Re: [PATCH v3 2/2] drm/bridge: ti-sn65dsi86: Implement the pwm_chip

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Wed 23 Jun 03:29 CDT 2021, Uwe Kleine-K?nig wrote:

> Hello Bjorn,
> 
> On Tue, Jun 22, 2021 at 08:12:48PM -0500, Bjorn Andersson wrote:
> > On Tue 22 Jun 15:29 CDT 2021, Uwe Kleine-K?nig wrote:
> > > On Mon, Jun 21, 2021 at 10:09:48PM -0500, Bjorn Andersson wrote:
> > > > +		/*
> > > > +		 * PWM duty cycle is given as:
> > > > +		 *
> > > > +		 *   duty = BACKLIGHT / (BACKLIGHT_SCALE + 1)
> > > > +		 *
> > > > +		 * The documentation is however inconsistent in its examples,
> > > > +		 * so the interpretation used here is that the duty cycle is
> > > > +		 * the period of BACKLIGHT * PRE_DIV / REFCLK_FREQ.
> > > 
> > > I don't understand this.
> > > 
> > > > +		 *
> > > > +		 * The ratio PRE_DIV / REFCLK_FREQ is rounded up to whole
> > > > +		 * nanoseconds in order to ensure that the calculations are
> > > > +		 * idempotent and gives results that are smaller than the
> > > > +		 * requested value.
> > > > +		 */
> > > > +		tick = DIV_ROUND_UP(NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);
> > > > +		backlight = state->duty_cycle / tick;
> > > 
> > > You're loosing precision here by dividing by the result of a division.
> > 
> > The actual period is also a result of a division and after spending too
> > many hours scratching my head I reached to conclusion that this was the
> > reason why I wasn't able to get the duty cycle calculation idempotent
> > over the ranges I tested.
> 
> How did you test? Using the sysfs interface?
>  

I primarily tested this by transplanting this into a user space thing
where I could sweep over various values for refclk, duty cycle and
period.

Then after that I tested it setting up pwm-backlight on top (as I don't
have access to the signal anyways) and try a few different periods and
for those test all possible brightness levels for those periods... (With
CONFIG_PWM_DEBUG enabled)

> > But in my effort to describe this to you here, I finally spotted the
> > error and will follow up with a new version that does:
> > 
> >                 actual = NSEC_PER_SEC * (pre_div * scale + 1) / pdata->pwm_refclk_freq);
> >                 backlight = state->duty_cycle * (scale + 1) / actual;
> 
> So the first term ("actual") is the period that you get for a given
> pre_div, scale and pwm_refclk_freq, right? And the 2nd ("backlight")
> defines the register value to configure the duty_cycle, right?
> 

Right, pre_div and pwm_refclk_freq defines the rate at which the PWM
ticks. "actual" is our estimate of the actual period that results in and
"backlight" is then the number of ticks (each prediv / refclk seconds
long) the signal should be high.

> I wonder: Is it possible to configure a 100% relative duty cycle? Then
> backlight would be scale + 1 which (at least if scale is 0xffff) would
> overflow the 16 bit register width?!
> 

The documentation gives two examples:
* backlight = 0x40, scale = 0xff results in 25% duty cycle, i.e. the
  duty is 0x40 / (0xff + 1).
* backlight = 0xff, scale = 0xff results in 100% duty cycle, i.e. the
  duty is 0xff / 0xff.

As you can see these are in  conflict and I think the latter is the one
that doesn't match the rest of what's described.

So I don't think it's possible to go beyond 99.6% - 99.998% duty cycle,
depending on BACKLIGHT_SCALE.

> > > > +static void ti_sn_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > +				struct pwm_state *state)
> > > > +{
> > > > +	struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
> > > > +	unsigned int pwm_en_inv;
> > > > +	unsigned int pre_div;
> > > > +	u16 backlight;
> > > > +	u16 scale;
> > > > +	int ret;
> > > > +
> > > > +	ret = regmap_read(pdata->regmap, SN_PWM_EN_INV_REG, &pwm_en_inv);
> > > > +	if (ret)
> > > > +		return;
> > > > +
> > > > +	ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_SCALE_REG, &scale);
> > > > +	if (ret)
> > > > +		return;
> > > > +
> > > > +	ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_REG, &backlight);
> > > > +	if (ret)
> > > > +		return;
> > > > +
> > > > +	ret = regmap_read(pdata->regmap, SN_PWM_PRE_DIV_REG, &pre_div);
> > > > +	if (ret)
> > > > +		return;
> > > > +
> > > > +	state->enabled = FIELD_GET(SN_PWM_EN_MASK, pwm_en_inv);
> > > > +	if (FIELD_GET(SN_PWM_INV_MASK, pwm_en_inv))
> > > > +		state->polarity = PWM_POLARITY_INVERSED;
> > > > +	else
> > > > +		state->polarity = PWM_POLARITY_NORMAL;
> > > > +
> > > > +	state->period = DIV_ROUND_UP(NSEC_PER_SEC * (pre_div * scale + 1), pdata->pwm_refclk_freq);
> > > > +	state->duty_cycle = backlight * DIV_ROUND_UP(NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);
> > > 
> > > If you use
> > > 
> > > 	state->duty_cycle = DIV_ROUND_UP(backlight * NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);
> > > 
> > > instead (with a cast to u64 to not yield an overflow) the result is more
> > > exact.
> > > 
> > 
> > The problem with this is that it sometimes yields duty_cycles larger
> > than what was requested... But going back to describing this as a ratio
> > of the period this becomes:
> > 
> >         state->duty_cycle = DIV_ROUND_UP_ULL(state->period * backlight, scale + 1);
> 
> I saw your next iteration of this patch set, but didn't look into it
> yet. Note that if it uses this formula it sill looses precision.
> Consider:
> 
> 	pwm_refclk_freq = 1333333
> 	pre_div = 4
> 	scale = 60000
> 	backlight = 59999
> 
> then you calculate:
> 
> 	state->period = 180000796 (exact value: 180000795.00019875)
> 	state->duty_cycle = 179994797 (exact value: 179994795.0736975)
> 
> so duty_cycle should actually be reported as 179994796. That happens
> because state->period is already the result of a division, you get the
> right value when doing:
> 
> 	state->duty_cycle = round_up(NSEC_PER_SEC * (pre_div * scale + 2) * backlight, (scale + 1) * pdata->pwm_refclk_freq)
> 

The problem (in addition to that being hideous) with that added
precision is that if I plug in that duty_cycle and period with
pwm_refclk_freq = 19200000 (one of the valid ones) the function is no
longer idempotent.

With period given as 180000796 i get 179998542 back as actual period,
but the duty cycle becomes 3186264 and if I throw that in I get 3185473.

> > > I still find this surprising, I'd expect that SCALE also matters for the
> > > duty_cycle. With the assumption implemented here modifying SCALE only
> > > affects the period. This should be easy to verify?! I would expect that
> > > changing SCALE doesn't affect the relative duty_cycle, so the brightness
> > > of an LED is unaffected (unless the period gets too big of course).
> > > 
> > 
> > I think the hardware is two nested counters, one (A) ticking at REFCLK_FREQ
> > and as that hits PRE_DIV, it kicks the second counter (B) (and resets).
> > 
> > As counter A is reset the output signal goes high, when A hits BACKLIGHT the
> > signal goes low and when A hits BACKLIGHT_SCALE it resets.
> 
> then we would probably have:
> 
> 	period = (scale + 1) * pre_div / refclk
> 
> but not
> 
> 	period = (scale * pre_div + 1) / refclk
> 
> . The former would be nicer because then in the calculation for
> duty_cycle the factor (scale + 1) would cancel.
> 

Not only does scale + 1 cancel, there's something entity that actually
divides the (BACKLIGHT_SCALE + 1) in the denominator of the duty cycle
ratio.

> > Per this implementation the actual length of the duty cycle would indeed
> > be independent of the BACKLIGHT_SCALE,
> 
> In your formula for duty_cycle scale actually does matter. So I think
> we're not there yet?
> 

Right, the relationship between pre_div, backlight and duty_cycle should
be independent of period. I think is misinterpreted what you said
yesterday, and thought you where looking for there to be a relationship.


So, if we decide that we have a typo in the datasheet and make the
formula:

          NSEC_PER_SEC * PRE_DIV * (BACKLIGHT_SCALE + 1)
period = -----------------------------------------------
                        REFCLK_FREQ

then given the formula for the duty ratio:

  duty           BACKLIGHT
-------- = ---------------------
 period     BACKLIGHT_SCALE + 1

with NSEC_PER_SEC * PRE_DIV / REFCLK_FREQ cancelled out, this fits
better together and we can deduce that:

              NSEC_PER_SEC * PRE_DIV * BACKLIGHT
duty_cycle = ------------------------------------
                        REFCLK_FREQ

So after adjusting the calculations for pre_div and scale I can
calculate backlight, without first calculating the actual period using:

             duty_cycle * REFCLK_FREQ
BACKLIGHT = --------------------------
              NSEC_PER_SEC * PRE_DIV

Which I now assume is what you where trying to say but I misunderstood
the other day?


PS. With refclk 19200000 and period 180000796 this satisfies the
PWM_DEBUG requirements for all possible duty_cycles.

Regards,
Bjorn

> > but the length of the low signal (and hence the ratio, which results
> > in the actual brightness) does depend on BACKLIGHT_SCALE.
> 
> Best regards
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |