regulator: core: Allow specific minimal selector for starting linear mapping

Some drivers (at least 3 drivers) have such variant of linear mapping that
the first few selectors are invalid and the reset are linear mapping.
Let's support this case in core.

This patch adds linear_min_sel in struct regulator_desc,
so we can allow specific minimal selector for starting linear mapping.
Then extends regulator_[map|list]_voltage_linear() to support this feature.

Note that for selectors less than min_linear_index, we need count them to
n_voltages so regulator_list_voltage() won't fail while checking the boundary
for selector before calling list_voltage callback.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/core.c

@@ -1897,6 +1897,10 @@ int regulator_list_voltage_linear(struct regulator_dev *rdev,
 {
 	if (selector >= rdev->desc->n_voltages)
 		return -EINVAL;
+	if (selector < rdev->desc->linear_min_sel)
+		return 0;
+
+	selector -= rdev->desc->linear_min_sel;
 
 	return rdev->desc->min_uV + (rdev->desc->uV_step * selector);
 }
@@ -2120,6 +2124,8 @@ int regulator_map_voltage_linear(struct regulator_dev *rdev,
 	if (ret < 0)
 		return ret;
 
+	ret += rdev->desc->linear_min_sel;
+
 	/* Map back into a voltage to verify we're still in bounds */
 	voltage = rdev->desc->ops->list_voltage(rdev, ret);
 	if (voltage < min_uV || voltage > max_uV)

include/linux/regulator/driver.h

@@ -185,6 +185,7 @@ enum regulator_type {
  *
  * @min_uV: Voltage given by the lowest selector (if linear mapping)
  * @uV_step: Voltage increase with each selector (if linear mapping)
+ * @linear_min_sel: Minimal selector for starting linear mapping
  * @ramp_delay: Time to settle down after voltage change (unit: uV/us)
  * @volt_table: Voltage mapping table (if table based mapping)
  *
@@ -207,6 +208,7 @@ struct regulator_desc {
 
 	unsigned int min_uV;
 	unsigned int uV_step;
+	unsigned int linear_min_sel;
 	unsigned int ramp_delay;
 
 	const unsigned int *volt_table;