regulator: core: Add option to prevent disabling unused regulators

This may be useful for debugging and develompent purposes, when there are
drivers that depend on regulators to be enabled but do not request them.

It is inspired from the clk_ignore_unused and pd_ignore_unused parameters,
that are used to keep firmware-enabled clocks and power domains on even if
these are not used by drivers.

The parameter is not expected to be used in normal cases and should not be
needed on a platform with proper driver support.

Signed-off-by: Javier Martinez Canillas <[e-mail]>
Reviewed-by: Brian Masney <[e-mail]>
Acked-by: Manivannan Sadhasivam <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			print every Nth verbose statement, where N is the value
 			specified.
 
+	regulator_ignore_unused
+			[REGULATOR]
+			Prevents regulator framework from disabling regulators
+			that are unused, due no driver claiming them. This may
+			be useful for debug and development, but should not be
+			needed on a platform with proper driver support.
+
 	relax_domain_level=
 			[KNL, SMP] Set scheduler's default relax_domain_level.
 			See Documentation/admin-guide/cgroup-v1/cpusets.rst.

drivers/regulator/core.c

···
 	return 0;
 }
 
+static bool regulator_ignore_unused;
+static int __init regulator_ignore_unused_setup(char *__unused)
+{
+	regulator_ignore_unused = true;
+	return 1;
+}
+__setup("regulator_ignore_unused", regulator_ignore_unused_setup);
+
 static void regulator_init_complete_work_function(struct work_struct *work)
 {
 	/*
···
 	 */
 	class_for_each_device(&regulator_class, NULL, NULL,
 			      regulator_register_resolve_supply);
+
+	/*
+	 * For debugging purposes, it may be useful to prevent unused
+	 * regulators from being disabled.
+	 */
+	if (regulator_ignore_unused) {
+		pr_warn("regulator: Not disabling unused regulators\n");
+		return;
+	}
 
 	/* If we have a full configuration then disable any regulators
 	 * we have permission to change the status for and which are