[PATCH 2/2] pwm: clk-pwm: add GPIO and pinctrl support for constant output levels

[Xilin Wu]

The clk-pwm driver cannot guarantee a defined output level when the
PWM is disabled or when 0%/100% duty cycle is requested, because the
pin state when the clock is stopped is hardware-dependent.

Add optional GPIO and pinctrl support: when a GPIO descriptor and
pinctrl states ("default" for clock mux, "gpio" for GPIO mode) are
provided in the device tree, the driver switches the pin to GPIO mode
and drives the appropriate level for disabled/0%/100% states. For
normal PWM output, the pin is switched back to its clock function mux.

If no GPIO is provided, the driver falls back to the original
clock-only behavior.

Signed-off-by: Xilin Wu <sophon@xxxxxxxxx>
---
 drivers/pwm/pwm-clk.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 66 insertions(+), 6 deletions(-)

diff --git a/drivers/pwm/pwm-clk.c b/drivers/pwm/pwm-clk.c
index f8f5af57acba..99821fae54e7 100644
--- a/drivers/pwm/pwm-clk.c
+++ b/drivers/pwm/pwm-clk.c
@@ -10,12 +10,15 @@
  * Limitations:
  * - Due to the fact that exact behavior depends on the underlying
  *   clock driver, various limitations are possible.
- * - Underlying clock may not be able to give 0% or 100% duty cycle
- *   (constant off or on), exact behavior will depend on the clock.
- * - When the PWM is disabled, the clock will be disabled as well,
- *   line state will depend on the clock.
  * - The clk API doesn't expose the necessary calls to implement
  *   .get_state().
+ *
+ * Optionally, a GPIO descriptor and pinctrl states ("default" and
+ * "gpio") can be provided. When a constant output level is needed
+ * (0% duty, 100% duty, or disabled), the driver switches the pin to
+ * GPIO mode and drives the appropriate level. For normal PWM output
+ * the pin is switched back to its clock function mux. If no GPIO is
+ * provided, the driver falls back to the original clock-only behavior.
  */
 
 #include <linux/kernel.h>
@@ -25,11 +28,17 @@
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/clk.h>
+#include <linux/gpio/consumer.h>
+#include <linux/pinctrl/consumer.h>
 #include <linux/pwm.h>
 
 struct pwm_clk_chip {
 	struct clk *clk;
 	bool clk_enabled;
+	struct pinctrl *pinctrl;
+	struct pinctrl_state *pins_default;  /* clock function mux */
+	struct pinctrl_state *pins_gpio;     /* GPIO mode */
+	struct gpio_desc *gpiod;
 };
 
 static inline struct pwm_clk_chip *to_pwm_clk_chip(struct pwm_chip *chip)
@@ -45,14 +54,36 @@ static int pwm_clk_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	u32 rate;
 	u64 period = state->period;
 	u64 duty_cycle = state->duty_cycle;
+	bool constant_level = false;
+	int gpio_value = 0;
 
 	if (!state->enabled) {
-		if (pwm->state.enabled) {
+		constant_level = true;
+		gpio_value = 0;
+	} else if (state->duty_cycle == 0) {
+		constant_level = true;
+		gpio_value = (state->polarity == PWM_POLARITY_INVERSED) ? 1 : 0;
+	} else if (state->duty_cycle >= state->period) {
+		constant_level = true;
+		gpio_value = (state->polarity == PWM_POLARITY_INVERSED) ? 0 : 1;
+	}
+
+	if (constant_level) {
+		if (pcchip->gpiod) {
+			pinctrl_select_state(pcchip->pinctrl, pcchip->pins_gpio);
+			gpiod_direction_output(pcchip->gpiod, gpio_value);
+		}
+		if (pcchip->clk_enabled) {
 			clk_disable(pcchip->clk);
 			pcchip->clk_enabled = false;
 		}
 		return 0;
-	} else if (!pwm->state.enabled) {
+	}
+
+	if (pcchip->gpiod)
+		pinctrl_select_state(pcchip->pinctrl, pcchip->pins_default);
+
+	if (!pcchip->clk_enabled) {
 		ret = clk_enable(pcchip->clk);
 		if (ret)
 			return ret;
@@ -97,6 +128,35 @@ static int pwm_clk_probe(struct platform_device *pdev)
 		return dev_err_probe(&pdev->dev, PTR_ERR(pcchip->clk),
 				     "Failed to get clock\n");
 
+	pcchip->pinctrl = devm_pinctrl_get(&pdev->dev);
+	if (IS_ERR(pcchip->pinctrl)) {
+		ret = PTR_ERR(pcchip->pinctrl);
+		pcchip->pinctrl = NULL;
+		if (ret == -EPROBE_DEFER)
+			return ret;
+	} else {
+		pcchip->pins_default = pinctrl_lookup_state(pcchip->pinctrl,
+							    PINCTRL_STATE_DEFAULT);
+		pcchip->pins_gpio = pinctrl_lookup_state(pcchip->pinctrl,
+							 "gpio");
+		if (IS_ERR(pcchip->pins_default) || IS_ERR(pcchip->pins_gpio))
+			pcchip->pinctrl = NULL;
+	}
+
+	/*
+	 * Switch to GPIO pinctrl state before requesting the GPIO.
+	 * The driver core has already applied the "default" state, which
+	 * muxes the pin to the clock function and claims it.  We must
+	 * release that claim first so that gpiolib can request the pin.
+	 */
+	if (pcchip->pinctrl)
+		pinctrl_select_state(pcchip->pinctrl, pcchip->pins_gpio);
+
+	pcchip->gpiod = devm_gpiod_get_optional(&pdev->dev, NULL, GPIOD_ASIS);
+	if (IS_ERR(pcchip->gpiod))
+		return dev_err_probe(&pdev->dev, PTR_ERR(pcchip->gpiod),
+				     "Failed to get gpio\n");
+
 	chip->ops = &pwm_clk_ops;
 
 	ret = pwmchip_add(chip);

-- 
2.53.0