Re: [v12 2/2] pwm: Add Aspeed ast2600 PWM support

From: Billy Tsai
Date: Fri Nov 26 2021 - 04:28:49 EST

Hi Uwe,

On 2021/10/11, 3:32 PM, "Uwe Kleine-König" <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:

On Mon, Sep 06, 2021 at 10:43:39AM +0800, Billy Tsai wrote:
> > This patch add the support of PWM controller which can be found at aspeed
> > ast2600 soc. The pwm supoorts up to 16 channels and it's part function
> > of multi-function device "pwm-tach controller".
> >
> > Signed-off-by: Billy Tsai <billy_tsai@xxxxxxxxxxxxxx>
> > ---
> > drivers/pwm/Kconfig | 10 +
> > drivers/pwm/Makefile | 1 +
> > drivers/pwm/pwm-aspeed-ast2600.c | 327 +++++++++++++++++++++++++++++++
> > 3 files changed, 338 insertions(+)
> > create mode 100644 drivers/pwm/pwm-aspeed-ast2600.c
> >
> > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> > index 63be5362fd3a..b0d26f6c2a8f 100644
> > --- a/drivers/pwm/Kconfig
> > +++ b/drivers/pwm/Kconfig
> > @@ -51,6 +51,16 @@ config PWM_AB8500
> > To compile this driver as a module, choose M here: the module
> > will be called pwm-ab8500.
> >
> > +config PWM_ASPEED_AST2600
> > + tristate "Aspeed ast2600 PWM support"
> > + depends on ARCH_ASPEED || COMPILE_TEST
> > + depends on HAVE_CLK && HAS_IOMEM
> > + help
> > + This driver provides support for Aspeed ast2600 PWM controllers.
> > +
> > + To compile this driver as a module, choose M here: the module
> > + will be called pwm-aspeed-ast2600.
> > +
> > config PWM_ATMEL
> > tristate "Atmel PWM support"
> > depends on OF
> > diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
> > index cbdcd55d69ee..ada454f9129a 100644
> > --- a/drivers/pwm/Makefile
> > +++ b/drivers/pwm/Makefile
> > @@ -2,6 +2,7 @@
> > obj-$(CONFIG_PWM) += core.o
> > obj-$(CONFIG_PWM_SYSFS) += sysfs.o
> > obj-$(CONFIG_PWM_AB8500) += pwm-ab8500.o
> > +obj-$(CONFIG_PWM_ASPEED_AST2600) += pwm-aspeed-ast2600.o
> > obj-$(CONFIG_PWM_ATMEL) += pwm-atmel.o
> > obj-$(CONFIG_PWM_ATMEL_HLCDC_PWM) += pwm-atmel-hlcdc.o
> > obj-$(CONFIG_PWM_ATMEL_TCB) += pwm-atmel-tcb.o
> > diff --git a/drivers/pwm/pwm-aspeed-ast2600.c b/drivers/pwm/pwm-aspeed-ast2600.c
> > new file mode 100644
> > index 000000000000..e4507a503698
> > --- /dev/null
> > +++ b/drivers/pwm/pwm-aspeed-ast2600.c
> > @@ -0,0 +1,327 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * Copyright (C) 2021 Aspeed Technology Inc.
> > + *
> > + * PWM controller driver for Aspeed ast2600 SoCs.
> > + * This drivers doesn't support earlier version of the IP.
> > + *
> > + * The formula of pwm period duration:
> > + * period duration = ((DIV_L + 1) * (PERIOD + 1) << DIV_H) / input-clk
> > + *
> > + * The formula of pwm duty cycle duration:
> > + * duty cycle duration = period duration * DUTY_CYCLE_FALLING_POINT / (PERIOD + 1)
> > + * = ((DIV_L + 1) * DUTY_CYCLE_FALLING_POINT << DIV_H) / input-clk
> > + *
> > + * The software driver fixes the period to 255, which causes the high-frequency
> > + * precision of the PWM to be coarse, in exchange for the fineness of the duty cycle.
> > + *

> From reading this it's not clear (to me that is) what's the difference
> between "period duration" and "PERIOD". Maybe rewrite the comment to
> something like:

> The hardware operates in time quantities of length

> Q := (DIV_L + 1) << DIV_H / input-clk

> The length of a PWM period is (DUTY_CYCLE_PERIOD + 1) * Q.
> The maximal value for DUTY_CYCLE_PERIOD is used here to provide
> a fine grained selection for the duty cycle.

> This driver uses DUTY_CYCLE_RISING_POINT = 0, so from the start of a
> period the output is active until DUTY_CYCLE_FALLING_POINT * Q. Note
> that if DUTY_CYCLE_RISING_POINT = DUTY_CYCLE_FALLING_POINT the output is
> always active.

> This is a bit more high-level and still gives all the details.

Thanks for your explain, it is a clearer for the behavior of our hardware.
I will rewrite the comment in next patch.

> Maybe the special case with DUTY_CYCLE_RISING_POINT =
> DUTY_CYCLE_FALLING_POINT would be a bit more natural if the driver used
> inverse logic: Assume the output to be inverted with CTRL_INVERSE = 0,
> fix DUTY_CYCLE_FALLING_POINT to 0 and adapt DUTY_CYCLE_RISING_POINT
> dependant on duty cycle? At least then DUTY_CYCLE_RISING_POINT = 0
> corresponds to duty_cycle = 0. Just an idea ... which might or might not
> work depending on the hardware.

This is a trade-off between
CTRL_INVERSE = 1 : Support duty_cycle from 0% to 99%
and
CTRL_INVERSE = 0: Support duty_cycle from 1% to 100%
The driver select the CTRL_INVERSE = 0 as the default state.

> > + * Register usage:
> > + * PIN_ENABLE: When it is unset the pwm controller will emit inactive level to the extern.
> > + * Use to determine whether the PWM channel is enabled or disabled
> > + * CLK_ENABLE: When it is unset the pwm controller will assert the duty counter reset and
> > + * emit inactive level to the PIN_ENABLE mux after that the driver can still change the pwm period
> > + * and duty and the value will apply when CLK_ENABLE be set again.
> > + * Use to determine whether duty_cycle bigger than 0.
> > + * PWM_ASPEED_CTRL_INVERSE: When it is toggled the output value will inverse immediately.
> > + * PWM_ASPEED_DUTY_CYCLE_FALLING_POINT/PWM_ASPEED_DUTY_CYCLE_RISING_POINT: When these two
> > + * values are equal it means the duty cycle = 100%.
> > + *
> > + * The glitch may generate at:
> > + * - Enabled changing when the duty_cycle bigger than 0% and less than 100%.
> > + * - Polarity changing when the duty_cycle bigger than 0% and less than 100%.
> > + * - Set duty cycle to 0% from other values.
> > + *
> > + * Limitations:
> > + * - When changing both duty cycle and period, we cannot prevent in
> > + * software that the output might produce a period with mixed
> > + * settings.
> > + * - Disabling the PWM doesn't complete the current period.
> > + *
> > + * Improvements:
> > + * - When only changing one of duty cycle or period, our pwm controller will not
> > + * generate the glitch, the configure will change at next cycle of pwm.
> > + * This improvement can disable/enable through PWM_ASPEED_CTRL_DUTY_SYNC_DISABLE.
> > + */
> > +
> > [...]
> > +static int aspeed_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > + const struct pwm_state *state)
> > +{
> > + struct device *dev = chip->dev;
> > + struct aspeed_pwm_data *priv = aspeed_pwm_chip_to_data(chip);
> > + u32 hwpwm = pwm->hwpwm, duty_pt;
> > + unsigned long rate;
> > + u64 div_h, div_l, divisor;
> > + bool clk_en;
> > +
> > + dev_dbg(dev, "expect period: %lldns, duty_cycle: %lldns", state->period,
> > + state->duty_cycle);
> > +
> > + rate = clk_get_rate(priv->clk);
> > + if (state->period > div64_u64(ULLONG_MAX, (u64)rate))
> > + return -ERANGE;

> Given that the biggest possible period <= requested period should be
> configured, this check + return is wrong.

> > + /*
> > + * Pick the smallest value for div_h so that div_l can be the biggest
> > + * which results in a finer resolution near the target period value.
> > + */
> > + divisor = (u64)NSEC_PER_SEC * (PWM_ASPEED_FIXED_PERIOD + 1) *
> > + (FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1);
> > + div_h = order_base_2(DIV64_U64_ROUND_UP(rate * state->period, divisor));

> > + if (div_h > 0xf)
> > + div_h = 0xf;
> > +
> > + divisor = ((u64)NSEC_PER_SEC * (PWM_ASPEED_FIXED_PERIOD + 1)) << div_h;
> > + div_l = div64_u64(rate * state->period, divisor);
> > +
> > + if (div_l == 0)
> > + return -ERANGE;
> > +
> > + div_l -= 1;
> > +
> > + if (div_l > 255)
> > + div_l = 255;
> > +
> > + [...]

Best Regards,
Billy Tsai