regulator: implement selector stepping

Some regulators require that the requested voltage be reached gradually
by setting all or some of the intermediate values. Implement a new field
in the regulator description struct that allows users to specify the
number of selectors by which the regulator API should step when ramping
the voltage up/down.

Signed-off-by: Bartosz Golaszewski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/regulator/core.c

···
 	return ret;
 }
 
+static int _regulator_set_voltage_sel_step(struct regulator_dev *rdev,
+					   int uV, int new_selector)
+{
+	const struct regulator_ops *ops = rdev->desc->ops;
+	int diff, old_sel, curr_sel, ret;
+
+	/* Stepping is only needed if the regulator is enabled. */
+	if (!_regulator_is_enabled(rdev))
+		goto final_set;
+
+	if (!ops->get_voltage_sel)
+		return -EINVAL;
+
+	old_sel = ops->get_voltage_sel(rdev);
+	if (old_sel < 0)
+		return old_sel;
+
+	diff = new_selector - old_sel;
+	if (diff == 0)
+		return 0; /* No change needed. */
+
+	if (diff > 0) {
+		/* Stepping up. */
+		for (curr_sel = old_sel + rdev->desc->vsel_step;
+		     curr_sel < new_selector;
+		     curr_sel += rdev->desc->vsel_step) {
+			/*
+			 * Call the callback directly instead of using
+			 * _regulator_call_set_voltage_sel() as we don't
+			 * want to notify anyone yet. Same in the branch
+			 * below.
+			 */
+			ret = ops->set_voltage_sel(rdev, curr_sel);
+			if (ret)
+				goto try_revert;
+		}
+	} else {
+		/* Stepping down. */
+		for (curr_sel = old_sel - rdev->desc->vsel_step;
+		     curr_sel > new_selector;
+		     curr_sel -= rdev->desc->vsel_step) {
+			ret = ops->set_voltage_sel(rdev, curr_sel);
+			if (ret)
+				goto try_revert;
+		}
+	}
+
+final_set:
+	/* The final selector will trigger the notifiers. */
+	return _regulator_call_set_voltage_sel(rdev, uV, new_selector);
+
+try_revert:
+	/*
+	 * At least try to return to the previous voltage if setting a new
+	 * one failed.
+	 */
+	(void)ops->set_voltage_sel(rdev, old_sel);
+	return ret;
+}
+
 static int _regulator_set_voltage_time(struct regulator_dev *rdev,
 				       int old_uV, int new_uV)
 {
···
 				selector = ret;
 				if (old_selector == selector)
 					ret = 0;
+				else if (rdev->desc->vsel_step)
+					ret = _regulator_set_voltage_sel_step(
+						rdev, best_val, selector);
 				else
 					ret = _regulator_call_set_voltage_sel(
 						rdev, best_val, selector);

include/linux/regulator/driver.h

···
  * @vsel_range_mask: Mask for register bitfield used for range selector
  * @vsel_reg: Register for selector when using regulator_regmap_X_voltage_
  * @vsel_mask: Mask for register bitfield used for selector
+ * @vsel_step: Specify the resolution of selector stepping when setting
+ *	       voltage. If 0, then no stepping is done (requested selector is
+ *	       set directly), if >0 then the regulator API will ramp the
+ *	       voltage up/down gradually each time increasing/decreasing the
+ *	       selector by the specified step value.
  * @csel_reg: Register for current limit selector using regmap set_current_limit
  * @csel_mask: Mask for register bitfield used for current limit selector
  * @apply_reg: Register for initiate voltage change on the output when
···
 	unsigned int vsel_range_mask;
 	unsigned int vsel_reg;
 	unsigned int vsel_mask;
+	unsigned int vsel_step;
 	unsigned int csel_reg;
 	unsigned int csel_mask;
 	unsigned int apply_reg;