Re: [PATCH 5/8] pwm: Add support for Azoteq IQS620A PWM generator

[Jeff LaBundy <jeff@labundy.com>]

Hi Uwe,

On Wed, Oct 23, 2019 at 09:23:04AM +0200, Uwe Kleine-König wrote:
> Hello Jeff,
> 
> On Tue, Oct 22, 2019 at 09:45:25PM -0500, Jeff LaBundy wrote:
> > On Tue, Oct 22, 2019 at 08:54:15AM +0200, Uwe Kleine-König wrote:
> > > On Mon, Oct 21, 2019 at 11:36:49PM -0500, Jeff LaBundy wrote:
> > > > On Mon, Oct 21, 2019 at 09:34:19AM +0200, Uwe Kleine-König wrote:
> > > > > > +{
> > > > > > +	struct iqs620_pwm_private *iqs620_pwm;
> > > > > > +	struct iqs62x_core *iqs62x;
> > > > > > +	int error;
> > > > > > +	int duty_calc = state->duty_cycle * 256 / IQS620_PWM_PERIOD_NS - 1;
> > > > > > +	u8 duty_clamp = clamp(duty_calc, 0, 0xFF);
> > > 
> > > Another problem that we have here is that the period is fixed to 1 ms
> > > and if a consumer requests for example:
> > > 
> > > 	.period = 5000000,
> > > 	.duty_cycle = 1000000,
> > > 
> > > the hardware is actually configured for
> > > 
> > > 	.period = 1000000,
> > > 	.duty_cycle = 1000000,
> > > 
> > > . I don't have a good suggestion how to fix this. We'd need to
> > > draw a line somewhere and decline a request that is too far from the
> > > result. But where this line should be is not obvious, it should
> > > definitively not be implemented in the driver itself IMHO.
> > > 
> > > (The only halfway sane approach would be to let lowlevel drivers
> > > implement a .round_state callback and then let the framework judge. But
> > > we're a long way from having that, so that's not a solution for today.)
> > > 
> > 
> > Agreed on all counts. For now, I will mention in the 'Limitations' heading that
> > the period cannot be adjusted.
> 
> Ack. My longterm plan is to require .apply_state() to round down both
> .period and .duty_cycle. This isn't wrong already today, so I suggest
> you decline a request to set the period to something smaller than 1 ms
> with an error code. (I think most drivers use -EINVAL here, conceptually
> -EDOM might be sensible. I'd stick to EINVAL for now.)
> 

Sure thing; will do.

> > > > > > +	iqs620_pwm = container_of(chip, struct iqs620_pwm_private, chip);
> > > > > > +	iqs62x = iqs620_pwm->iqs62x;
> > > > > > +
> > > > > > +	error = regmap_write(iqs62x->map, IQS620_PWM_DUTY_CYCLE, duty_clamp);
> > > > > > +	if (error)
> > > > > > +		return error;
> > > > > > +
> > > > > > +	state->period = IQS620_PWM_PERIOD_NS;
> > > > > > +	state->duty_cycle = (duty_clamp + 1) * IQS620_PWM_PERIOD_NS / 256;
> > > > > 
> > > > > This suggests that if the value in the IQS620_PWM_DUTY_CYCLE is 0 the
> > > > > duty cycle is 1/256 ms with a period of 1 ms and the output cannot be
> > > > > constant inactive. If this is right please add a paragraph in the
> > > > > driver's comment at the top:
> > > > > 
> > > > > 	* Limitations:
> > > > > 	* - The hardware cannot generate a 0% duty cycle
> > > > > 
> > > > > (Please stick to this format, other drivers use it, too.)
> > > > 
> > > > That's correct; the lowest duty cycle that can be achieved using only the
> > > > IQS620_PWM_DUTY_CYCLE register is 0.4%. We can, however, generate 0% duty
> > > > cycle by disabling the output altogether using a separate register. Would
> > > > that be better than flat-out saying it's impossible?
> > > 
> > > There is (maybe) a small difference between disabled and 0% duty cycle,
> > > at least from the framework's POV: If you do:
> > > 
> > > 	pwm_apply_state(pwm, { .enabled = true, .period = 1000000, .duty_cycle = 1000000, });
> > > 	pwm_apply_state(pwm, { .enabled = false, .period = $DC, .duty_cycle = $DC, });
> > > 	pwm_apply_state(pwm, { .enabled = true, .period = 1000000, .duty_cycle = 1000000, });
> > > 
> > > and compare it to the expected result of
> > > 
> > > 	pwm_apply_state(pwm, { .enabled = true, .period = 1000000, .duty_cycle = 1000000, });
> > > 	pwm_apply_state(pwm, { .enabled = true, .period = 1000000, .duty_cycle = 0, });
> > > 	pwm_apply_state(pwm, { .enabled = true, .period = 1000000, .duty_cycle = 1000000, });
> > > 
> > > the difference is that the duration of the inactive phase in the latter
> > > case is a multiple of 1 ms.
> > > 
> > > There is no policy for lowlevel drivers what to do, but disabling when
> > > 0% is requested is at least not unseen and probably more what consumers
> > > expect.
> > > 
> > 
> > With the change I am proposing, the output will be driven to zero if enabled = false
> > OR duty_cycle < 4000 ns. Stated another way:
> > 
> > enable duty_cycle IQS620_PWR_SETTINGS[7] IQS620_PWM_DUTY_CYCLE
> > ------ ---------- ---------------------- ---------------------
> >   0    don't care           0                  don't care
> >   1    0 ... 3999           0                  don't care
> >   1    4000 ... x           1                      0
> >   1    x+1  ... y           1                      1
> > 
> > ...and so on. For context, if IQS620_PWR_SETTINGS[7] = 0 then the output is held to
> > zero. If IQS620_PWR_SETTINGS[7] = 1 then the output toggles at a duty cycle between
> > 0.4% and 100% as a function of IQS620_PWM_DUTY_CYCLE.
> 
> Your table isn't accurate. IQS620_PWM_DUTY_CYCLE=0 results in a
> duty_cycle of 3906.25 ns so the table should look as follows:
> 
> enable  duty_cycle  IQS620_PWR_SETTINGS[7] IQS620_PWM_DUTY_CYCLE
> ------ ------------ ---------------------- ---------------------
>   0     don't care           0                  don't care
>   1       [0, 3906]          0                  don't care
>   1    [3907, 7812]          1                      0
>   1    [7813,11718]          1                      1
> 
> In general:
> 
> 	dc = state->duty_cycle * 256 / 1000000
> 	if state->enabled == false or dc == 0:
> 	    IQS620_PWR_SETTINGS[7] = 0
> 
> 	else:
> 	    IQS620_PWM_DUTY_CYCLE = min(dc - 1, 0xff)
> 	    IQS620_PWR_SETTINGS[7] = 1
> 

Sure thing; will do. Thank you for catching that!

> > Based on how the device behaves in response to its two available
> > registers, I think your two examples will appear equal, but please let
> > me know if I have understood.
> 
> Yeah, that's the expectation.
> 
> With the rounding as I suggested above this yields strange effects like
> if
> 
> 	.period = 1 s, .duty_cycle = 0.5 s
> 
> is requested you end up in
> 
> 	.period = 1 ms, .duty_cycle = 1 ms
> 
> but I think there is nothing we can reasonably do about this.
> 

Acknowledged on all counts. FWIW, I expect the most common consumer of this PWM
to be leds-pwm. That is to say, I think the limitations in this case are pretty
harmless. Users will typically pin the period to 1000000 ns like the example in
patch [1/8].

> Best regards
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
> 

Kind regards,
Jeff LaBundy