| Message ID | 20191014135303.2944058-1-megous@megous.com |
|---|---|
| State | Accepted |
| Headers | show |
| Series | pwm: sun4i: Fix incorrect calculation of duty_cycle/period | expand |
On Mon, Oct 14, 2019 at 03:53:03PM +0200, megous@megous.com wrote: > From: Ondrej Jirman <megous@megous.com> > > Since 5.4-rc1, pwm_apply_state calls ->get_state after ->apply > if available, and this revealed an issue with integer precision > when calculating duty_cycle and period for the currently set > state in ->get_state callback. > > This issue manifested in broken backlight on several Allwinner > based devices. > > Previously this worked, because ->apply updated the passed state > directly. > > Fixes: deb9c462f4e53 ("pwm: sun4i: Don't update the state for the caller of pwm_apply_state") > Signed-off-by: Ondrej Jirman <megous@megous.com> > --- > drivers/pwm/pwm-sun4i.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c > index 6f5840a1a82d..05273725a9ff 100644 > --- a/drivers/pwm/pwm-sun4i.c > +++ b/drivers/pwm/pwm-sun4i.c > @@ -137,10 +137,10 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip, > > val = sun4i_pwm_readl(sun4i_pwm, PWM_CH_PRD(pwm->hwpwm)); > > - tmp = prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); > + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); > state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); > > - tmp = prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); > + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); > state->period = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); The issue is real and the fix looks right. So take my Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> There are a few more issues though: - The dty value is calculated as (simplified): dty = prd * state->duty_cycle / state->period which gives suboptimal values in some cases. - The algorithm does 2 divisions (and one too early) instead of a single one at the end, which is both ineffective and gives away precision. - the test in sun4i_pwm_apply about cstate.period != state->period and the same for duty is probably less useful now because it compares requested values with actually implemented ones. - it's unclear what the "surprising values" are that sun4i_pwm_calculate talks about in a comment. (And IMHO you should always round down.) - Having a comment describing the implemented duty_cycle and period depending on how the registers are set would be helpful to understand the implemented algorithm. - If there is a publically available datasheet adding a link to it in the header of the driver would be great. Best regards Uwe
On Mon, Oct 14, 2019 at 03:53:03PM +0200, megous@megous.com wrote: > From: Ondrej Jirman <megous@megous.com> > > Since 5.4-rc1, pwm_apply_state calls ->get_state after ->apply > if available, and this revealed an issue with integer precision > when calculating duty_cycle and period for the currently set > state in ->get_state callback. > > This issue manifested in broken backlight on several Allwinner > based devices. > > Previously this worked, because ->apply updated the passed state > directly. > > Fixes: deb9c462f4e53 ("pwm: sun4i: Don't update the state for the caller of pwm_apply_state") > Signed-off-by: Ondrej Jirman <megous@megous.com> > --- > drivers/pwm/pwm-sun4i.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) Applied, thanks. Thierry > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c > index 6f5840a1a82d..05273725a9ff 100644 > --- a/drivers/pwm/pwm-sun4i.c > +++ b/drivers/pwm/pwm-sun4i.c > @@ -137,10 +137,10 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip, > > val = sun4i_pwm_readl(sun4i_pwm, PWM_CH_PRD(pwm->hwpwm)); > > - tmp = prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); > + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); > state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); > > - tmp = prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); > + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); > state->period = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); > } > > -- > 2.23.0 >
diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c index 6f5840a1a82d..05273725a9ff 100644 --- a/drivers/pwm/pwm-sun4i.c +++ b/drivers/pwm/pwm-sun4i.c @@ -137,10 +137,10 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip, val = sun4i_pwm_readl(sun4i_pwm, PWM_CH_PRD(pwm->hwpwm)); - tmp = prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_DTY(val); state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); - tmp = prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); + tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_PRD(val); state->period = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate); }