Re: [PATCH v4 3/3] pwm: Add support for Xilinx AXI Timer

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Sean,

On Mon, Jun 28, 2021 at 01:41:43PM -0400, Sean Anderson wrote:
> On 6/28/21 1:20 PM, Uwe Kleine-König wrote:
> > On Mon, Jun 28, 2021 at 12:35:19PM -0400, Sean Anderson wrote:
> >> On 6/28/21 12:24 PM, Uwe Kleine-König wrote:
> >> > On Mon, Jun 28, 2021 at 11:50:33AM -0400, Sean Anderson wrote:
> >> > > On 6/27/21 2:19 PM, Uwe Kleine-König wrote:
> >> > > > On Fri, Jun 25, 2021 at 01:46:26PM -0400, Sean Anderson wrote:
> >> > > > > So for the moment, why not give an error? This will be legal code both
> >> > > > > now and after round_state is implemented.
> >> > > >
> >> > > > The problem is where to draw the line. To stay with your example: If a
> >> > > > request for period = 150 ns comes in, and let X be the biggest period <=
> >> > > > 150 ns that the hardware can configure. For which values of X should an
> >> > > > error be returned and for which values the setting should be
> >> > > > implemented.
> >> > > >
> >> > > > In my eyes the only sensible thing to implement here is to tell the
> >> > > > consumer about X and let it decide if it's good enough. If you have a
> >> > > > better idea let me hear about it.
> >> > >
> >> > > Sure. And I think it's ok to tell the consumer that X is the best we can
> >> > > do. But if they go along and request an unconfigurable state anyway, we
> >> > > should tell them as much.
> >> >
> >> > I have the impression you didn't understand where I see the problem. If
> >> > you request 150 ns and the controller can only do 149 ns (or 149.6667 ns)
> >> > should we refuse? If yes: This is very unusable, e.g. the led-pwm driver
> >> > expects that it can configure the duty_cycle in 1/256 steps of the
> >> > period, and then maybe only steps 27 and 213 of the 256 possible steps
> >> > work. (This example doesn't really match because the led-pwm driver
> >> > varies duty_cycle and not period, but the principle becomes clear I
> >> > assume.) If no: Should we accept 151 ns? Isn't that ridiculous?
> >>
> >> I am fine with this sort of rounding. The part I take issue with is when
> >> the consumer requests (e.g.) a 10ns period, but the best we can do is
> >> 20ns. Or at the other end if they request a 4s period but the best we
> >> can do is 2s. Here, there is no obvious way to round it, so I think we
> >> should just say "come back with a reasonable period" and let whoever
> >> wrote the device tree pick a better period.
> >
> > Note that giving ridiculus examples is easy, but this doesn't help to
> > actually implement something sensible. Please tell us for your example
> > where the driver can only implement 20 ns what is the smallest requested
> > period the driver should accept.
> 
> 20ns :)
> 
> In the case of this device, that would result in 0% duty cycle with a
> 100MHz input. So the smallest reasonable period is 30ns with a duty
> cycle of 20ns.

I took the time to understand the hardware a bit better, also to be able
to reply to your formulae below. So to recap (and simplify slightly
assuming TCSR_UDT = 1):


              TLR0 + 2
 period     = --------
              clkrate

              TLR1 + 2
 duty_cycle = -------- if TLR1 < TLR0, else 0
              clkrate


where TLRx has the range [0..0xffffffff] (for some devices the range is
smaller). So clkrate seems to be 100 MHz?

> >> > > IMO, this is the best way to prevent surprising results in the API.
> >> >
> >> > I think it's not possible in practise to refuse "near" misses and every
> >> > definition of "near" is in some case ridiculous. Also if you consider
> >> > the pwm_round_state() case you don't want to refuse any request to tell
> >> > as much as possible about your controller's capabilities. And then it's
> >> > straight forward to let apply behave in the same way to keep complexity
> >> > low.
> >> >
> >> > > The real issue here is that it is impossible to determine the correct
> >> > > way to round the PWM a priori, and in particular, without considering
> >> > > both duty_cycle and period. If a consumer requests very small
> >> > > period/duty cycle which we cannot produce, how should it be rounded?
> >> >
> >> > Yeah, because there is no obviously right one, I picked one that is as
> >> > wrong as the other possibilities but is easy to work with.
> >> >
> >> > > Should we just set TLR0=1 and TLR1=0 to give them 66% duty cycle with
> >> > > the least period? Or should we try and increase the period to better
> >> > > approximate the % duty cycle? And both of these decisions must be made
> >> > > knowing both parameters. We cannot (for example) just always round up,
> >> > > since we may produce a configuration with TLR0 == TLR1, which would
> >> > > produce 0% duty cycle instead of whatever was requested. Rounding rate
> >> > > will introduce significant complexity into the driver. Most of the time
> >> > > if a consumer requests an invalid rate, it is due to misconfiguration
> >> > > which is best solved by fixing the configuration.
> >> >
> >> > In the first step pick the biggest period not bigger than the requested
> >> > and then pick the biggest duty cycle that is not bigger than the
> >> > requested and that can be set with the just picked period. That is the
> >> > behaviour that all new drivers should do. This is somewhat arbitrary but
> >> > after quite some thought the most sensible in my eyes.
> >>
> >> And if there are no periods smaller than the requested period?
> >
> > Then return -ERANGE.
> 
> Ok, so instead of
> 
> 	if (cycles < 2 || cycles > priv->max + 2)
> 		return -ERANGE;
> 
> you would prefer
> 
> 	if (cycles < 2)
> 		return -ERANGE;
> 	else if (cycles > priv->max + 2)
> 		cycles = priv->max;

The actual calculation is a bit harder to handle TCSR_UDT = 0 but in
principle, yes, but see below.

> But if we do the above clamping for TLR0, then we have to recalculate
> the duty cycle for TLR1. Which I guess means doing something like
> 
> 	ret = xilinx_timer_tlr_period(priv, &tlr0, tcsr0, state->period);
> 	if (ret)
> 		return ret;
> 
> 	state->duty_cycle = mult_frac(state->duty_cycle,
> 				      xilinx_timer_get_period(priv, tlr0, tcsr0),
> 				      state->period);
> 
> 	ret = xilinx_timer_tlr_period(priv, &tlr1, tcsr1, state->duty_cycle);
> 	if (ret)
> 		return ret;

No, you need something like:

	/*
	 * The multiplication cannot overflow as both priv_max and
	 * NSEC_PER_SEC fit into an u32.
	 */
	max_period = div64_ul((u64)priv->max * NSEC_PER_SEC, clkrate);

	/* cap period to the maximal possible value */
	if (state->period > max_period)
		period = max_period;
	else
		period = state->period;

	/* cap duty_cycle to the maximal possible value */
	if (state->duty_cycle > max_period)
		duty_cycle = max_period;
	else
		duty_cycle = state->duty_cycle;

	period_cycles = period * clkrate / NSEC_PER_SEC;

	if (period_cycles < 2)
		return -ERANGE;

	duty_cycles = duty_cycle * clkrate / NSEC_PER_SEC;

	/*
	 * The hardware cannot emit a 100% relative duty cycle, if
	 * duty_cycle >= period_cycles is programmed the hardware emits
	 * a 0% relative duty cycle.
	 */
	if (duty_cycle == period_cycles)
		duty_cycles = period_cycles - 1;

	/*
	 * The hardware cannot emit a duty_cycle of one clk step, so
	 * emit 0 instead.
	 */
	if (duty_cycles < 2)
		duty_cycles = period_cycles;

> >> > > > > Perhaps I should add
> >> > > > >
> >> > > > > 	if (tlr0 <= tlr1)
> >> > > > > 		return -EINVAL;
> >> > > > >
> >> > > > > here to prevent accidentally getting 0% duty cycle.
> >> > > >
> >> > > > You can assume that duty_cycle <= period when .apply is called.
> >> > >
> >> > > Ok, I will only check for == then.
> >> >
> >> > You just have to pay attention to the case that you had to decrement
> >> > .period to the next possible value. Then .duty_cycle might be bigger
> >> > than the corrected period.
> >>
> >> This is specifically to prevent 100% duty cycle from turning into 0%. My
> >> current draft is
> >>
> >> 	/*
> >> 	 * If TLR0 == TLR1, then we will produce 0% duty cycle instead of 100%
> >> 	 * duty cycle. Try and reduce the high time to compensate. If we can't
> >> 	 * do that because the high time is already 0 cycles, then just error
> >> 	 * out.
> >> 	 */
> >> 	if (tlr0 == tlr1 && !tlr1--)
> >> 		return -EINVAL;
> >
> > If you follow my suggested policy this isn't an error and you should
> > yield the biggest duty_cycle here even if it is zero.
> 
> So like this?
> 
> 	if (tlr0 == tlr1) {
> 		if (tlr1)
> 			tlr1--;
> 		else if (tlr0 != priv->max)
> 			tlr0++;
> 		else
> 			return -ERANGE;
> 	}

No, this is wrong as it configures a longer period than requested in
some cases.

> And I would really appreciate if you could write up some documentation
> with common errors and how to handle them. It's not at all obvious to me
> what all the implications of the above guidelines are.

Yes, I fully agree this should be documented and doing that is on my
todo list. Until I come around to do this, enabling PWM_DEBUG should
help you getting this right (assuming you test extensively and read the
resulting kernel messages).

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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