[PATCH v10 06/13] pwm: add support for sl28cpld PWM controller

[Michael Walle <michael@xxxxxxxx>]

Add support for the PWM controller of the sl28cpld board management
controller. This is part of a multi-function device driver.

The controller has one PWM channel and can just generate four distinct
frequencies.

Signed-off-by: Michael Walle <michael@xxxxxxxx>
Acked-by: Thierry Reding <thierry.reding@xxxxxxxxx>
---
Changes since v9:
 - fixed double whitespace in Kconfig
 - improved comment about division

Changes since v8:
 - none

Changes since v7:
 - added "depends on MFD_SL28CPLD || COMPILE_TEST"
 - corrected comment about divison
 - sanitize pwm values before handing them over to the PWM core
 - check wether we need to write the the duty cycle first to avoid
   glitches

Changes since v6:
 - added new row "period length"
 - fixed parenthesis mistake in the description of a calculation
 - added sl28cpld_pwm_{read/write}()
 - added more error messages

Changes since v5:
 - added brief description of the PWM hardware implementation
 - added hardware limitations
 - dropped the frequency mode table, instead calculate the prescaler
   value on the fly.
 - round the requested parameters instead of support just distinct
   periods.
 - prefix the macros by SL28CPLD_ to make them less generic
 - set polarity to PWM_POLARITY_NORMAL and reject inverted polarity
   requests.
 - apply the workaround just for prescaler value of 0.
 - make errors during probing more verbose

Changes since v4:
 - update copyright year
 - remove #include <linux/of_device.h>, suggested by Andy.
 - make the pwm mode table look nicer, suggested by Lee.
 - use dev_get_drvdata(chip->dev) instead of container_of(), suggested by
   Lee.
 - use whole sentence in comments, suggested by Lee.
 - renamed the local "struct sl28cpld_pwm" variable to "priv" everywhere,
   suggested by Lee.
 - use pwm_{get,set}_relative_duty_cycle(), suggested by Andy.
 - make the comment about the 250Hz hardware limitation clearer
 - don't use "if (ret < 0)", but only "if (ret)", suggested by Andy.
 - don't use KBUID_MODNAME
 - remove comma in terminator line of the compatible strings list
 - remove the platform device table

Changes since v3:
 - see cover letter

 drivers/pwm/Kconfig        |  10 ++
 drivers/pwm/Makefile       |   1 +
 drivers/pwm/pwm-sl28cpld.c | 270 +++++++++++++++++++++++++++++++++++++
 3 files changed, 281 insertions(+)
 create mode 100644 drivers/pwm/pwm-sl28cpld.c

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 9448e4ca8c73..63be5362fd3a 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -428,6 +428,16 @@ config PWM_SIFIVE
 	  To compile this driver as a module, choose M here: the module
 	  will be called pwm-sifive.
 
+config PWM_SL28CPLD
+	tristate "Kontron sl28cpld PWM support"
+	depends on MFD_SL28CPLD || COMPILE_TEST
+	help
+	  Generic PWM framework driver for board management controller
+	  found on the Kontron sl28 CPLD.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called pwm-sl28cpld.
+
 config PWM_SPEAR
 	tristate "STMicroelectronics SPEAr PWM support"
 	depends on PLAT_SPEAR || COMPILE_TEST
diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index 2c2ba0a03557..cbdcd55d69ee 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -40,6 +40,7 @@ obj-$(CONFIG_PWM_RENESAS_TPU)	+= pwm-renesas-tpu.o
 obj-$(CONFIG_PWM_ROCKCHIP)	+= pwm-rockchip.o
 obj-$(CONFIG_PWM_SAMSUNG)	+= pwm-samsung.o
 obj-$(CONFIG_PWM_SIFIVE)	+= pwm-sifive.o
+obj-$(CONFIG_PWM_SL28CPLD)	+= pwm-sl28cpld.o
 obj-$(CONFIG_PWM_SPEAR)		+= pwm-spear.o
 obj-$(CONFIG_PWM_SPRD)		+= pwm-sprd.o
 obj-$(CONFIG_PWM_STI)		+= pwm-sti.o
diff --git a/drivers/pwm/pwm-sl28cpld.c b/drivers/pwm/pwm-sl28cpld.c
new file mode 100644
index 000000000000..5046b6b7fd35
--- /dev/null
+++ b/drivers/pwm/pwm-sl28cpld.c
@@ -0,0 +1,270 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * sl28cpld PWM driver
+ *
+ * Copyright (c) 2020 Michael Walle <michael@xxxxxxxx>
+ *
+ * There is no public datasheet available for this PWM core. But it is easy
+ * enough to be briefly explained. It consists of one 8-bit counter. The PWM
+ * supports four distinct frequencies by selecting when to reset the counter.
+ * With the prescaler setting you can select which bit of the counter is used
+ * to reset it. This implies that the higher the frequency the less remaining
+ * bits are available for the actual counter.
+ *
+ * Let cnt[7:0] be the counter, clocked at 32kHz:
+ * +-----------+--------+--------------+-----------+---------------+
+ * | prescaler |  reset | counter bits | frequency | period length |
+ * +-----------+--------+--------------+-----------+---------------+
+ * |         0 | cnt[7] |     cnt[6:0] |    250 Hz |    4000000 ns |
+ * |         1 | cnt[6] |     cnt[5:0] |    500 Hz |    2000000 ns |
+ * |         2 | cnt[5] |     cnt[4:0] |     1 kHz |    1000000 ns |
+ * |         3 | cnt[4] |     cnt[3:0] |     2 kHz |     500000 ns |
+ * +-----------+--------+--------------+-----------+---------------+
+ *
+ * Limitations:
+ * - The hardware cannot generate a 100% duty cycle if the prescaler is 0.
+ * - The hardware cannot atomically set the prescaler and the counter value,
+ *   which might lead to glitches and inconsistent states if a write fails.
+ * - The counter is not reset if you switch the prescaler which leads
+ *   to glitches, too.
+ * - The duty cycle will switch immediately and not after a complete cycle.
+ * - Depending on the actual implementation, disabling the PWM might have
+ *   side effects. For example, if the output pin is shared with a GPIO pin
+ *   it will automatically switch back to GPIO mode.
+ */
+
+#include <linux/bitfield.h>
+#include <linux/kernel.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/pwm.h>
+#include <linux/regmap.h>
+
+/*
+ * PWM timer block registers.
+ */
+#define SL28CPLD_PWM_CTRL			0x00
+#define   SL28CPLD_PWM_CTRL_ENABLE		BIT(7)
+#define   SL28CPLD_PWM_CTRL_PRESCALER_MASK	GENMASK(1, 0)
+#define SL28CPLD_PWM_CYCLE			0x01
+#define   SL28CPLD_PWM_CYCLE_MAX		GENMASK(6, 0)
+
+#define SL28CPLD_PWM_CLK			32000 /* 32 kHz */
+#define SL28CPLD_PWM_MAX_DUTY_CYCLE(prescaler)	(1 << (7 - (prescaler)))
+#define SL28CPLD_PWM_PERIOD(prescaler) \
+	(NSEC_PER_SEC / SL28CPLD_PWM_CLK * SL28CPLD_PWM_MAX_DUTY_CYCLE(prescaler))
+
+/*
+ * We calculate the duty cycle like this:
+ *   duty_cycle_ns = pwm_cycle_reg * max_period_ns / max_duty_cycle
+ *
+ * With
+ *   max_period_ns = 1 << (7 - prescaler) / SL28CPLD_PWM_CLK * NSEC_PER_SEC
+ *   max_duty_cycle = 1 << (7 - prescaler)
+ * this then simplifies to:
+ *   duty_cycle_ns = pwm_cycle_reg / SL28CPLD_PWM_CLK * NSEC_PER_SEC
+ *                 = NSEC_PER_SEC / SL28CPLD_PWM_CLK * pwm_cycle_reg
+ *
+ * NSEC_PER_SEC is a multiple of SL28CPLD_PWM_CLK, therefore we're not losing
+ * precision by doing the divison first.
+ */
+#define SL28CPLD_PWM_TO_DUTY_CYCLE(reg) \
+	(NSEC_PER_SEC / SL28CPLD_PWM_CLK * (reg))
+#define SL28CPLD_PWM_FROM_DUTY_CYCLE(duty_cycle) \
+	(DIV_ROUND_DOWN_ULL((duty_cycle), NSEC_PER_SEC / SL28CPLD_PWM_CLK))
+
+#define sl28cpld_pwm_read(priv, reg, val) \
+	regmap_read((priv)->regmap, (priv)->offset + (reg), (val))
+#define sl28cpld_pwm_write(priv, reg, val) \
+	regmap_write((priv)->regmap, (priv)->offset + (reg), (val))
+
+struct sl28cpld_pwm {
+	struct pwm_chip pwm_chip;
+	struct regmap *regmap;
+	u32 offset;
+};
+
+static void sl28cpld_pwm_get_state(struct pwm_chip *chip,
+				   struct pwm_device *pwm,
+				   struct pwm_state *state)
+{
+	struct sl28cpld_pwm *priv = dev_get_drvdata(chip->dev);
+	unsigned int reg;
+	int prescaler;
+
+	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CTRL, &reg);
+
+	state->enabled = reg & SL28CPLD_PWM_CTRL_ENABLE;
+
+	prescaler = FIELD_GET(SL28CPLD_PWM_CTRL_PRESCALER_MASK, reg);
+	state->period = SL28CPLD_PWM_PERIOD(prescaler);
+
+	sl28cpld_pwm_read(priv, SL28CPLD_PWM_CYCLE, &reg);
+	state->duty_cycle = SL28CPLD_PWM_TO_DUTY_CYCLE(reg);
+	state->polarity = PWM_POLARITY_NORMAL;
+
+	/*
+	 * Sanitize values for the PWM core. Depending on the prescaler it
+	 * might happen that we calculate a duty_cycle greater than the actual
+	 * period. This might happen if someone (e.g. the bootloader) sets an
+	 * invalid combination of values. The behavior of the hardware is
+	 * undefined in this case. But we need to report sane values back to
+	 * the PWM core.
+	 */
+	state->duty_cycle = min(state->duty_cycle, state->period);
+}
+
+static int sl28cpld_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
+			      const struct pwm_state *state)
+{
+	struct sl28cpld_pwm *priv = dev_get_drvdata(chip->dev);
+	unsigned int cycle, prescaler;
+	bool write_duty_cycle_first;
+	int ret;
+	u8 ctrl;
+
+	/* Polarity inversion is not supported */
+	if (state->polarity != PWM_POLARITY_NORMAL)
+		return -EINVAL;
+
+	/*
+	 * Calculate the prescaler. Pick the biggest period that isn't
+	 * bigger than the requested period.
+	 */
+	prescaler = DIV_ROUND_UP_ULL(SL28CPLD_PWM_PERIOD(0), state->period);
+	prescaler = order_base_2(prescaler);
+
+	if (prescaler > field_max(SL28CPLD_PWM_CTRL_PRESCALER_MASK))
+		return -ERANGE;
+
+	ctrl = FIELD_PREP(SL28CPLD_PWM_CTRL_PRESCALER_MASK, prescaler);
+	if (state->enabled)
+		ctrl |= SL28CPLD_PWM_CTRL_ENABLE;
+
+	cycle = SL28CPLD_PWM_FROM_DUTY_CYCLE(state->duty_cycle);
+	cycle = min_t(unsigned int, cycle, SL28CPLD_PWM_MAX_DUTY_CYCLE(prescaler));
+
+	/*
+	 * Work around the hardware limitation. See also above. Trap 100% duty
+	 * cycle if the prescaler is 0. Set prescaler to 1 instead. We don't
+	 * care about the frequency because its "all-one" in either case.
+	 *
+	 * We don't need to check the actual prescaler setting, because only
+	 * if the prescaler is 0 we can have this particular value.
+	 */
+	if (cycle == SL28CPLD_PWM_MAX_DUTY_CYCLE(0)) {
+		ctrl &= ~SL28CPLD_PWM_CTRL_PRESCALER_MASK;
+		ctrl |= FIELD_PREP(SL28CPLD_PWM_CTRL_PRESCALER_MASK, 1);
+		cycle = SL28CPLD_PWM_MAX_DUTY_CYCLE(1);
+	}
+
+	/*
+	 * To avoid glitches when we switch the prescaler, we have to make sure
+	 * we have a valid duty cycle for the new mode.
+	 *
+	 * Take the current prescaler (or the current period length) into
+	 * account to decide whether we have to write the duty cycle or the new
+	 * prescaler first. If the period length is decreasing we have to
+	 * write the duty cycle first.
+	 */
+	write_duty_cycle_first = pwm->state.period > state->period;
+
+	if (write_duty_cycle_first) {
+		ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CYCLE, cycle);
+		if (ret)
+			return ret;
+	}
+
+	ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CTRL, ctrl);
+	if (ret)
+		return ret;
+
+	if (!write_duty_cycle_first) {
+		ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CYCLE, cycle);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static const struct pwm_ops sl28cpld_pwm_ops = {
+	.apply = sl28cpld_pwm_apply,
+	.get_state = sl28cpld_pwm_get_state,
+	.owner = THIS_MODULE,
+};
+
+static int sl28cpld_pwm_probe(struct platform_device *pdev)
+{
+	struct sl28cpld_pwm *priv;
+	struct pwm_chip *chip;
+	int ret;
+
+	if (!pdev->dev.parent) {
+		dev_err(&pdev->dev, "no parent device\n");
+		return -ENODEV;
+	}
+
+	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->regmap = dev_get_regmap(pdev->dev.parent, NULL);
+	if (!priv->regmap) {
+		dev_err(&pdev->dev, "could not get parent regmap\n");
+		return -ENODEV;
+	}
+
+	ret = device_property_read_u32(&pdev->dev, "reg", &priv->offset);
+	if (ret) {
+		dev_err(&pdev->dev, "no 'reg' property found (%pe)\n",
+			ERR_PTR(ret));
+		return -EINVAL;
+	}
+
+	/* Initialize the pwm_chip structure */
+	chip = &priv->pwm_chip;
+	chip->dev = &pdev->dev;
+	chip->ops = &sl28cpld_pwm_ops;
+	chip->base = -1;
+	chip->npwm = 1;
+
+	ret = pwmchip_add(&priv->pwm_chip);
+	if (ret) {
+		dev_err(&pdev->dev, "failed to add PWM chip (%pe)",
+			ERR_PTR(ret));
+		return ret;
+	}
+
+	platform_set_drvdata(pdev, priv);
+
+	return 0;
+}
+
+static int sl28cpld_pwm_remove(struct platform_device *pdev)
+{
+	struct sl28cpld_pwm *priv = platform_get_drvdata(pdev);
+
+	return pwmchip_remove(&priv->pwm_chip);
+}
+
+static const struct of_device_id sl28cpld_pwm_of_match[] = {
+	{ .compatible = "kontron,sl28cpld-pwm" },
+	{}
+};
+MODULE_DEVICE_TABLE(of, sl28cpld_pwm_of_match);
+
+static struct platform_driver sl28cpld_pwm_driver = {
+	.probe = sl28cpld_pwm_probe,
+	.remove	= sl28cpld_pwm_remove,
+	.driver = {
+		.name = "sl28cpld-pwm",
+		.of_match_table = sl28cpld_pwm_of_match,
+	},
+};
+module_platform_driver(sl28cpld_pwm_driver);
+
+MODULE_DESCRIPTION("sl28cpld PWM Driver");
+MODULE_AUTHOR("Michael Walle <michael@xxxxxxxx>");
+MODULE_LICENSE("GPL");
-- 
2.20.1