On Thu, 08 Nov 2012 10:51:35 +0100
Roland Stigge <sti...@antcom.de> wrote:
> On 07/11/12 16:25, Alban Bedel wrote:
> > Signed-off-by: Alban Bedel <alban.be...@avionic-design.de>
> > ---
> > drivers/pwm/pwm-lpc32xx.c | 6 +++++-
> > 1 files changed, 5 insertions(+), 1 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-lpc32xx.c b/drivers/pwm/pwm-lpc32xx.c
> > index adb87f0..0dc278d 100644
> > --- a/drivers/pwm/pwm-lpc32xx.c
> > +++ b/drivers/pwm/pwm-lpc32xx.c
> > @@ -51,7 +51,11 @@ static int lpc32xx_pwm_config(struct pwm_chip *chip,
> > struct pwm_device *pwm,
> >
> > c = 256 * duty_ns;
> > do_div(c, period_ns);
> > - duty_cycles = c;
> > + if (c == 0)
> > + c = 256;
> > + if (c > 255)
> > + c = 255;
> > + duty_cycles = 256 - c;
>
> Except for the range check (for the original c > 255), this results in:
>
> duty_cycles = 256 - c
>
> except for (c == 0) where
>
> duty_cycles = 1
No it lead to duty_cycles = 0
> which actually is
>
> duty_cycles = (256 - c) - 255
>
> (think with the original c)
>
> i.e. nearly a polarity inversion in the case of (c == 0).
>
> Why is the case (c == 0) so special here? Maybe you can document this,
> if it is really intended?
It is intended, the formular for duty value in the register is:
duty = (256 - 256*duty_ns/period_ns) % 256
But the code avoid the modulo by clamping '256*duty_ns/period_ns' to 1-256.
Perhaps something like:
if (c > 255)
c = 255;
duty_cycles = (256 - c) % 256;
would be easier to understand.
Alban
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