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

[Brian Norris]

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) ...

Seems much saner to me.

Now, I note that in patch 10 you're now using pwm_prepare_new_state() to
avoid this propagation problem entirely (good idea anyway, IMO), but I
just wanted to further note what kind of real problems we can see when
we don't round to the closest value.

> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Brian Norris <briannorris@xxxxxxxxxxxx>
Tested-by: Brian Norris <briannorris@xxxxxxxxxxxx>

Tested this whole series on rk3399's PWM regulators used for the CPUs,
to clarify what my Tested-by means.

Thanks for the patches.

> ---
>  drivers/pwm/pwm-rockchip.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c
> index 7d9cc90..68d72ce 100644
> --- a/drivers/pwm/pwm-rockchip.c
> +++ b/drivers/pwm/pwm-rockchip.c
> @@ -114,12 +114,11 @@ static int rockchip_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  	 * default prescaler value for all practical clock rate values.
>  	 */
>  	div = clk_rate * period_ns;
> -	do_div(div, pc->data->prescaler * NSEC_PER_SEC);
> -	period = div;
> +	period = DIV_ROUND_CLOSEST_ULL(div,
> +				       pc->data->prescaler * NSEC_PER_SEC);
>  
>  	div = clk_rate * duty_ns;
> -	do_div(div, pc->data->prescaler * NSEC_PER_SEC);
> -	duty = div;
> +	duty = DIV_ROUND_CLOSEST_ULL(div, pc->data->prescaler * NSEC_PER_SEC);
>  
>  	ret = clk_enable(pc->clk);
>  	if (ret)
> -- 
> 2.7.4
>