linux-kernel - Re: [PATCH 03/14] pwm: rockchip: Fix period and duty

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Message-ID: <20160604081955.47393878@bbrezillon>
Date:	Sat, 4 Jun 2016 08:19:55 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Brian Norris <briannorris@...omium.org>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Heiko Stuebner <heiko@...ech.de>,
	linux-rockchip@...ts.infradead.org,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Milo Kim <milo.kim@...com>,
	Doug Anderson <dianders@...gle.com>,
	Caesar Wang <wxt@...k-chips.com>,
	Stephen Barber <smbarber@...omium.org>,
	Ajit Pal Singh <ajitpal.singh@...com>,
	Srinivas Kandagatla <srinivas.kandagatla@...il.com>,
	Maxime Coquelin <maxime.coquelin@...com>,
	Patrice Chotard <patrice.chotard@...com>, kernel@...inux.com
Subject: Re: [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle
 approximation

On Fri, 3 Jun 2016 13:03:26 -0700
Brian Norris <briannorris@...omium.org> wrote:

> On Fri, Jun 03, 2016 at 10:23:01AM +0200, Boris Brezillon wrote:
> > The current implementation always round down the duty and period
> > values, while it would be better to round them to the closest integer.  
> 
> Agreed. As I noted to you elsewhere, not having this change can cause
> problems where doing a series of pwm_get_state() / modify /
> pwm_apply_state() will propagate rounding errors, which will change the
> period unexpectedly. e.g., I have an expected period of 3.333 us and a
> clk rate of 112.666667 MHz -- the clock frequency doesn't divide evenly,
> so the period (stashed in nanoseconds) shrinks when we convert to the
> register value and back, as follows:
> 
>  pwm_apply_state(): register = period * 112666667 / 1000000000;
>  pwm_get_state(): period = register * 1000000000 / 112666667;
> 
> or in other words:
> 
>  period = period * 112666667 / 1000000000 * 1000000000 / 112666667;
> 
> which yields a sequence like:
> 
>  3333 -> 3328
>  3328 -> 3319
>  3319 -> 3310
>  3310 -> 3301
>  3301 -> 3292
>  3292 -> ... (etc) ...
> 
> With this patch, we'd see instead:
> 
>  period = div_round_closest(period * 112666667, 1000000000) * 1000000000 / 112666667;
> 
> which yields a stable sequence:
> 
>  3333 -> 3337
>  3337 -> 3337
>  3337 -> ... (etc) ...

Woh! Thanks for the detailed explanation. Do you want me to put that in
a comment explaining why we're using DIV_ROUND_CLOSEST_ULL()?


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com