Message-ID: <vvpfehfcvcimghnzzeejhs5qklgvfkpbjioqlmrl64kiaaxjrg@lw2t4djyvt36>
Date: Tue, 29 Oct 2024 09:41:44 +0100
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Aleksandr Shubin <privatesub2@...il.com>
Cc: linux-kernel@...r.kernel.org, 
	Brandon Cheo Fusi <fusibrandon13@...il.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Philipp Zabel <p.zabel@...gutronix.de>, 
	linux-pwm@...r.kernel.org, devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-sunxi@...ts.linux.dev, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 2/3] pwm: Add Allwinner's D1/T113-S3/R329 SoCs PWM
 support

Hello,

On Fri, Oct 11, 2024 at 01:27:33PM +0300, Aleksandr Shubin wrote:
> [...]
> diff --git a/drivers/pwm/pwm-sun20i.c b/drivers/pwm/pwm-sun20i.c
> new file mode 100644
> index 000000000000..7d1b47843bb6
> --- /dev/null
> +++ b/drivers/pwm/pwm-sun20i.c
> @@ -0,0 +1,379 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * PWM Controller Driver for sunxi platforms (D1, T113-S3 and R329)
> + *
> + * Limitations:
> + * - When the parameters change, current running period will not be completed
> + *   and run new settings immediately.
> + * - It output HIGH-Z state when PWM channel disabled.
> + *
> + * Copyright (c) 2023 Aleksandr Shubin <privatesub2@...il.com>
> + */
> +
> +#include <linux/bitfield.h>
> +#include <linux/clk.h>
> +#include <linux/err.h>
> +#include <linux/io.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/pwm.h>
> +#include <linux/reset.h>
> +
> +#define SUN20I_PWM_CLK_CFG(chan)		(0x20 + ((chan) * 0x4))
> +#define SUN20I_PWM_CLK_CFG_SRC			GENMASK(8, 7)
> +#define SUN20I_PWM_CLK_CFG_DIV_M		GENMASK(3, 0)
> +#define SUN20I_PWM_CLK_DIV_M_MAX		8
> +
> +#define SUN20I_PWM_CLK_GATE			0x40
> +#define SUN20I_PWM_CLK_GATE_BYPASS(chan)	BIT((chan) + 16)
> +#define SUN20I_PWM_CLK_GATE_GATING(chan)	BIT(chan)
> +
> +#define SUN20I_PWM_ENABLE			0x80
> +#define SUN20I_PWM_ENABLE_EN(chan)		BIT(chan)
> +
> +#define SUN20I_PWM_CTL(chan)			(0x100 + (chan) * 0x20)
> +#define SUN20I_PWM_CTL_ACT_STA			BIT(8)
> +#define SUN20I_PWM_CTL_PRESCAL_K		GENMASK(7, 0)
> +#define SUN20I_PWM_CTL_PRESCAL_K_MAX		field_max(SUN20I_PWM_CTL_PRESCAL_K)
> +
> +#define SUN20I_PWM_PERIOD(chan)			(0x104 + (chan) * 0x20)
> +#define SUN20I_PWM_PERIOD_ENTIRE_CYCLE		GENMASK(31, 16)
> +#define SUN20I_PWM_PERIOD_ACT_CYCLE		GENMASK(15, 0)
> +
> +#define SUN20I_PWM_PCNTR_SIZE			BIT(16)

It's a bit unfortunate that SUN20I_PWM_CLK_CFG is passed hwpwm/2 while
SUN20I_PWM_CTL gets a plain hwpwm. I suggest to at least name the
parameters differently.

> +/*
> + * SUN20I_PWM_MAGIC is used to quickly compute the values of the clock dividers
> + * div_m (SUN20I_PWM_CLK_CFG_DIV_M) & prescale_k (SUN20I_PWM_CTL_PRESCAL_K)
> + * without using a loop. These dividers limit the # of cycles in a period
> + * to SUN20I_PWM_PCNTR_SIZE by applying a scaling factor of
> + * 1/(div_m * (prescale_k + 1)) to the clock source.
> + *
> + * SUN20I_PWM_MAGIC is derived by solving for div_m and prescale_k
> + * such that for a given requested period,
> + *
> + * i) div_m is minimized for any prescale_k ≤ SUN20I_PWM_CTL_PRESCAL_K_MAX,
> + * ii) prescale_k is minimized.
> + *
> + * The derivation proceeds as follows, with val = # of cycles for requested
> + * period:
> + *
> + * for a given value of div_m we want the smallest prescale_k such that
> + *
> + * (val >> div_m) // (prescale_k + 1) ≤ 65536 (SUN20I_PWM_PCNTR_SIZE)
                                                  ^
I'd add a = here ---------------------------------' to make it clear
that SUN20I_PWM_PCNTR_SIZE is a description for 65536 and this isn't a
multiplication.

> + *
> + * This is equivalent to:
> + *
> + * (val >> div_m) ≤ 65536 * (prescale_k + 1) + prescale_k
> + * ⟺ (val >> div_m) ≤ 65537 * prescale_k + 65536
> + * ⟺ (val >> div_m) - 65536 ≤ 65537 * prescale_k
> + * ⟺ ((val >> div_m) - 65536) / 65537 ≤ prescale_k
> + *
> + * As prescale_k is integer, this becomes
> + *
> + * ((val >> div_m) - 65536) // 65537 ≤ prescale_k
> + *
> + * And is minimized at
> + *
> + * ((val >> div_m) - 65536) // 65537
> + *
> + * Now we pick the smallest div_m that satifies prescale_k ≤ 255
> + * (i.e SUN20I_PWM_CTL_PRESCAL_K_MAX),
> + *
> + * ((val >> div_m) - 65536) // 65537 ≤ 255
> + * ⟺ (val >> div_m) - 65536 ≤ 255 * 65537 + 65536
> + * ⟺ val >> div_m ≤ 255 * 65537 + 2 * 65536
> + * ⟺ val >> div_m < (255 * 65537 + 2 * 65536 + 1)
> + * ⟺ div_m = fls((val) / (255 * 65537 + 2 * 65536 + 1))
> + *
> + * Suggested by Uwe Kleine-König
> + */
> +#define SUN20I_PWM_MAGIC			(255 * 65537 + 2 * 65536 + 1)
> +
> +struct sun20i_pwm_chip {
> +	struct clk *clk_bus, *clk_hosc, *clk_apb;
> +	struct reset_control *rst;

clk_bus and rst are only used in probe and so can be a local variable there.

> +	void __iomem *base;
> +	struct mutex mutex; /* Protect PWM apply state */

Since commit 1cc2e1faafb3 ("pwm: Add more locking") that currently waits
in next for the merge window the callbacks for a single chip are
serialized, so (unless I miss something) this mutex can be dropped.

> +};
> +
> +static inline struct sun20i_pwm_chip *to_sun20i_pwm_chip(struct pwm_chip *chip)
> +{
> +	return pwmchip_get_drvdata(chip);
> +}
> +
> +static inline u32 sun20i_pwm_readl(struct sun20i_pwm_chip *chip,
> +				   unsigned long offset)
> +{
> +	return readl(chip->base + offset);
> +}
> +
> +static inline void sun20i_pwm_writel(struct sun20i_pwm_chip *chip,
> +				     u32 val, unsigned long offset)
> +{
> +	writel(val, chip->base + offset);
> +}
> +
> +static int sun20i_pwm_get_state(struct pwm_chip *chip,
> +				struct pwm_device *pwm,
> +				struct pwm_state *state)
> +{
> +	struct sun20i_pwm_chip *sun20i_chip = to_sun20i_pwm_chip(chip);
> +	u16 ent_cycle, act_cycle, prescale_k;
> +	u64 clk_rate, tmp;
> +	u8 div_m;
> +	u32 val;
> +
> +	mutex_lock(&sun20i_chip->mutex);
> +
> +	val = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_CLK_CFG(pwm->hwpwm / 2));
> +	div_m = FIELD_GET(SUN20I_PWM_CLK_CFG_DIV_M, val);
> +	if (div_m > SUN20I_PWM_CLK_DIV_M_MAX)
> +		div_m = SUN20I_PWM_CLK_DIV_M_MAX;
> +
> +	if (FIELD_GET(SUN20I_PWM_CLK_CFG_SRC, val) == 0)

SUN20I_PWM_CLK_CFG_SRC is two bits wide. Do all values != 0 mean APB?

> +		clk_rate = clk_get_rate(sun20i_chip->clk_hosc);
> +	else
> +		clk_rate = clk_get_rate(sun20i_chip->clk_apb);
> +
> +	val = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_CTL(pwm->hwpwm));
> +	state->polarity = (SUN20I_PWM_CTL_ACT_STA & val) ?
> +			   PWM_POLARITY_NORMAL : PWM_POLARITY_INVERSED;
> +
> +	prescale_k = FIELD_GET(SUN20I_PWM_CTL_PRESCAL_K, val) + 1;
> +
> +	val = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_ENABLE);
> +	state->enabled = (SUN20I_PWM_ENABLE_EN(pwm->hwpwm) & val) ? true : false;
> +
> +	val = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_PERIOD(pwm->hwpwm));
> +
> +	mutex_unlock(&sun20i_chip->mutex);
> +
> +	act_cycle = FIELD_GET(SUN20I_PWM_PERIOD_ACT_CYCLE, val);
> +	ent_cycle = FIELD_GET(SUN20I_PWM_PERIOD_ENTIRE_CYCLE, val);
> +
> +	/*
> +	 * The duration of the active phase should not be longer
> +	 * than the duration of the period
> +	 */
> +	if (act_cycle > ent_cycle)
> +		act_cycle = ent_cycle;
> +
> +	/*
> +	 * We have act_cycle <= ent_cycle <= 0xffff, prescale_k <= 0x100,
> +	 * div_m <= 8. So the multiplication fits into an u64 without
> +	 * overflow.
> +	 */
> +	tmp = ((u64)(act_cycle) * prescale_k << div_m) * NSEC_PER_SEC;
> +	state->duty_cycle = DIV_ROUND_UP_ULL(tmp, clk_rate);
> +	tmp = ((u64)(ent_cycle) * prescale_k << div_m) * NSEC_PER_SEC;
> +	state->period = DIV_ROUND_UP_ULL(tmp, clk_rate);
> +
> +	return 0;
> +}
> +
> +static int sun20i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> +			    const struct pwm_state *state)
> +{
> +	struct sun20i_pwm_chip *sun20i_chip = to_sun20i_pwm_chip(chip);
> +	u64 bus_rate, hosc_rate, val, ent_cycle, act_cycle;
> +	u32 clk_gate, clk_cfg, pwm_en, ctl, reg_period;
> +	u32 prescale_k, div_m;
> +	bool use_bus_clk;
> +
> +	guard(mutex)(&sun20i_chip->mutex);
> +
> +	pwm_en = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_ENABLE);
> +
> +	if (state->enabled != pwm->state.enabled) {
> +		clk_gate = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_CLK_GATE);
> +
> +		if (!state->enabled) {
> +			clk_gate &= ~SUN20I_PWM_CLK_GATE_GATING(pwm->hwpwm);
> +			pwm_en &= ~SUN20I_PWM_ENABLE_EN(pwm->hwpwm);
> +			sun20i_pwm_writel(sun20i_chip, pwm_en, SUN20I_PWM_ENABLE);
> +			sun20i_pwm_writel(sun20i_chip, clk_gate, SUN20I_PWM_CLK_GATE);
> +
> +			return 0;
> +		}
> +	}
> +
> +	ctl = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_CTL(pwm->hwpwm));
> +	clk_cfg = sun20i_pwm_readl(sun20i_chip, SUN20I_PWM_CLK_CFG(pwm->hwpwm / 2));
> +	hosc_rate = clk_get_rate(sun20i_chip->clk_hosc);
> +	bus_rate = clk_get_rate(sun20i_chip->clk_apb);
> +	if (pwm_en & SUN20I_PWM_ENABLE_EN(pwm->hwpwm ^ 1)) {
> +		/* if the neighbor channel is enabled, check period only */
> +		use_bus_clk = FIELD_GET(SUN20I_PWM_CLK_CFG_SRC, clk_cfg) != 0;
> +		val = mul_u64_u64_div_u64(state->period,
> +					  (use_bus_clk ? bus_rate : hosc_rate),
> +					  NSEC_PER_SEC);
> +
> +		div_m = FIELD_GET(SUN20I_PWM_CLK_CFG_DIV_M, clk_cfg);
> +	} else {
> +		/*
> +		 * Select the clock source based on the period,
> +		 * since bus_rate > hosc_rate, which means bus_rate
> +		 * can provide a higher frequency than hosc_rate.
> +		 */
> +		use_bus_clk = false;
> +		val = mul_u64_u64_div_u64(state->period, hosc_rate, NSEC_PER_SEC);
> +		if (val <= 1) {

Why is val == 1 already problematic?

> +			use_bus_clk = true;
> +			val = mul_u64_u64_div_u64(state->period, bus_rate, NSEC_PER_SEC);
> +			if (val <= 1)
> +				return -EINVAL;
> +		}
> +		div_m = fls(DIV_ROUND_DOWN_ULL(val, SUN20I_PWM_MAGIC));
> +		if (div_m > SUN20I_PWM_CLK_DIV_M_MAX)
> +			return -EINVAL;
> +
> +		/* set up the CLK_DIV_M and clock CLK_SRC */
> +		clk_cfg = FIELD_PREP(SUN20I_PWM_CLK_CFG_DIV_M, div_m);
> +		clk_cfg |= FIELD_PREP(SUN20I_PWM_CLK_CFG_SRC, use_bus_clk);
> +
> +		sun20i_pwm_writel(sun20i_chip, clk_cfg, SUN20I_PWM_CLK_CFG(pwm->hwpwm / 2));
> +	}

Does writing SUN20I_PWM_CLK_CFG already influence the output? If so this
needs mentioning in the Limitations paragraph at the driver's top as the
output might glitch more than suggested there currently.

> +	/* calculate prescale_k, PWM entire cycle */
> +	ent_cycle = val >> div_m;
> +	prescale_k = DIV_ROUND_DOWN_ULL(ent_cycle, 65537);

A #define for 65537?

> +	if (prescale_k > SUN20I_PWM_CTL_PRESCAL_K_MAX)
> +		prescale_k = SUN20I_PWM_CTL_PRESCAL_K_MAX;
> +
> +	do_div(ent_cycle, prescale_k + 1);
> +
> +	/* for N cycles, PPRx.PWM_ENTIRE_CYCLE = (N-1) */
> +	reg_period = FIELD_PREP(SUN20I_PWM_PERIOD_ENTIRE_CYCLE, ent_cycle - 1);

Is ent_cycle known to be ≥ 1?

> +	/* set duty cycle */
> +	val = mul_u64_u64_div_u64(state->duty_cycle,
> +				  (use_bus_clk ? bus_rate : hosc_rate),
> +				  NSEC_PER_SEC);
> +	act_cycle = val >> div_m;
> +	do_div(act_cycle, prescale_k + 1);
> +
> +	/*
> +	 * The formula of the output period and the duty-cycle for PWM are as follows.
> +	 * T period = PWM0_PRESCALE_K / PWM01_CLK * (PPR0.PWM_ENTIRE_CYCLE + 1)
> +	 * T high-level = PWM0_PRESCALE_K / PWM01_CLK * PPR0.PWM_ACT_CYCLE
> +	 * Duty-cycle = T high-level / T period
> +	 */
> +	reg_period |= FIELD_PREP(SUN20I_PWM_PERIOD_ACT_CYCLE, act_cycle);
> +	sun20i_pwm_writel(sun20i_chip, reg_period, SUN20I_PWM_PERIOD(pwm->hwpwm));
> +
> +	ctl = FIELD_PREP(SUN20I_PWM_CTL_PRESCAL_K, prescale_k);
> +	if (state->polarity == PWM_POLARITY_NORMAL)
> +		ctl |= SUN20I_PWM_CTL_ACT_STA;
> +
> +	sun20i_pwm_writel(sun20i_chip, ctl, SUN20I_PWM_CTL(pwm->hwpwm));
> +
> +	if (state->enabled != pwm->state.enabled && state->enabled) {
> +		clk_gate &= ~SUN20I_PWM_CLK_GATE_BYPASS(pwm->hwpwm);
> +		clk_gate |= SUN20I_PWM_CLK_GATE_GATING(pwm->hwpwm);
> +		pwm_en |= SUN20I_PWM_ENABLE_EN(pwm->hwpwm);
> +		sun20i_pwm_writel(sun20i_chip, pwm_en, SUN20I_PWM_ENABLE);
> +		sun20i_pwm_writel(sun20i_chip, clk_gate, SUN20I_PWM_CLK_GATE);
> +	}

There is an early return at the start of the function for
state->enabled != pwm->state.enabled && !state->enabled
. So just checking for

	if (state->enabled != pwm->state.enabled)

is enough.

> +
> +	return 0;
> +}
> +
> +static const struct pwm_ops sun20i_pwm_ops = {
> +	.apply = sun20i_pwm_apply,
> +	.get_state = sun20i_pwm_get_state,
> +};
> +
> +static const struct of_device_id sun20i_pwm_dt_ids[] = {
> +	{ .compatible = "allwinner,sun20i-d1-pwm" },
> +	{ },

No comma after the sentinel entry please.

> +};
> +MODULE_DEVICE_TABLE(of, sun20i_pwm_dt_ids);
> +
> +static void sun20i_pwm_reset_ctrl_release(void *data)
> +{
> +	struct reset_control *rst = data;
> +
> +	reset_control_assert(rst);
> +}
> +
> +static int sun20i_pwm_probe(struct platform_device *pdev)
> +{
> +	struct pwm_chip *chip;
> +	struct sun20i_pwm_chip *sun20i_chip;
> +	int ret;
> +
> +	chip = devm_pwmchip_alloc(&pdev->dev, 8, sizeof(*sun20i_chip));
> +	if (IS_ERR(chip))
> +		return PTR_ERR(chip);
> +	sun20i_chip = to_sun20i_pwm_chip(chip);
> +
> +	sun20i_chip->base = devm_platform_ioremap_resource(pdev, 0);
> +	if (IS_ERR(sun20i_chip->base))
> +		return PTR_ERR(sun20i_chip->base);
> +
> +	sun20i_chip->clk_bus = devm_clk_get_enabled(&pdev->dev, "bus");
> +	if (IS_ERR(sun20i_chip->clk_bus))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(sun20i_chip->clk_bus),
> +				     "failed to get bus clock\n");

I like error messages starting with a capital letter. Would you mind
converting accordingly?

> +	sun20i_chip->clk_hosc = devm_clk_get_enabled(&pdev->dev, "hosc");
> +	if (IS_ERR(sun20i_chip->clk_hosc))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(sun20i_chip->clk_hosc),
> +				     "failed to get hosc clock\n");
> +
> +	sun20i_chip->clk_apb = devm_clk_get_enabled(&pdev->dev, "apb");
> +	if (IS_ERR(sun20i_chip->clk_apb))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(sun20i_chip->clk_apb),
> +				     "failed to get apb clock\n");
> +
> +	if (clk_get_rate(sun20i_chip->clk_apb) > clk_get_rate(sun20i_chip->clk_hosc))
> +		dev_info(&pdev->dev, "apb clock must be greater than hosc clock");
> +
> +	sun20i_chip->rst = devm_reset_control_get_exclusive(&pdev->dev, NULL);
> +	if (IS_ERR(sun20i_chip->rst))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(sun20i_chip->rst),
> +				     "failed to get bus reset\n");
> +
> +	ret = of_property_read_u32(pdev->dev.of_node, "allwinner,pwm-channels",
> +				   &chip->npwm);

error checking for ret?

> +	if (chip->npwm > 16) {
> +		dev_info(&pdev->dev, "limiting number of PWM lines from %u to 16",
> +			 chip->npwm);
> +		chip->npwm = 16;

Layer violation; drivers are not supposed to assign npwm. Also above you
only allocated 8. Better check allwinner,pwm-channels before calling
devm_pwmchip_alloc().

> +	}
> +
> +	/* Deassert reset */
> +	ret = reset_control_deassert(sun20i_chip->rst);
> +	if (ret)
> +		return dev_err_probe(&pdev->dev, ret, "failed to deassert reset\n");
> +
> +	ret = devm_add_action_or_reset(&pdev->dev, sun20i_pwm_reset_ctrl_release, sun20i_chip->rst);
> +	if (ret)
> +		return ret;

There is devm_reset_control_get_exclusive_asserted() scheduled to go
into v6.13-rc1. Please group the operations concerning rst together such
that it can be converted trivially to that function.

> +	chip->ops = &sun20i_pwm_ops;
> +
> +	mutex_init(&sun20i_chip->mutex);
> +
> +	ret = devm_pwmchip_add(&pdev->dev, chip);
> +	if (ret < 0)
> +		return dev_err_probe(&pdev->dev, ret, "failed to add PWM chip\n");
> +
> +	return 0;
> +}

Best regards
Uwe

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