
+	}
+
+	if (init_data && init_data->supply_regulator)
+		rdev->supply_name = init_data->supply_regulator;
+	else if (regulator_desc->supply_name)
+		rdev->supply_name = regulator_desc->supply_name;
+
+	if ((rdev->supply_name && !rdev->supply) &&
+			(rdev->constraints->always_on ||
+			 rdev->constraints->boot_on)) {
+		/* Try to resolve the name of the supplying regulator here first
+		 * so we prevent double-initializing the regulator, which may
+		 * cause timing-specific voltage brownouts/glitches that are
+		 * hard to debug.
+		 */
+		ret = regulator_resolve_supply(rdev);
+		if (ret)
+			rdev_dbg(rdev, "unable to resolve supply early: %pe\n",
+					 ERR_PTR(ret));
+	}
+
+	/* perform any regulator specific init */
 	if (init_data && init_data->regulator_init) {
 		ret = init_data->regulator_init(rdev->reg_data);
 		if (ret < 0)
@@ -5522,24 +5554,6 @@ regulator_register(const struct regulator_desc *regulator_desc,
 		    (unsigned long) atomic_inc_return(&regulator_no));
 	dev_set_drvdata(&rdev->dev, rdev);
 
-	/* set regulator constraints */
-	if (init_data)
-		rdev->constraints = kmemdup(&init_data->constraints,
-					    sizeof(*rdev->constraints),
-					    GFP_KERNEL);
-	else
-		rdev->constraints = kzalloc(sizeof(*rdev->constraints),
-					    GFP_KERNEL);
-	if (!rdev->constraints) {
-		ret = -ENOMEM;
-		goto wash;
-	}
-
-	if (init_data && init_data->supply_regulator)
-		rdev->supply_name = init_data->supply_regulator;
-	else if (regulator_desc->supply_name)
-		rdev->supply_name = regulator_desc->supply_name;
-
 	ret = set_machine_constraints(rdev);
 	if (ret == -EPROBE_DEFER) {
 		/* Regulator might be in bypass mode and so needs its supply