Re: [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle approximation

Boris Brezillon
2016-06-04T06:19:55Z

On Fri, 3 Jun 2016 13:03:26 -0700
Brian Norris <briannorris@chromium.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