RE: [PATCH] pwm: tegra: dynamic clk freq configuration by PWM driver

[Sandipan Patra]

Hello,

> -----Original Message-----
> From: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
> Sent: Friday, April 17, 2020 7:20 PM
> To: Sandipan Patra <spatra@xxxxxxxxxx>
> Cc: Thierry Reding <treding@xxxxxxxxxx>; robh+dt@xxxxxxxxxx; Jonathan
> Hunter <jonathanh@xxxxxxxxxx>; Bibek Basu <bbasu@xxxxxxxxxx>; Laxman
> Dewangan <ldewangan@xxxxxxxxxx>; linux-pwm@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] pwm: tegra: dynamic clk freq configuration by PWM driver
> 
> External email: Use caution opening links or attachments
> 
> 
> 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 â implementablePeriods | x <= requestedPeriod }
> 
> ?

I think I got your question.
Should tegra_pwm_config() not return error (EINVAL) when the requested period is
invalid but it should configure to a nearest possible value?
 
> 
> > 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).)
>
 
1. I will add the below comments as Limitations:
-	When PWM is disabled, the output is driven to 0 and
-	It does not allow the current PWM period to complete and stops abruptly.
2. Yes. Right.
If the register is updated while the pwm is running, It does not complete the
currently running period.

Hope this clarifies the concerns.
If they are clarified and acknowledged, I will be able to send the Patch for V2.


Thanks & Regards,
Sandipan

> Best regards
> Uwe
> 
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> Pengutronix e.K.                           | Uwe Kleine-KÃnig            |
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