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Date:   Fri, 17 Apr 2020 15:50:27 +0200
From:   Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>
To:     Sandipan Patra <spatra@nvidia.com>
Cc:     Thierry Reding <treding@nvidia.com>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Bibek Basu <bbasu@nvidia.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        "linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] pwm: tegra: dynamic clk freq configuration by PWM driver
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Hello,

On Fri, Apr 17, 2020 at 10:06:28AM +0000, Sandipan Patra wrote:
> > On Wed, Apr 15, 2020 at 09:03:35AM +0000, Sandipan Patra wrote:
> > > > On Fri, Apr 03, 2020 at 06:05:03PM +0530, Sandipan Patra wrote:
> > > > > Added support for dynamic clock freq configuration in pwm kernel driver.
> > > > > Earlier the pwm driver used to cache boot time clock rate by pwm
> > > > > clock parent during probe. Hence dynamically changing pwm
> > > > > frequency was not possible for all the possible ranges. With this
> > > > > change, dynamic calculation is enabled and it is able to set the
> > > > > requested period from sysfs knob provided the value is supported by clock source.
> > > >
> > > > Without having looked closely at the patch (yet), just for my
> > > > understanding: If the PWM is running and the frequency changes, the
> > > > output changes, too, right? If so, do we need a notifier that
> > > > prevents a frequency change when the PWM is running?
> > >
> > > Yes, frequency can be changed anytime but by the same process who has
> > > acquired the channel. So if a process is already running/using the
> > > channel, same process can only modify the frequency.
> > 
> > How is this enforced? Does some other peripheral get its input clock from the
> > clock in question? What is the motivation to modify the frequency other than
> > modifying the PWM output?
>  
> PWM instance uses a derived clock and sets the divider for further division of rate.
> Regarding modifying frequency: it was my wrong interpretation. I mean, to modify
> the PWM output the driver first sets the clock rate which allows to configure the
> requested PWM output.

The point here is: It should not happen that some other driver modifies
a clock that results in a change of the output wave form. Also ideally
if the PWM is running you should not modify the clock as this results
in a non-atomic update. this is however not always possible and there is
no general guideline what to do then. In practise it probably matters
only little.

> > > > > +     /*
> > > > > +      *  Period in nano second has to be <= highest allowed period
> > > > > +      *  based on the max clock rate of the pwm controller.
> > > > > +      *
> > > > > +      *  higher limit = max clock limit >> PWM_DUTY_WIDTH
> > > > > +      */
> > > > > +     if (rate > (pc->soc->max_frequency >> PWM_DUTY_WIDTH))
> > > > > +             return -EINVAL;
> > > >
> > > > Related to my question above: What happens if the rate increases
> > > > after this check?
> > >
> > > Discussed above with my understanding. Please help me understand if
> > > you are referring to any other possibilities that rate can be changed.
> > 
> > The goal to reach is: The only way to modify the PWM output should be to call
> > pwm_apply_state() (or its legacy relatives).
> 
> I see with current settings, pwm output gets modified by .config() which
> comes from pwm_apply_state(). I think it suffices the purpose
> or I am still missing anything?

I assume, you don't miss something.

> > > > Also the division above is just done to compare the requested period
> > > > value with the allowed range.
> > > >
> > > > Your check is:
> > > >
> > > >         NSEC_PER_SEC / period_ns > (max_frequency >> PWM_DUTY_WIDTH)
> > > >
> > > > This is equivalent to
> > > >
> > > >         period_ns <= NSEC_PER_SEC / (max_frequency >>
> > > > PWM_DUTY_WIDTH)
> > > >
> > > > where the right side is constant per PWM type. (Rounding might need
> > > > addressing.)
> > >
> > > I will update this calculation in the probe since max_frequency value
> > > is Different for each chip. Also please note that at this point the
> > > rate is not the actual pwm output rate. It's just a reference for what
> > > should be the source clock rate and then requested with
> > > clk_set_rate(); Actual rounding is required while setting pwm
> > > controller output rate is done later down in same function.
> > 
> > I think I understood. Will check again in your next patch round.
> > 
> > > > > +              * clk_set_rate() can not be called again in config because
> > > > > +              * T210 or any prior chip supports one pwm-controller and
> > > > > +              * multiple channels. Hence in this case cached clock rate
> > > > > +              * will be considered which was stored during probe.
> > > >
> > > > I don't understand that. If
> > >
> > > The if part is for SoCs which have single channel per pwm instance.
> > > i.e. T186,
> > > T194 etc. For controllers with single channel, dynamic clock rate
> > > configuration is possible. The other part is for legacy controller
> > > which has multiple channels for single pwm instance. The pwm
> > > controllers having multiple channels share the source clock. So it
> > > does not allow dynamic clock configuration since it will affect users on the
> > other channels.
> > 
> > The usual approach here is to allow changes iff all other channels are off or
> > unused.
> > 
> 
> This is handled in the if part, where pwm instances have only one channel
> and only the dynamic clock configuration can be done. On the other side
> (under else part), the rate is stored during probe and it does not get modified
> during run time.
> 
> > > > > +              */
> > > > > +             rate = pc->clk_rate >> PWM_DUTY_WIDTH;
> > > > > +     }
> > > > >
> > > > >       /* Consider precision in PWM_SCALE_WIDTH rate calculation */
> > > > >       hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC,
> > > > > period_ns);
> > > >
> > > > I took a deeper look into the driver now. Just to ensure, I
> > > > understood the PWMs behaviour right:
> > > >
> > > > There is an ENABLE bit (with obvious semantics), a 13-bit SCALE
> > > > value and an 8- bit DUTY value. There is an internal counter
> > > > incrementing by one each (SCALE +
> > > > 1) clock cycles and resets at 256. The counter going from 0 to 256
> > > > defines the period length. On counter reset the output gets active
> > > > and on reaching DUTY the output gets inactive.
> > > >
> > > > So we have:
> > > >
> > > >         .period = 256 * (SCALE + 1) / clkrate
> > > >         .duty_cycle = DUTY * (SCALE + 1) / clkrate
> > > >
> > > > Right?
> > >
> > > Yes. Right.
> > 
> > Ideally this would be described in a code comment.
> 
> Ok.
> I will add adequate comments to help providing the register insights.
> 
> > 
> > > >  - When .duty_ns == .period the assignment of DUTY overflows.
> > > >    (Can the PWM provide 100% duty cycle at all?)
> > >
> > > Yes, PWM controller is capable to provide 100% duty cycle.
> > > Bits 30:16 are dedicated for pulse width out of which only 24:16 (9
> > > bits) are used. Only 8 bits are usable [23:16] for varying pulse width.
> > > To achieve 100% duty cycle, Bit [24] needs to be programmed of this
> > > register to 1'b1.
> > 
> > This needs to be documented in a driver comment to be understandable for
> > people being interested in this driver later.
> >
> 
> Sure. As stated above, I will add the details in code comment. And for further
> Understanding Tegra documents and specifications can be followed.

If they are publically available, having a link at the top of the driver
would be great.

> > If Bit[24] is 1, should [23:16] be zero, or is it "don't care" then?
> >
> 
> Once the 24th bit is set, all other bits are considered to be don't care.

ok.

> > > >  - The comment "Since the actual PWM divider is the register's frequency
> > > >    divider field minus 1, we need to decrement to get the correct value
> > > >    to write to the register." seems wrong. If I understand correctly, we
> > > >    need to do s/minus/plus/. If the register holds a 0, the divider
> > > >    isn't -1 for sure?!
> > >
> > > Yes, you are right. The comment needs a correction. It will be plus 1
> > > instead of minus 1. I will update the comment in the follow up patch.
> > > Otherwise the calculation is correct.
> > > rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz); here rate is the
> > > divider value to be set.
> > 
> > If a certain duty+period is requested the driver is supposed to provide an output
> > such that:
> > 
> >         implemented_period = max{ possible periods <= requested period }
> >         implemented_duty = max{ possible duty <= requested duty }
> > 
> 
> I am not clear if I understood the question correctly.

It was not a question :-)

> implemented_period = max{ possible periods <= requested period }
> should it be, min { possible periods, requested period } ?

To put my expression in words: pick the maximum of the possible periods
that are less or equal to the requested value.  Maybe this is better
understandable:

	max { x ∊ implementablePeriods | x <= requestedPeriod }

?

> If you are asking for requested parameters to fall inside range, this is taken care
> at below checks.
> if (period_ns < min_period_ns) //lower bound
> And if (rate >> PWM_SCALE_WIDTH) //higher bound
> 
> If I am not clear with the question, please help me understanding.

Also not sure if your problem is resolved with my words. I hope so,
please ask if something is still unclear. Maybe also look at the
PWM_DEBUG checks to understand.

> > so I think DIV_ROUND_CLOSEST_ULL is wrong.
> > (If the driver provided the modern callback instead of .config/.enable/.disable
> > CONFIG_PWM_DEBUG would help you here.)
> 
> FYI, I will further be working on a separate change sets for tegra pwm driver
> to use atomic callbacks.

That's good. If you do these first, you can benefit from PWM_DEBUG
checks.

> > > > How does the PWM behave when it gets disabled? Does it complete the
> > > > currently running period? Does the output stop at the inactive
> > > > level, or where it just happens to be? How does a running PWM behave
> > > > when the register is updated? Does it complete the currently running period?
> > >
> > > Yes, it allows to write the bit during any active and inactive time of
> > > the width. Hence the pwm gets disabled as soon as the enable bit is set to 0.
> > 
> > OK, so the output stops oscillating as soon as the PWM_ENABLE bit is cleared in
> > hardware. How does the output behave then? (Does the output become
> > inactive? Or does it drive the output level where it just happens to be?) I assume
> > that the register write in tegra_pwm_config() also results in aborting the
> > currently running period and start of a new one with the new settings?
>  
> Yes, the output stops as soon as the PWM_ENABLE bit is cleared in hardware. Then
> The output is set to 0 (which is inactive).
> Once .disable() => tegra_pwm_disable() gets invoked, enable bit is cleared and hence
> PWM will possess no output signal.
> tegra_pwm_config() will be invoked for any new configuration request.

Some drivers already have a "Limitations" section in their header.
Please take a look at the existing examples and provide something
similar. (Note you still didn't answer "How does a running PWM behave
when the register is updated? Does it complete the currently running
period?". I assume the answer to the second question is "No" (and the
first is only there for rhetoric reasons).)

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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