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    Subject[PATCH v4 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC
    Date
    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>
    ---

    v4:
    - Document hardware/driver limitations
    - Only accept normal polarity
    - Fix a typo ("zone" -> "zero")
    - change MAX_PERIOD_NS to 0xffff * 125
    - Clamp period to the maximum rather than returning an error
    - Rename private struct pointer to priv
    - Rearrage control flow in _probe to save a few lines and a temporary variable
    - Add missing MODULE_ALIAS line
    - Spell out ODM

    v3:
    - https://lore.kernel.org/lkml/20200924192455.2484005-5-j.neuschaefer@gmx.net/
    - 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 | 166 ++++++++++++++++++++++++++++++++++++++++
    3 files changed, 175 insertions(+)
    create mode 100644 drivers/pwm/pwm-ntxec.c

    diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
    index 63be5362fd3a5..815f329ed5b46 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 original
    + design manufacturer 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 cbdcd55d69eef..1deb29e6ae8e5 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..4f4f736d71aba
    --- /dev/null
    +++ b/drivers/pwm/pwm-ntxec.c
    @@ -0,0 +1,166 @@
    +// 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 <j.neuschaefer@gmx.net>
    + *
    + * 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.
    + */
    +
    +#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_priv(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_apply(struct pwm_chip *chip, struct pwm_device *pwm_dev,
    + const struct pwm_state *state)
    +{
    + struct ntxec_pwm *priv = pwmchip_to_priv(pwm_dev->chip);
    + unsigned int duty = state->duty_cycle;
    + unsigned int period = state->period;
    + int res = 0;
    +
    + if (state->polarity != PWM_POLARITY_NORMAL)
    + return -EINVAL;
    +
    + if (period > MAX_PERIOD_NS) {
    + period = MAX_PERIOD_NS;
    +
    + if (duty > period)
    + duty = period;
    + }
    +
    + period /= TIME_BASE_NS;
    + duty /= TIME_BASE_NS;
    +
    + res = regmap_write(priv->ec->regmap, NTXEC_REG_PERIOD_HIGH, ntxec_reg8(period >> 8));
    + if (res)
    + return res;
    +
    + res = regmap_write(priv->ec->regmap, NTXEC_REG_PERIOD_LOW, ntxec_reg8(period));
    + if (res)
    + return res;
    +
    + res = regmap_write(priv->ec->regmap, NTXEC_REG_DUTY_HIGH, ntxec_reg8(duty >> 8));
    + if (res)
    + return res;
    +
    + res = regmap_write(priv->ec->regmap, NTXEC_REG_DUTY_LOW, ntxec_reg8(duty));
    + if (res)
    + return res;
    +
    + /*
    + * 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.
    + */
    + if (state->enabled && duty != 0) {
    + 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 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 *priv;
    + struct pwm_chip *chip;
    +
    + 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 <j.neuschaefer@gmx.net>");
    +MODULE_DESCRIPTION("PWM driver for Netronix EC");
    +MODULE_LICENSE("GPL");
    +MODULE_ALIAS("platform:ntxec-pwm");
    --
    2.29.2
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