pwm: pca9685: Make use of register caching in regmap

This essentially only caches the PRESCALE register because the per channel
registers are affected by the ALL configuration that is used by the virtual
pwm #16. The PRESCALE register is read often so caching it saves quite some
i2c transfers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/dc25361908ad1dd790f108599bc9dbcc752288a5.1753784092.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

Author: ukleinek

drivers/pwm/pwm-pca9685.c

@@ -521,11 +521,36 @@ static const struct pwm_ops pca9685_pwm_ops = {
 	.free = pca9685_pwm_free,
 };
 
+static bool pca9685_readable_reg(struct device *dev, unsigned int reg)
+{
+	/* The ALL_LED registers are readable but read as zero */
+	return reg <= REG_OFF_H(15) || reg >= PCA9685_PRESCALE;
+}
+
+static bool pca9685_writeable_reg(struct device *dev, unsigned int reg)
+{
+	return reg <= REG_OFF_H(15) || reg >= PCA9685_ALL_LED_ON_L;
+}
+
+static bool pca9685_volatile_reg(struct device *dev, unsigned int reg)
+{
+	/*
+	 * Writing to an ALL_LED register affects all LEDi registers, so they
+	 * are not cachable. :-\
+	 */
+	return reg < PCA9685_PRESCALE;
+}
+
 static const struct regmap_config pca9685_regmap_i2c_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
+
+	.readable_reg = pca9685_readable_reg,
+	.writeable_reg = pca9685_writeable_reg,
+	.volatile_reg = pca9685_volatile_reg,
+
 	.max_register = PCA9685_NUMREGS,
-	.cache_type = REGCACHE_NONE,
+	.cache_type = REGCACHE_MAPLE,
 };
 
 static int pca9685_pwm_probe(struct i2c_client *client)