[PATCH V4] pwm: tegra: dynamic clk freq configuration by PWM driver

[Sandipan Patra <spatra@nvidia.com>]

Added support for dynamic clock freq configuration in pwm kernel driver.
Earlier the pwm driver used to cache boot time clock rate by pwm clock
parent during probe. Hence dynamically changing pwm frequency was not
possible for all the possible ranges. With this change, dynamic calculation
is enabled and it is able to set the requested period from sysfs knob
provided the value is supported by clock source.

Changes mainly have 2 parts:
  - T186 and later chips [1]
  - T210 and prior chips [2]

For [1] - Changes implemented to set pwm period dynamically and
          also checks added to allow only if requested period(ns) is
          below or equals to higher range.

For [2] - Only checks if the requested period(ns) is below or equals
          to higher range defined by max clock limit. The limitation
          in T210 or prior chips are due to the reason of having only
          one pwm-controller supporting multiple channels. But later
          chips have multiple pwm controller instances each having
	  single channel support.

Signed-off-by: Sandipan Patra <spatra@nvidia.com>
---
PATCH V4:
1. Code comments fixes

PATCH V3:
1. Return -EINVAL if requested period does not fall inside limit.
2. Store the new clock rate for further references.
3. Variable name change reverted.
4. Comments corrected and new comments are added.

PATCH V2:
1. Maximum frequency calculation is moved to probe.
2. Added descriptions for PWM register bits and functional behavior
   of the controller when new configuration is applied.
3. Setting period with possible value when supplied period is below limit.
4. Corrected the earlier code comment:
   plus 1 instead of minus 1 during pwm calculation

 drivers/pwm/pwm-tegra.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 76 insertions(+), 4 deletions(-)

diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
index d26ed8f..1daf591 100644
--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -4,8 +4,36 @@
  *
  * Tegra pulse-width-modulation controller driver
  *
- * Copyright (c) 2010, NVIDIA Corporation.
+ * Copyright (c) 2010-2020, NVIDIA Corporation.
  * Based on arch/arm/plat-mxc/pwm.c by Sascha Hauer <s.hauer@pengutronix.de>
+ *
+ * Overview of Tegra Pulse Width Modulator Register:
+ * 1. 13-bit: Frequency division (SCALE)
+ * 2. 8-bit : Pulse division (DUTY)
+ * 3. 1-bit : Enable bit
+ *
+ * The PWM clock frequency is divided by 256 before subdividing it based
+ * on the programmable frequency division value to generate the required
+ * frequency for PWM output. The maximum output frequency that can be
+ * achieved is (max rate of source clock) / 256.
+ * e.g. if source clock rate is 408 MHz, maximum output frequency can be:
+ * 408 MHz/256 = 1.6 MHz.
+ * This 1.6 MHz frequency can further be divided using SCALE value in PWM.
+ *
+ * PWM pulse width: 8 bits are usable [23:16] for varying pulse width.
+ * To achieve 100% duty cycle, program Bit [24] of this register to
+ * 1’b1. In which case the other bits [23:16] are set to don't care.
+ *
+ * Limitations:
+ * -	When PWM is disabled, the output is driven to inactive.
+ * -	It does not allow the current PWM period to complete and
+ *	stops abruptly.
+ *
+ * -	If the register is reconfigured while PWM is running,
+ *	it does not complete the currently running period.
+ *
+ * -	If the user input duty is beyond acceptible limits,
+ *	-EINVAL is returned.
  */
 
 #include <linux/clk.h>
@@ -41,6 +69,7 @@ struct tegra_pwm_chip {
 	struct reset_control*rst;
 
 	unsigned long clk_rate;
+	unsigned long min_period_ns;
 
 	void __iomem *regs;
 
@@ -68,7 +97,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 {
 	struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
 	unsigned long long c = duty_ns, hz;
-	unsigned long rate;
+	unsigned long rate, required_clk_rate;
 	u32 val = 0;
 	int err;
 
@@ -83,9 +112,47 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	val = (u32)c << PWM_DUTY_SHIFT;
 
 	/*
+	 *  min period = max clock limit >> PWM_DUTY_WIDTH
+	 */
+	if (period_ns < pc->min_period_ns)
+		return -EINVAL;
+
+	/*
 	 * Compute the prescaler value for which (1 << PWM_DUTY_WIDTH)
 	 * cycles at the PWM clock rate will take period_ns nanoseconds.
+	 *
+	 * num_channels: If single instance of PWM controller has multiple
+	 * channels (e.g. Tegra210 or older) then it is not possible to
+	 * configure separate clock rates to each of the channels, in such
+	 * case the value stored during probe will be referred.
+	 *
+	 * If every PWM controller instance has one channel respectively, i.e.
+	 * nums_channels == 1 then only the clock rate can be modified
+	 * dynamically (e.g. Tegra186 or Tegra194).
 	 */
+	if (pc->soc->num_channels == 1) {
+		/*
+		 * Rate is multiplied with 2^PWM_DUTY_WIDTH so that it matches
+		 * with the maximum possible rate that the controller can
+		 * provide. Any further lower value can be derived by setting
+		 * PFM bits[0:12].
+		 *
+		 * required_clk_rate is a reference rate for source clock and
+		 * it is derived based on user requested period. By setting the
+		 * source clock rate as required_clk_rate, PWM controller will
+		 * be able to configure the requested period.
+		 */
+		required_clk_rate =
+			(NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;
+
+		err = clk_set_rate(pc->clk, required_clk_rate);
+		if (err < 0)
+			return -EINVAL;
+
+		/* Store the new rate for further references */
+		pc->clk_rate = clk_get_rate(pc->clk);
+	}
+
 	rate = pc->clk_rate >> PWM_DUTY_WIDTH;
 
 	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
@@ -94,7 +161,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	/*
 	 * Since the actual PWM divider is the register's frequency divider
-	 * field minus 1, we need to decrement to get the correct value to
+	 * field plus 1, we need to decrement to get the correct value to
 	 * write to the register.
 	 */
 	if (rate > 0)
@@ -205,6 +272,10 @@ static int tegra_pwm_probe(struct platform_device *pdev)
 	 */
 	pwm->clk_rate = clk_get_rate(pwm->clk);
 
+	/* Set minimum limit of PWM period for the IP */
+	pwm->min_period_ns =
+	    (NSEC_PER_SEC / (pwm->soc->max_frequency >> PWM_DUTY_WIDTH)) + 1;
+
 	pwm->rst = devm_reset_control_get_exclusive(&pdev->dev, "pwm");
 	if (IS_ERR(pwm->rst)) {
 		ret = PTR_ERR(pwm->rst);
@@ -312,5 +383,6 @@ static struct platform_driver tegra_pwm_driver = {
 module_platform_driver(tegra_pwm_driver);
 
 MODULE_LICENSE("GPL");
-MODULE_AUTHOR("NVIDIA Corporation");
+MODULE_AUTHOR("Sandipan Patra <spatra@nvidia.com>");
+MODULE_DESCRIPTION("Tegra PWM controller driver");
 MODULE_ALIAS("platform:tegra-pwm");
-- 
2.7.4