[RFC PATCH v3 2/2] pwm: imx: Configure output to GPIO in disabled state

[VokÃÄ Michal]

Normally the PWM output is held LOW when PWM is disabled. This can cause
problems when inverted PWM signal polarity is needed. With this behavior
the connected circuit is fed by 100% duty cycle instead of being shut-off.

Allow users to define a "pwm" and a "gpio" pinctrl states. The pwm pinctrl
state is selected when PWM is enabled and the gpio pinctrl state is
selected when PWM is disabled. In the gpio state the new pwm-gpios GPIO is
configured as input and the internal pull-up resistor is used to pull the
output level high.

If all the pinctrl states and the pwm-gpios GPIO are not correctly
specified in DT the PWM work as usual.

As an example, with this patch a PWM controlled backlight with inversed
signal polarity can be used in full brightness range. Without this patch
the backlight can not be turned off as brightness = 0 disables the PWM
and that in turn set PWM output LOW, that is full brightness.

Inverted output of the PWM with "default" and with "pwm"+"gpio" pinctrl:

+--------------+------------+---------------+----------- +-------------+
| After reset  | Bootloader |   PWM probe   |     PWM    |     PWM     |
| 100k pull-up |            |               | enable 30% |   disable   |
+--------------+------------+---------------+------------+-------------+
| pinctrl      |    none    |    default    |   default  |   default   |
| out H   __________________                   __   __                 |
| out L                     \_________________/  \_/  \_/\____________ |
|                                           ^    ^    ^                |
+--------------+------------+---------------+------------+-------------+
| pinctrl      |    none    |      gpio     |     pwm    |     gpio    |
| out H   __________________________________   __   __   _____________ |
| out L                                     \_/  \_/  \_/              |
|                                           ^    ^    ^                |
+----------------------------------------------------------------------+

Signed-off-by: Michal VokÃÄ <michal.vokac@xxxxxxxxx>
---
Changes in v3:
 - Commit message update.
 - Minor fix in code comment (Uwe)
 - Align function arguments to the opening parentheses. (Uwe)
 - Do not test devm_pinctrl_get for NULL. (Thierry)
 - Convert all messages to dev_dbg() (Thierry)
 - Do not actively drive the pin in gpio state. Configure it as input
   and rely solely on the internal pull-up. (Thierry)

Changes in v2:
 - Utilize the "pwm" and "gpio" pinctrl states.
 - Use the pwm-gpios signal to drive the output in "gpio" pinctrl state.
 - Select the right pinctrl state in probe.

 drivers/pwm/pwm-imx.c | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
index 1d5242c..84eaec8 100644
--- a/drivers/pwm/pwm-imx.c
+++ b/drivers/pwm/pwm-imx.c
@@ -12,10 +12,12 @@
 #include <linux/err.h>
 #include <linux/clk.h>
 #include <linux/delay.h>
+#include <linux/gpio/consumer.h>
 #include <linux/io.h>
 #include <linux/pwm.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
+#include <linux/pinctrl/consumer.h>
 
 /* i.MX1 and i.MX21 share the same PWM function block: */
 
@@ -52,10 +54,44 @@ struct imx_chip {
 	void __iomem	*mmio_base;
 
 	struct pwm_chip	chip;
+
+	struct pinctrl *pinctrl;
+	struct pinctrl_state *pinctrl_pins_gpio;
+	struct pinctrl_state *pinctrl_pins_pwm;
+	struct gpio_desc *pwm_gpiod;
 };
 
 #define to_imx_chip(chip)	container_of(chip, struct imx_chip, chip)
 
+static int imx_pwm_init_pinctrl_info(struct imx_chip *imx_chip,
+		struct platform_device *pdev)
+{
+	imx_chip->pinctrl = devm_pinctrl_get(&pdev->dev);
+	if (IS_ERR(imx_chip->pinctrl)) {
+		dev_dbg(&pdev->dev, "can not get pinctrl\n");
+		return PTR_ERR(imx_chip->pinctrl);
+	}
+
+	imx_chip->pinctrl_pins_pwm = pinctrl_lookup_state(imx_chip->pinctrl,
+							  "pwm");
+	imx_chip->pinctrl_pins_gpio = pinctrl_lookup_state(imx_chip->pinctrl,
+							   "gpio");
+	imx_chip->pwm_gpiod = devm_gpiod_get_optional(&pdev->dev, "pwm",
+						      GPIOD_IN);
+
+	if (PTR_ERR(imx_chip->pwm_gpiod) == -EPROBE_DEFER) {
+		return -EPROBE_DEFER;
+	} else if (IS_ERR(imx_chip->pwm_gpiod) ||
+		   IS_ERR(imx_chip->pinctrl_pins_pwm) ||
+		   IS_ERR(imx_chip->pinctrl_pins_gpio)) {
+		dev_dbg(&pdev->dev, "PWM pinctrl information incomplete\n");
+		devm_pinctrl_put(imx_chip->pinctrl);
+		imx_chip->pinctrl = NULL;
+	}
+
+	return 0;
+}
+
 static int imx_pwm_config_v1(struct pwm_chip *chip,
 		struct pwm_device *pwm, int duty_ns, int period_ns)
 {
@@ -216,7 +252,25 @@ static int imx_pwm_apply_v2(struct pwm_chip *chip, struct pwm_device *pwm,
 			cr |= MX3_PWMCR_POUTC;
 
 		writel(cr, imx->mmio_base + MX3_PWMCR);
+
+		/*
+		 * If we are in charge of pinctrl then switch output to
+		 * the PWM signal.
+		 */
+		if (imx->pinctrl)
+			pinctrl_select_state(imx->pinctrl,
+					     imx->pinctrl_pins_pwm);
 	} else if (cstate.enabled) {
+		/*
+		 * PWM block will be disabled. Normally its output will be set
+		 * low no matter what output polarity is configured. Let's use
+		 * pinctrl to switch the output pin to GPIO functon and keep
+		 * the output at the same level as for duty-cycle = 0.
+		 */
+		if (imx->pinctrl)
+			pinctrl_select_state(imx->pinctrl,
+					     imx->pinctrl_pins_gpio);
+
 		writel(0, imx->mmio_base + MX3_PWMCR);
 
 		clk_disable_unprepare(imx->clk_per);
@@ -263,6 +317,7 @@ static int imx_pwm_probe(struct platform_device *pdev)
 	const struct of_device_id *of_id =
 			of_match_device(imx_pwm_dt_ids, &pdev->dev);
 	const struct imx_pwm_data *data;
+	struct pwm_state cstate;
 	struct imx_chip *imx;
 	struct resource *r;
 	int ret = 0;
@@ -294,6 +349,10 @@ static int imx_pwm_probe(struct platform_device *pdev)
 		imx->chip.of_pwm_n_cells = 3;
 	}
 
+	ret = imx_pwm_init_pinctrl_info(imx, pdev);
+	if (ret)
+		return ret;
+
 	r = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	imx->mmio_base = devm_ioremap_resource(&pdev->dev, r);
 	if (IS_ERR(imx->mmio_base))
@@ -303,6 +362,24 @@ static int imx_pwm_probe(struct platform_device *pdev)
 	if (ret < 0)
 		return ret;
 
+	if (imx->pinctrl) {
+		/*
+		 * Update cstate after pwmchip_add() call as the core might
+		 * call the get_state() function to read the PWM registers
+		 * to get the actual HWÂstate.
+		 */
+		pwm_get_state(imx->chip.pwms, &cstate);
+		if (cstate.enabled) {
+			dev_dbg(&pdev->dev,
+				"PWM entered probe in enabled state\n");
+			pinctrl_select_state(imx->pinctrl,
+					     imx->pinctrl_pins_pwm);
+		} else {
+			pinctrl_select_state(imx->pinctrl,
+					     imx->pinctrl_pins_gpio);
+		}
+	}
+
 	platform_set_drvdata(pdev, imx);
 	return 0;
 }
-- 
2.1.4