pwm: Add a driver for Chrome OS EC PWM

This PWM is used in rk3399-gru-bob and rk3399-gru-kevin to control
the display brightness. We can only change the duty cycle, so on
set_config() we just try to match the duty cycle that dividing duty_ns
by period_ns gives us. To disable, we set the duty cycle to zero while
keeping the old value for when we want to re-enable it.

The cros_ec_set_pwm_duty() function is taken from Depthcharge's
cros_ec_set_bl_pwm_duty() but modified to use the generic pwm type.
The driver itself is very loosely based on rk_pwm.c for the general pwm
driver structure.

The devicetree binding file is from Linux, before it was converted to
YAML at 5df5a577a6b4 ("dt-bindings: pwm: Convert google,cros-ec-pwm.txt
to YAML format") in their repo.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/pwm/cros-ec-pwm.txt

@@ -0,0 +1,23 @@
+PWM controlled by ChromeOS EC
+
+Google's ChromeOS EC PWM is a simple PWM attached to the Embedded Controller
+(EC) and controlled via a host-command interface.
+
+An EC PWM node should be only found as a sub-node of the EC node (see
+doc/device-tree-bindings/misc/cros-ec.txt).
+
+Required properties:
+- compatible: Must contain "google,cros-ec-pwm"
+- #pwm-cells: Should be 1. The cell specifies the PWM index.
+
+Example:
+	cros-ec@0 {
+		compatible = "google,cros-ec-spi";
+
+		...
+
+		cros_ec_pwm: ec-pwm {
+			compatible = "google,cros-ec-pwm";
+			#pwm-cells = <1>;
+		};
+	};

drivers/misc/cros_ec.c

@@ -1170,6 +1170,23 @@ int cros_ec_battery_cutoff(struct udevice *dev, uint8_t flags)
 	return 0;
 }
 
+int cros_ec_set_pwm_duty(struct udevice *dev, uint8_t index, uint16_t duty)
+{
+	struct ec_params_pwm_set_duty p;
+	int ret;
+
+	p.duty = duty;
+	p.pwm_type = EC_PWM_TYPE_GENERIC;
+	p.index = index;
+
+	ret = ec_command(dev, EC_CMD_PWM_SET_DUTY, 0, &p, sizeof(p),
+			 NULL, 0);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
 int cros_ec_set_ldo(struct udevice *dev, uint8_t index, uint8_t state)
 {
 	struct ec_params_ldo_set params;

drivers/pwm/Kconfig

@@ -9,6 +9,15 @@ config DM_PWM
 	  frequency/period can be controlled along with the proportion of that
 	  time that the signal is high.
 
+config PWM_CROS_EC
+	bool "Enable support for the Chrome OS EC PWM"
+	depends on DM_PWM
+	help
+	  This PWM is found on several Chrome OS devices and controlled by
+	  the Chrome OS embedded controller. It may be used to control the
+	  screen brightness and/or the keyboard backlight depending on the
+	  device.
+
 config PWM_EXYNOS
 	bool "Enable support for the Exynos PWM"
 	depends on DM_PWM

drivers/pwm/Makefile

@@ -10,6 +10,7 @@
 
 obj-$(CONFIG_DM_PWM)		+= pwm-uclass.o
 
+obj-$(CONFIG_PWM_CROS_EC)	+= cros_ec_pwm.o
 obj-$(CONFIG_PWM_EXYNOS)	+= exynos_pwm.o
 obj-$(CONFIG_PWM_IMX)		+= pwm-imx.o pwm-imx-util.o
 obj-$(CONFIG_PWM_MESON)		+= pwm-meson.o

drivers/pwm/cros_ec_pwm.c

@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <common.h>
+#include <cros_ec.h>
+#include <dm.h>
+#include <errno.h>
+#include <log.h>
+#include <pwm.h>
+
+struct cros_ec_pwm_priv {
+	bool enabled;
+	uint duty;
+};
+
+static int cros_ec_pwm_set_config(struct udevice *dev, uint channel,
+				  uint period_ns, uint duty_ns)
+{
+	struct cros_ec_pwm_priv *priv = dev_get_priv(dev);
+	uint duty;
+	int ret;
+
+	debug("%s: period_ns=%u, duty_ns=%u asked\n", __func__,
+	      period_ns, duty_ns);
+
+	/* No way to set the period, only a relative duty cycle */
+	duty = EC_PWM_MAX_DUTY * duty_ns / period_ns;
+	if (duty > EC_PWM_MAX_DUTY)
+		duty = EC_PWM_MAX_DUTY;
+
+	if (!priv->enabled) {
+		priv->duty = duty;
+		debug("%s: duty=%#x to-be-set\n", __func__, duty);
+		return 0;
+	}
+
+	ret = cros_ec_set_pwm_duty(dev->parent, channel, duty);
+	if (ret) {
+		debug("%s: duty=%#x failed\n", __func__, duty);
+		return ret;
+	}
+
+	priv->duty = duty;
+	debug("%s: duty=%#x set\n", __func__, duty);
+
+	return 0;
+}
+
+static int cros_ec_pwm_set_enable(struct udevice *dev, uint channel,
+				  bool enable)
+{
+	struct cros_ec_pwm_priv *priv = dev_get_priv(dev);
+	int ret;
+
+	ret = cros_ec_set_pwm_duty(dev->parent, channel,
+				   enable ? priv->duty : 0);
+	if (ret) {
+		debug("%s: enable=%d failed\n", __func__, enable);
+		return ret;
+	}
+
+	priv->enabled = enable;
+	debug("%s: enable=%d (duty=%#x) set\n", __func__,
+	      enable, priv->duty);
+
+	return 0;
+}
+
+static const struct pwm_ops cros_ec_pwm_ops = {
+	.set_config	= cros_ec_pwm_set_config,
+	.set_enable	= cros_ec_pwm_set_enable,
+};
+
+static const struct udevice_id cros_ec_pwm_ids[] = {
+	{ .compatible = "google,cros-ec-pwm" },
+	{ }
+};
+
+U_BOOT_DRIVER(cros_ec_pwm) = {
+	.name	= "cros_ec_pwm",
+	.id	= UCLASS_PWM,
+	.of_match = cros_ec_pwm_ids,
+	.ops	= &cros_ec_pwm_ops,
+	.priv_auto_alloc_size	= sizeof(struct cros_ec_pwm_priv),
+};

include/cros_ec.h

@@ -512,6 +512,19 @@ int cros_ec_efs_verify(struct udevice *dev, enum ec_flash_region region);
  */
 int cros_ec_battery_cutoff(struct udevice *dev, uint8_t flags);
 
+/**
+ * cros_ec_set_pwm_duty() - Set duty cycle of a generic pwm
+ *
+ * Note that duty value needs to be passed to the EC as a 16 bit number
+ * for increased precision.
+ *
+ * @param dev		CROS-EC device
+ * @param index		Index of the pwm
+ * @param duty		Desired duty cycle, in 0..EC_PWM_MAX_DUTY range.
+ * @return 0 if OK, -ve on error
+ */
+int cros_ec_set_pwm_duty(struct udevice *dev, uint8_t index, uint16_t duty);
+
 /**
  * cros_ec_read_limit_power() - Check if power is limited by batter/charger
  *