On Fri, Apr 22, 2022 at 06:19:03PM +0200, Thierry Reding wrote:
> On Thu, Mar 03, 2022 at 06:50:12PM +0100, Uwe Kleine-König wrote:
> > Dividing by the result of a division looses precision because the result is
> > rounded twice. E.g. with clk_rate = 48000000 and period = 32760033 the
> > following numbers result:
> >
> > rate = pc->clk_rate >> PWM_DUTY_WIDTH = 187500
> > hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC, period_ns) = 3052
> > rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz) = 6144
> >
> > The exact result would be 6142.5061875 and (apart from rounding) this is
> > found by using a single division. As a side effect is also a tad
> > cheaper to calculate.
> >
> > Also using clk_rate >> PWM_DUTY_WIDTH looses precision. Consider for
> > example clk_rate = 47999999 and period = 106667:
> >
> > mul_u64_u64_div_u64(pc->clk_rate >> PWM_DUTY_WIDTH, period_ns,
> > NSEC_PER_SEC) = 19
> >
> > mul_u64_u64_div_u64(pc->clk_rate, period_ns,
> > NSEC_PER_SEC << PWM_DUTY_WIDTH) = 20
> >
> > (The exact result is 20.000062083332033.)
> >
> > With this optimizations also switch from round-closest to round-down. Given
> > that the calculations were non-optimal for quite some time now which
> > nobody reported as a problem, this is the opportunity to align the driver's
> > behavior to the requirements of new drivers. (Note however that the
> > duty_cycle calculation isn't aligned yet.)
> >
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> > ---
> > drivers/pwm/pwm-tegra.c | 10 +++++-----
> > 1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
> > index e5a9ffef4a71..7fc03a9ec154 100644
> > --- a/drivers/pwm/pwm-tegra.c
> > +++ b/drivers/pwm/pwm-tegra.c
> > @@ -99,7 +99,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > int duty_ns, int period_ns)
> > {
> > struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
> > - unsigned long long c = duty_ns, hz;
> > + unsigned long long c = duty_ns;
> > unsigned long rate, required_clk_rate;
> > u32 val = 0;
> > int err;
> > @@ -156,11 +156,9 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > pc->clk_rate = clk_get_rate(pc->clk);
> > }
> >
> > - rate = pc->clk_rate >> PWM_DUTY_WIDTH;
> > -
> > /* Consider precision in PWM_SCALE_WIDTH rate calculation */
> > - hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC, period_ns);
> > - rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz);
> > + rate = mul_u64_u64_div_u64(pc->clk_rate, period_ns,
> > + (u64)NSEC_PER_SEC << PWM_DUTY_WIDTH);
> >
> > /*
> > * Since the actual PWM divider is the register's frequency divider
> > @@ -169,6 +167,8 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > */
> > if (rate > 0)
> > rate--;
> > + else
> > + return -EINVAL;
>
> Can you elaborate on why this is needed? Previously rate == 0 was a
> valid case and this could still happen with the above calculations.
If the calculations before the if block result in rate = 0 this means
that the requested period is smaller than the minimal possible period.
So refusing this setting is part of the switch from
something-like-round-closest to round-down.
I will send a v2 explaining that in the commit log.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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