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

Michael Walle
2020-09-14T21:46:02Z

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@walle.cc>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
---
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@walle.cc>
+ *
+ * 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@walle.cc>");
+MODULE_LICENSE("GPL");
--
2.20.1