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 > -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
