pwm: ntxec: Add driver for PWM function in Netronix EC

The Netronix EC provides a PWM output which is used for the backlight
on some ebook readers. This patches adds a driver for the PWM output.

The .get_state callback is not implemented, because the PWM state can't
be read back from the hardware.

Signed-off-by: Jonathan Neuschäfer <[email protected]>
Reviewed-by: Uwe Kleine-König <[email protected]>
Acked-by: Thierry Reding <[email protected]>
Signed-off-by: Lee Jones <[email protected]>

drivers/pwm/Kconfig

@@ -393,6 +393,14 @@ config PWM_MXS
 	  To compile this driver as a module, choose M here: the module
 	  will be called pwm-mxs.
 
+config PWM_NTXEC
+	tristate "Netronix embedded controller PWM support"
+	depends on MFD_NTXEC
+	help
+	  Say yes here if you want to support the PWM output of the embedded
+	  controller found in certain e-book readers designed by the original
+	  design manufacturer Netronix.
+
 config PWM_OMAP_DMTIMER
 	tristate "OMAP Dual-Mode Timer PWM support"
 	depends on OF

drivers/pwm/Makefile

@@ -35,6 +35,7 @@ obj-$(CONFIG_PWM_MESON)		+= pwm-meson.o
 obj-$(CONFIG_PWM_MEDIATEK)	+= pwm-mediatek.o
 obj-$(CONFIG_PWM_MTK_DISP)	+= pwm-mtk-disp.o
 obj-$(CONFIG_PWM_MXS)		+= pwm-mxs.o
+obj-$(CONFIG_PWM_NTXEC)		+= pwm-ntxec.o
 obj-$(CONFIG_PWM_OMAP_DMTIMER)	+= pwm-omap-dmtimer.o
 obj-$(CONFIG_PWM_PCA9685)	+= pwm-pca9685.o
 obj-$(CONFIG_PWM_PXA)		+= pwm-pxa.o

drivers/pwm/pwm-ntxec.c

@@ -0,0 +1,184 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The Netronix embedded controller is a microcontroller found in some
+ * e-book readers designed by the original design manufacturer Netronix, Inc.
+ * It contains RTC, battery monitoring, system power management, and PWM
+ * functionality.
+ *
+ * This driver implements PWM output.
+ *
+ * Copyright 2020 Jonathan Neuschäfer <[email protected]>
+ *
+ * Limitations:
+ * - The get_state callback is not implemented, because the current state of
+ *   the PWM output can't be read back from the hardware.
+ * - The hardware can only generate normal polarity output.
+ * - The period and duty cycle can't be changed together in one atomic action.
+ */
+
+#include <linux/mfd/ntxec.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/pwm.h>
+#include <linux/regmap.h>
+#include <linux/types.h>
+
+struct ntxec_pwm {
+	struct device *dev;
+	struct ntxec *ec;
+	struct pwm_chip chip;
+};
+
+static struct ntxec_pwm *ntxec_pwm_from_chip(struct pwm_chip *chip)
+{
+	return container_of(chip, struct ntxec_pwm, chip);
+}
+
+#define NTXEC_REG_AUTO_OFF_HI	0xa1
+#define NTXEC_REG_AUTO_OFF_LO	0xa2
+#define NTXEC_REG_ENABLE	0xa3
+#define NTXEC_REG_PERIOD_LOW	0xa4
+#define NTXEC_REG_PERIOD_HIGH	0xa5
+#define NTXEC_REG_DUTY_LOW	0xa6
+#define NTXEC_REG_DUTY_HIGH	0xa7
+
+/*
+ * The time base used in the EC is 8MHz, or 125ns. Period and duty cycle are
+ * measured in this unit.
+ */
+#define TIME_BASE_NS 125
+
+/*
+ * The maximum input value (in nanoseconds) is determined by the time base and
+ * the range of the hardware registers that hold the converted value.
+ * It fits into 32 bits, so we can do our calculations in 32 bits as well.
+ */
+#define MAX_PERIOD_NS (TIME_BASE_NS * 0xffff)
+
+static int ntxec_pwm_set_raw_period_and_duty_cycle(struct pwm_chip *chip,
+						   int period, int duty)
+{
+	struct ntxec_pwm *priv = ntxec_pwm_from_chip(chip);
+
+	/*
+	 * Changes to the period and duty cycle take effect as soon as the
+	 * corresponding low byte is written, so the hardware may be configured
+	 * to an inconsistent state after the period is written and before the
+	 * duty cycle is fully written. If, in such a case, the old duty cycle
+	 * is longer than the new period, the EC may output 100% for a moment.
+	 *
+	 * To minimize the time between the changes to period and duty cycle
+	 * taking effect, the writes are interleaved.
+	 */
+
+	struct reg_sequence regs[] = {
+		{ NTXEC_REG_PERIOD_HIGH, ntxec_reg8(period >> 8) },
+		{ NTXEC_REG_DUTY_HIGH, ntxec_reg8(duty >> 8) },
+		{ NTXEC_REG_PERIOD_LOW, ntxec_reg8(period) },
+		{ NTXEC_REG_DUTY_LOW, ntxec_reg8(duty) },
+	};
+
+	return regmap_multi_reg_write(priv->ec->regmap, regs, ARRAY_SIZE(regs));
+}
+
+static int ntxec_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm_dev,
+			   const struct pwm_state *state)
+{
+	struct ntxec_pwm *priv = ntxec_pwm_from_chip(chip);
+	unsigned int period, duty;
+	int res;
+
+	if (state->polarity != PWM_POLARITY_NORMAL)
+		return -EINVAL;
+
+	period = min_t(u64, state->period, MAX_PERIOD_NS);
+	duty   = min_t(u64, state->duty_cycle, period);
+
+	period /= TIME_BASE_NS;
+	duty   /= TIME_BASE_NS;
+
+	/*
+	 * Writing a duty cycle of zero puts the device into a state where
+	 * writing a higher duty cycle doesn't result in the brightness that it
+	 * usually results in. This can be fixed by cycling the ENABLE register.
+	 *
+	 * As a workaround, write ENABLE=0 when the duty cycle is zero.
+	 * The case that something has previously set the duty cycle to zero
+	 * but ENABLE=1, is not handled.
+	 */
+	if (state->enabled && duty != 0) {
+		res = ntxec_pwm_set_raw_period_and_duty_cycle(chip, period, duty);
+		if (res)
+			return res;
+
+		res = regmap_write(priv->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(1));
+		if (res)
+			return res;
+
+		/* Disable the auto-off timer */
+		res = regmap_write(priv->ec->regmap, NTXEC_REG_AUTO_OFF_HI, ntxec_reg8(0xff));
+		if (res)
+			return res;
+
+		return regmap_write(priv->ec->regmap, NTXEC_REG_AUTO_OFF_LO, ntxec_reg8(0xff));
+	} else {
+		return regmap_write(priv->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(0));
+	}
+}
+
+static const struct pwm_ops ntxec_pwm_ops = {
+	.owner = THIS_MODULE,
+	.apply = ntxec_pwm_apply,
+	/*
+	 * No .get_state callback, because the current state cannot be read
+	 * back from the hardware.
+	 */
+};
+
+static int ntxec_pwm_probe(struct platform_device *pdev)
+{
+	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
+	struct ntxec_pwm *priv;
+	struct pwm_chip *chip;
+
+	pdev->dev.of_node = pdev->dev.parent->of_node;
+
+	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->ec = ec;
+	priv->dev = &pdev->dev;
+
+	platform_set_drvdata(pdev, priv);
+
+	chip = &priv->chip;
+	chip->dev = &pdev->dev;
+	chip->ops = &ntxec_pwm_ops;
+	chip->base = -1;
+	chip->npwm = 1;
+
+	return pwmchip_add(chip);
+}
+
+static int ntxec_pwm_remove(struct platform_device *pdev)
+{
+	struct ntxec_pwm *priv = platform_get_drvdata(pdev);
+	struct pwm_chip *chip = &priv->chip;
+
+	return pwmchip_remove(chip);
+}
+
+static struct platform_driver ntxec_pwm_driver = {
+	.driver = {
+		.name = "ntxec-pwm",
+	},
+	.probe = ntxec_pwm_probe,
+	.remove = ntxec_pwm_remove,
+};
+module_platform_driver(ntxec_pwm_driver);
+
+MODULE_AUTHOR("Jonathan Neuschäfer <[email protected]>");
+MODULE_DESCRIPTION("PWM driver for Netronix EC");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:ntxec-pwm");