Re: [PATCH 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

[Yash Shah <yash.shah@sifive.com>]

On Wed, Jan 16, 2019 at 10:16 PM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>
> Hello,
>
> On Wed, Jan 16, 2019 at 04:40:42PM +0530, Yash Shah wrote:
> > On Wed, Jan 16, 2019 at 3:30 AM Uwe Kleine-König
> > <u.kleine-koenig@pengutronix.de> wrote:
> > > On Fri, Jan 11, 2019 at 01:52:44PM +0530, Yash Shah wrote:
> > > > Adds a PWM driver for PWM chip present in SiFive's HiFive Unleashed SoC.
> > > >
> > > > Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>
> > > > [Atish: Various fixes and code cleanup]
> > > > Signed-off-by: Atish Patra <atish.patra@wdc.com>
> > > > Signed-off-by: Yash Shah <yash.shah@sifive.com>
> > > > ---
> > > >  drivers/pwm/Kconfig      |  10 ++
> > > >  drivers/pwm/Makefile     |   1 +
> > > >  drivers/pwm/pwm-sifive.c | 246 +++++++++++++++++++++++++++++++++++++++++++++++
> > > >  3 files changed, 257 insertions(+)
> > > >  create mode 100644 drivers/pwm/pwm-sifive.c
> > > >
> > > > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> > > > index a8f47df..3bcaf6a 100644
> > > > --- a/drivers/pwm/Kconfig
> > > > +++ b/drivers/pwm/Kconfig
> > > > @@ -380,6 +380,16 @@ config PWM_SAMSUNG
> > > >         To compile this driver as a module, choose M here: the module
> > > >         will be called pwm-samsung.
> > > >
> > > > +config PWM_SIFIVE
> > > > +     tristate "SiFive PWM support"
> > > > +     depends on OF
> > > > +     depends on COMMON_CLK
> > >
> > > I'd say add:
> > >
> > >         depends on MACH_SIFIVE || COMPILE_TEST
> > >
> > > (I guess "MACH_SIFIVE" is wrong, but I assume you get what I mean.)
> >
> > As of now, MACH_SIFIVE/ARCH_SIFIVE isn't available.
> > @Paul, Do you have any comments on this?
>
> If this is not going to be available at least protect it by
>
>         depends RISCV || COMPILE_TEST
>
> > > I wonder if such an instance should be only a single PWM instead of
> > > four. Then you were more flexible with the period lengths (using
> > > pwmcfg.pwmzerocmp) and could do stuff like inverted and uninverted mode.
> > >
> > > I didn't understand how the deglitch logic works yet. Currently it is
> > > not used which might result in four edges in a single period (which is
> > > bad).
> >
> > I can enable deglitch logic by just setting a bit (BIT_PWM_DEGLITCH) in
> > REG_PWMCFG. Will do that.
>
> This only works for the first pwm output though.
>
> > > > +struct sifive_pwm_device {
> > > > +     struct pwm_chip chip;
> > > > +     struct notifier_block notifier;
> > > > +     struct clk *clk;
> > > > +     void __iomem *regs;
> > > > +     unsigned int approx_period;
>
> When thinking about this a bit more, I think the better approach would
> be to let the consumer change the period iff there is only one consumer.
> Then you can drop that approx-period stuff and the result is more
> flexible. (Having said that I still prefer making the driver provide
> only a single PWM with the ability to have periods other than powers of
> two.)
>

I am not confident about the implementation of the way you are suggesting.
As of now, I am going to stick with the current implementation.

> > > > +     writel(frac, pwm->regs + REG_PWMCMP0 + dev->hwpwm * SIZE_PWMCMP);
> > >
> > > If you get a constant inactive output with frac=0 and a constant active
> > > output with frac=0xffff the calculation above seems wrong to me.
> > > (A value i written to the pwmcmpX register means a duty cycle of
> > > (i * period / 0xffff). Your calculation assumes a divisor of 0x10000
> > > however.)
> >
> > Not sure if I get you completely. But, if divisor of 0xffff is your concern then
> > does the below look ok?
> >
> > frac = div_u64(((u64)duty_cycle << 16) - duty_cycle, state->period);
>
> This works (I think, didn't redo the maths), but I would prefer
>
>         frac = div_u64((u64)duty_cycle * 0xffff, state->period);
>
> for better readability. (Maybe the compiler is even clever enough to see
> this can be calculated as you suggested.)

Sure, will multiply it with 0xffff for better readability.

>
> > > > +static int sifive_pwm_clock_notifier(struct notifier_block *nb,
> > > > +                                  unsigned long event, void *data)
> > > > +{
> > > > +     struct clk_notifier_data *ndata = data;
> > > > +     struct sifive_pwm_device *pwm =
> > > > +             container_of(nb, struct sifive_pwm_device, notifier);
> > > > +
> > > > +     if (event == POST_RATE_CHANGE)
> > > > +             sifive_pwm_update_clock(pwm, ndata->new_rate);
> > >
> > > Does this need locking? (Maybe not with the current state.)
> >
> > No. We can add it when required.
>
> My thought was, that the clk freq might change while .apply is active.
> But given that you only configure the relative duty cycle this is
> independent of the actual clk freq.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
>
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