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

From: John Watts <contact@jookia.org>
To: Aleksandr Shubin <privatesub2@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	"Brandon Cheo Fusi" <fusibrandon13@gmail.com>,
	"Uwe Kleine-König" <ukleinek@kernel.org>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzk+dt@kernel.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Skrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Philipp Zabel" <p.zabel@pengutronix.de>,
	"Marc Kleine-Budde" <mkl@pengutronix.de>,
	"Maksim Kiselev" <bigunclemax@gmail.com>,
	linux-pwm@vger.kernel.org, devicetree@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-riscv@lists.infradead.org
Subject: Re: [PATCH v9 2/3] pwm: Add Allwinner's D1/T113-S3/R329 SoCs PWM support
Date: Thu, 23 May 2024 13:16:47 +1000	[thread overview]
Message-ID: <Zk61H05-BTVUarFV@titan> (raw)
In-Reply-To: <20240520184227.120956-3-privatesub2@gmail.com>

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.