Re: [PATCH v10 3/4] pwm: add microchip soft ip corePWM driver

[Conor Dooley <conor.dooley@microchip.com>]

On Mon, Sep 19, 2022 at 03:29:19PM +0100, Conor Dooley wrote:
> Hey Uwe,
> 
> On Mon, Sep 19, 2022 at 03:50:08PM +0200, Uwe Kleine-König wrote:
> > On Mon, Sep 19, 2022 at 01:53:56PM +0100, Conor Dooley wrote:
> > > Hey Uwe,
> > > Thanks (as always). I've switched up my email setup a bit so I hope
> > > that I've not mangled anything here.
> > > 
> > > On Thu, Sep 15, 2022 at 09:21:52AM +0200, Uwe Kleine-König wrote:
> > > > Hello,
> > > > 
> > > > On Wed, Aug 24, 2022 at 10:12:14AM +0100, Conor Dooley wrote:
> > > > > Add a driver that supports the Microchip FPGA "soft" PWM IP core.
> > > > > 
> > > > > Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> > > > > ---

> > > > $ ./test 255 65535
> > > > period_steps = 255
> > > > prescale = 65535
> > > > period = 18446744073018591744
> > > > 
> > > > The problem is that the result of 16711425 * 1000000000L isn't affected
> > > > by the type of period and so it's promoted to L which isn't big enough
> > > > to hold 16711425000000000 where longs are only 32 bit wide.
> > > 
> > > I don't think this is ever going to be hit in the wild, since prescale
> > > comes from the hardware where it is limited to 255 - but preventing the
> > > issue seems trivially done by splitting the multiplication so no reason
> > > not to. Thanks for providing the test program btw :)
> > 
> > Even 255 * 255 * 1000000000 overflows. With a maintainer's hat on, it is
> > very valuable to prevent such issues because your driver might be used
> > as a template for the next driver.
> > 
> > > > > +	state->period = DIV64_U64_ROUND_UP(state->period, clk_get_rate(mchp_core_pwm->clk));
> > > > > +
> > > > > +	posedge = readb_relaxed(mchp_core_pwm->base + MCHPCOREPWM_POSEDGE(pwm->hwpwm));
> > > > > +	negedge = readb_relaxed(mchp_core_pwm->base + MCHPCOREPWM_NEGEDGE(pwm->hwpwm));
> > > > > +
> > > > > +	if ((negedge == posedge) && state->enabled) {
> > > > 
> > > > Why do you need that state->enabled?
> > > 
> > > Because I was running into conflicts between the reporting here and some
> > > of the checks that I have added to prevent the PWM being put into an
> > > invalid state. On boot both negedge and posedge will be zero & this was
> > > preventing me from setting the period at all.
> > 
> > I don't understood that.
> 
> On startup, (negedge == posedge) is true as both are zero, but the reset
> values for prescale and period are actually 0x8. If on reset I try to
> set a small period, say "echo 1000 > period" apply() returns -EINVAL
> because of a check in the pwm core in pwm_apply_state() as I am
> attempting to set the period to lower than the out-of-reset duty cycle.
> 
> I considered zeroing the registers, but if something below Linux had
> been using the PWM I felt that may not be the right thing to do. Can I
> continue to check for the enablement here or would you rather I did
> something different?

Hey Uwe,
Just bumping here ICYMI. Should I leave the behaviour as-was and just
document what the default values out of reset may be? That would leave
the check here making more sense & head off confusion about why apply()
fails?

Thanks,
Conor.