On Wed, Jan 06, 2021 at 09:37:37AM +0200, Baruch Siach wrote:
> The period is the sum of on and off values.
>
> Reported-by: Russell King <linux@xxxxxxxxxxxxxxx>
> Fixes: 757642f9a584e ("gpio: mvebu: Add limited PWM support")
> Signed-off-by: Baruch Siach <baruch@xxxxxxxxxx>
> ---
> v6: divide (on + off) sum to reduce rounding error (RMK)
> ---
> drivers/gpio/gpio-mvebu.c | 19 ++++++++-----------
> 1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpio/gpio-mvebu.c b/drivers/gpio/gpio-mvebu.c
> index 672681a976f5..a912a8fed197 100644
> --- a/drivers/gpio/gpio-mvebu.c
> +++ b/drivers/gpio/gpio-mvebu.c
> @@ -676,20 +676,17 @@ static void mvebu_pwm_get_state(struct pwm_chip *chip,
> else
> state->duty_cycle = 1;
>
> + val = (unsigned long long) u; /* on duration */
> regmap_read(mvpwm->regs, mvebu_pwmreg_blink_off_duration(mvpwm), &u);
> - val = (unsigned long long) u * NSEC_PER_SEC;
> + val += (unsigned long long) u; /* period = on + off duration */
> + val *= NSEC_PER_SEC;
> do_div(val, mvpwm->clk_rate);
> - if (val < state->duty_cycle) {
> + if (val > UINT_MAX)
> + state->period = UINT_MAX;
state->period is an u64, so there is no reason to not use values greater
than UINT_MAX.
> + else if (val)
> + state->period = val;
> + else
> state->period = 1;
This case assigning 1 looks strange. An explanation in a comment would
be great. I wonder if this is a hardware property or if it is only used
to not report 0 in case that mvpwm->clk_rate is high.
I found a few further shortcommings in the mvebu_pwm implementation while
looking through it:
a) The rounding problem that RMK found is also present in .apply
There we have:
val = clk_rate * (period - duty_cycle) / NSEC_PER_SEC
while
val = clk_rate * period / NSEC_PER_SEC - on
would be more exact.
b) To make
pwm_get_state(pwm, &state);
pwm_apply_state(pwm, &state);
idempotent .get_state should round up the division results.
c) .apply also has a check for val being zero and configures at least 1
cycle for the on and off intervals. Is this a hardware imposed
limitation?
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
Attachment:
signature.asc
Description: PGP signature
