linux-kernel - Re: [PATCH v9 2/3] pwm: Add Allwinner's D1/T113-S3/R329 SoCs PWM support

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Date: Thu, 23 May 2024 13:16:47 +1000
From: John Watts <contact@...kia.org>
To: Aleksandr Shubin <privatesub2@...il.com>
Cc: linux-kernel@...r.kernel.org,
	Brandon Cheo Fusi <fusibrandon13@...il.com>,
	Uwe Kleine-König <ukleinek@...nel.org>,
	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>,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	Maksim Kiselev <bigunclemax@...il.com>, 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 v9 2/3] pwm: Add Allwinner's D1/T113-S3/R329 SoCs PWM
 support

Hi,

Here's a quick review based on the experience of me writing my own driver.

On Mon, May 20, 2024 at 09:42:20PM +0300, Aleksandr Shubin wrote:
> +	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);

Doesn't ent_cycle require a + 1 here?
Shouldn't act_cycle be > ent_cycle on 0% duty cycles?

> +			/* if the neighbor channel is enable, 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);

It would be nice if it reclocked both channels.

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

I'm not sure about this code. I don't quite get where the 65537 comes from or
what's really happening here.

To my understanding you either want to limit PWM_ENTIRE_CYCLE to 0xFFFE so and
scale PWM_ACTIVE_CYCLE from 0 to 65535 so it can be 0x0 at 100% duty cycles and
0xFFFF at 0% duty cycles, OR you want to scale it from 0 to 65536 and check if
the value is 65536, and if it is wrap it around to 0 and flip the polarity.

Thanks,
John.