linux-kernel - Re: [PATCH v2] pwm: rockchip: round period/duty down on apply, up on get

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Message-ID: <vb2wmbbbmwheovxkac36ab4omw6dj3owb4vhz7r7woumvawkp5@eeejjh6jt7wd>
Date: Tue, 17 Jun 2025 16:28:00 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Cc: Heiko Stuebner <heiko@...ech.de>, 
	Brian Norris <briannorris@...omium.org>, Boris Brezillon <bbrezillon@...nel.org>, 
	Thierry Reding <thierry.reding@...il.com>, kernel@...labora.com, linux-pwm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] pwm: rockchip: round period/duty down on apply, up on
 get

Hello Nicolas,

On Mon, Jun 16, 2025 at 05:14:17PM +0200, Nicolas Frattaroli wrote:
> With CONFIG_PWM_DEBUG=y, the rockchip PWM driver produces warnings like
> this:
> 
>   rockchip-pwm fd8b0010.pwm: .apply is supposed to round down
>   duty_cycle (requested: 23529/50000, applied: 23542/50000)
> 
> This is because the driver chooses ROUND_CLOSEST for purported
> idempotency reasons. However, it's possible to keep idempotency while
> always rounding down in .apply.
> 
> Do this by making get_state always round up, and making apply always
> round down. This is done with u64 maths, and setting both period and
> duty to U32_MAX (the biggest the hardware can support) if they would
> exceed their 32 bits confines.
> 
> Fixes: 12f9ce4a5198 ("pwm: rockchip: Fix period and duty cycle approximation")
> Fixes: 1ebb74cf3537 ("pwm: rockchip: Add support for hardware readout")
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>

I pondered if I should drop the Fixes: lines because I think this patch
shouldn't get backported to stable. But I think it's correct to keep
them and then just veto the backport if the stable guys pick it up.

Applied with a few cosmetic changes to the commit log to

	https://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux.git pwm/for-next

Thanks
Uwe

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