[PATCH v3 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC

["Jonathan Neuschäfer" <j.neuschaefer@gmx.net>]

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 <j.neuschaefer@gmx.net>
---

v3:
- Relicense as GPLv2 or later
- Add email address to copyright line
- Remove OF compatible string and don't include linux/of_device.h
- Fix bogus ?: in return line
- Don't use a comma after sentinels
- Avoid ret |= ... pattern
- Move 8-bit register conversion to ntxec.h

v2:
- https://lore.kernel.org/lkml/20200905133230.1014581-6-j.neuschaefer@gmx.net/
- Various grammar and style improvements, as suggested by Uwe Kleine-König,
  Lee Jones, and Alexandre Belloni
- Switch to regmap
- Prefix registers with NTXEC_REG_
- Add help text to the Kconfig option
- Use the .apply callback instead of the old API
- Add a #define for the time base (125ns)
- Don't change device state in .probe; this avoids multiple problems
- Rework division and overflow check logic to perform divisions in 32 bits
- Avoid setting duty cycle to zero, to work around a hardware quirk
---
 drivers/pwm/Kconfig     |   8 ++
 drivers/pwm/Makefile    |   1 +
 drivers/pwm/pwm-ntxec.c | 161 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 drivers/pwm/pwm-ntxec.c

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 7dbcf6973d335..530dfda38d65e 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -350,6 +350,14 @@ 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 ODM
+	  Netronix.
+
 config PWM_OMAP_DMTIMER
 	tristate "OMAP Dual-Mode Timer PWM support"
 	depends on OF
diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index 2c2ba0a035577..1cc50dba22d1b 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -32,6 +32,7 @@ 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
diff --git a/drivers/pwm/pwm-ntxec.c b/drivers/pwm/pwm-ntxec.c
new file mode 100644
index 0000000000000..50da2dc14bb03
--- /dev/null
+++ b/drivers/pwm/pwm-ntxec.c
@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The Netronix embedded controller is a microcontroller found in some
+ * e-book readers designed by the ODM Netronix, Inc. It contains RTC,
+ * battery monitoring, system power management, and PWM functionality.
+ *
+ * This driver implements PWM output.
+ *
+ * Copyright 2020 Jonathan Neuschäfer <j.neuschaefer@gmx.net>
+ */
+
+#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 *pwmchip_to_pwm(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 * 0x10000 - 1)
+
+static int ntxec_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm_dev,
+			   const struct pwm_state *state)
+{
+	struct ntxec_pwm *pwm = pwmchip_to_pwm(pwm_dev->chip);
+	unsigned int duty = state->duty_cycle;
+	unsigned int period = state->period;
+	int res = 0;
+
+	if (period > MAX_PERIOD_NS) {
+		dev_warn(pwm->dev,
+			 "Period is not representable in 16 bits after division by %u: %u\n",
+			 TIME_BASE_NS, period);
+		return -ERANGE;
+	}
+
+	period /= TIME_BASE_NS;
+	duty /= TIME_BASE_NS;
+
+	res = regmap_write(pwm->ec->regmap, NTXEC_REG_PERIOD_HIGH, ntxec_reg8(period >> 8));
+	if (res)
+		return res;
+
+	res = regmap_write(pwm->ec->regmap, NTXEC_REG_PERIOD_LOW, ntxec_reg8(period));
+	if (res)
+		return res;
+
+	res = regmap_write(pwm->ec->regmap, NTXEC_REG_DUTY_HIGH, ntxec_reg8(duty >> 8));
+	if (res)
+		return res;
+
+	res = regmap_write(pwm->ec->regmap, NTXEC_REG_DUTY_LOW, ntxec_reg8(duty));
+	if (res)
+		return res;
+
+	/*
+	 * Writing a duty cycle of zone 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.
+	 */
+	if (state->enabled && duty != 0) {
+		res = regmap_write(pwm->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(1));
+		if (res)
+			return res;
+
+		/* Disable the auto-off timer */
+		res = regmap_write(pwm->ec->regmap, NTXEC_REG_AUTO_OFF_HI, ntxec_reg8(0xff));
+		if (res)
+			return res;
+
+		return regmap_write(pwm->ec->regmap, NTXEC_REG_AUTO_OFF_LO, ntxec_reg8(0xff));
+	} else {
+		return regmap_write(pwm->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(0));
+	}
+}
+
+static struct pwm_ops ntxec_pwm_ops = {
+	.apply = ntxec_pwm_apply,
+	.owner = THIS_MODULE,
+};
+
+static int ntxec_pwm_probe(struct platform_device *pdev)
+{
+	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
+	struct ntxec_pwm *pwm;
+	struct pwm_chip *chip;
+	int res;
+
+	pwm = devm_kzalloc(&pdev->dev, sizeof(*pwm), GFP_KERNEL);
+	if (!pwm)
+		return -ENOMEM;
+
+	pwm->ec = ec;
+	pwm->dev = &pdev->dev;
+
+	chip = &pwm->chip;
+	chip->dev = &pdev->dev;
+	chip->ops = &ntxec_pwm_ops;
+	chip->base = -1;
+	chip->npwm = 1;
+
+	res = pwmchip_add(chip);
+	if (res < 0)
+		return res;
+
+	platform_set_drvdata(pdev, pwm);
+
+	return 0;
+}
+
+static int ntxec_pwm_remove(struct platform_device *pdev)
+{
+	struct ntxec_pwm *pwm = platform_get_drvdata(pdev);
+	struct pwm_chip *chip = &pwm->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 <j.neuschaefer@gmx.net>");
+MODULE_DESCRIPTION("PWM driver for Netronix EC");
+MODULE_LICENSE("GPL");
--
2.28.0