[PATCH v2 1/5] pwm: core: add pwm_get_waveform_step_might_sleep()

[Nicolae-Daniel Deaconescu]

A consumer that has to meet minimum times, or place an edge relative to
the cycles of a clock that a PWM gates, cannot do that with the waveform
API alone. The rounding functions round down, so a minimum has to be
requested with some margin, and the size of that margin is the step of
the PWM, which only the provider knows.

A provider can report the rate of the clock its timing is derived from
and how many cycles of that clock one step lasts. A consumer can then
work in whole steps, convert them to nanoseconds rounding up, and get
that waveform back from the hardware. Without this, a consumer has to
read the rate from a clock it gets on its own, which describes
a relationship the hardware doesn't have, and is wrong if
PWM divides the clock.

Signed-off-by: Nicolae-Daniel Deaconescu <Nicolae-daniel.Deaconescu@xxxxxxxxxx>
---
 drivers/pwm/core.c  | 63 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/pwm.h | 32 +++++++++++++++++++++++
 2 files changed, 95 insertions(+)

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index 2a050ff4608b..6184f17497f0 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -325,6 +325,66 @@ int pwm_get_waveform_might_sleep(struct pwm_device *pwm, struct pwm_waveform *wf
 }
 EXPORT_SYMBOL_GPL(pwm_get_waveform_might_sleep);
 
+/**
+ * pwm_get_waveform_step_might_sleep - Query the granularity of the waveforms
+ * Cannot be used in atomic context.
+ * @pwm: PWM device
+ * @step: output parameter
+ *
+ * Stores in @step the clock rate the timing of @pwm is derived from and how
+ * many cycles of that clock one step lasts. Period, duty length and duty
+ * offset are multiples of that step, so a waveform asked for in whole steps
+ * is not changed by rounding. Compute the length in nanoseconds of n steps
+ * as n * @step->clk_cycles_per_step * NSEC_PER_SEC / @step->clk_rate_hz and
+ * round it up, because pwm_round_waveform_might_sleep() and
+ * pwm_set_waveform_might_sleep() round down.
+ *
+ * The values can change when a clock changes its rate, so they are only valid
+ * as long as the provider's input clocks are not reconfigured.
+ *
+ * Returns: 0 on success, -EOPNOTSUPP if the provider doesn't report it or a
+ * negative errno
+ * Context: May sleep.
+ */
+int pwm_get_waveform_step_might_sleep(struct pwm_device *pwm,
+				      struct pwm_waveform_step *step)
+{
+	struct pwm_chip *chip = pwm->chip;
+	const struct pwm_ops *ops = chip->ops;
+	int ret;
+
+	if (!pwmchip_supports_waveform(chip) || !ops->get_waveform_step)
+		return -EOPNOTSUPP;
+
+	might_sleep();
+
+	guard(pwmchip)(chip);
+
+	if (!chip->operational)
+		return -ENODEV;
+
+	if (IS_ENABLED(CONFIG_PWM_DEBUG) && chip->atomic) {
+		/*
+		 * Catch any drivers that have been marked as atomic but
+		 * that will sleep anyway.
+		 */
+		non_block_start();
+		ret = ops->get_waveform_step(chip, pwm, step);
+		non_block_end();
+	} else {
+		ret = ops->get_waveform_step(chip, pwm, step);
+	}
+	if (ret)
+		return ret;
+
+	if (!step->clk_rate_hz || !step->clk_cycles_per_step ||
+	    step->clk_rate_hz > (u64)step->clk_cycles_per_step * NSEC_PER_SEC)
+		return -EINVAL;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(pwm_get_waveform_step_might_sleep);
+
 /* Called with the pwmchip lock held */
 static int __pwm_set_waveform(struct pwm_device *pwm,
 			      const struct pwm_waveform *wf,
@@ -1710,6 +1770,9 @@ static bool pwm_ops_check(const struct pwm_chip *chip)
 		if (!ops->apply)
 			return false;
 
+		if (ops->get_waveform_step)
+			return false;
+
 		if (IS_ENABLED(CONFIG_PWM_DEBUG) && !ops->get_state)
 			dev_warn(pwmchip_parent(chip),
 				 "Please implement the .get_state() callback\n");
diff --git a/include/linux/pwm.h b/include/linux/pwm.h
index b11ae91723f8..f6e943607542 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -76,6 +76,32 @@ struct pwm_waveform {
 	u64 duty_offset_ns;
 };
 
+/**
+ * struct pwm_waveform_step - granularity of the waveforms a PWM can generate
+ * @clk_rate_hz: rate of the clock the PWM's timing is derived from
+ * @clk_cycles_per_step: number of those clock cycles one step lasts
+ *
+ * The period, the duty length and the duty offset of a waveform are multiples
+ * of one step, which lasts @clk_cycles_per_step / @clk_rate_hz seconds. A PWM
+ * that divides its input clock in hardware has @clk_cycles_per_step > 1.
+ *
+ * The step is the same for every waveform and for the period, the duty length
+ * and the duty offset, so a provider whose granularity depends on the
+ * requested period (for example through a prescaler) can't describe itself
+ * with it. One step also has to last at least 1 ns, otherwise rounding a
+ * length in nanoseconds up can add a whole step.
+ *
+ * A consumer that needs a minimum time, or that has to place an edge relative
+ * to the cycles of the clock a PWM channel gates, can use this to ask for
+ * the next possible value instead of one that is rounded down, and to compute
+ * the frequency a rounded period results in without the error of the
+ * nanosecond resolution of struct pwm_waveform.
+ */
+struct pwm_waveform_step {
+	unsigned long clk_rate_hz;
+	unsigned int clk_cycles_per_step;
+};
+
 /*
  * struct pwm_state - state of a PWM channel
  * @period: PWM period (in nanoseconds)
@@ -287,6 +313,9 @@ struct pwm_capture {
  * @round_waveform_fromhw: convert a driver specific waveform presentation to struct pwm_waveform
  * @read_waveform: read driver specific waveform presentation from hardware
  * @write_waveform: write driver specific waveform presentation to hardware
+ * @get_waveform_step: optional, report the granularity of the waveforms. Called
+ *	with the chip locked, so it must not sleep on an atomic chip and should
+ *	report a cached clock rate instead of calling clk_get_rate()
  * @apply: atomically apply a new PWM config
  * @get_state: get the current PWM state.
  */
@@ -305,6 +334,8 @@ struct pwm_ops {
 			    void *wfhw);
 	int (*write_waveform)(struct pwm_chip *chip, struct pwm_device *pwm,
 			      const void *wfhw);
+	int (*get_waveform_step)(struct pwm_chip *chip, struct pwm_device *pwm,
+				 struct pwm_waveform_step *step);
 
 	int (*apply)(struct pwm_chip *chip, struct pwm_device *pwm,
 		     const struct pwm_state *state);
@@ -394,6 +425,7 @@ static inline void pwmchip_set_drvdata(struct pwm_chip *chip, void *data)
 /* PWM consumer APIs */
 int pwm_round_waveform_might_sleep(struct pwm_device *pwm, struct pwm_waveform *wf);
 int pwm_get_waveform_might_sleep(struct pwm_device *pwm, struct pwm_waveform *wf);
+int pwm_get_waveform_step_might_sleep(struct pwm_device *pwm, struct pwm_waveform_step *step);
 int pwm_set_waveform_might_sleep(struct pwm_device *pwm, const struct pwm_waveform *wf, bool exact);
 int pwm_apply_might_sleep(struct pwm_device *pwm, const struct pwm_state *state);
 int pwm_apply_atomic(struct pwm_device *pwm, const struct pwm_state *state);
-- 
2.34.1