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SubjectRe: [RESEND PATCH v5 0/9] extend PWM framework to support PWM modes
Hello Claudiu,

On Fri, Oct 26, 2018 at 10:44:43AM +0000, Claudiu.Beznea@microchip.com wrote:
> Thank you for your inputs and sorry for the late response. Please see my
> answers inline.

No problems, I didn't held my breath :-)

> On 22.10.2018 11:29, Uwe Kleine-König wrote:
> > On Tue, Aug 28, 2018 at 04:01:17PM +0300, Claudiu Beznea wrote:
> >> Since PWMs with more than one output per channel could be used as one
> >> output PWM the normal mode is the default mode for all PWMs (if not
> >> specified otherwise).
> >>
> >> Complementary mode - for PWM channels with two outputs; output waveforms
> >> for a PWM channel in complementary mode looks line this:
> >> __ __ __ __
> >> PWMH1 __| |__| |__| |__| |__
> >> __ __ __ __ __
> >> PWML1 |__| |__| |__| |__|
> >> <--T-->
> >>
> >> Where T is the signal period.
> >
> > So this translates to (I think):
> >
> > __ __ __ __ __
> > PWMH __/ \_____/ \_____/ \_____/ \_____/ \_____/
> > __ _____ _____ _____ _____ _____
> > PWML \__/ \__/ \__/ \__/ \__/ \
> > ^ ^ ^ ^ ^ ^
> >
> > That is PWML always pulls in the opposite direction of PWMH. Maybe we
> > could come up with better terms than PWMH and PWML (which might be
> > specific for the Atmel implementation?).
>
> Yes, this is Atmel implementation naming.
>
> > Maybe "normal" and
> > "complement"?
>
> I will think about it try to come with new naming. Normal and Complement
> may be confusing for users with regards to PWM modes.
>
> >
> >> Push-pull mode - for PWM channels with two outputs; output waveforms for a
> >> PWM channel in push-pull mode with normal polarity looks like this:
> >> __ __
> >> PWMH __| |________| |________
> >> __ __
> >> PWML ________| |________| |__
> >> <--T-->
> >
> > That again with the alternative display method and duty cycle 1/3:
> >
> > __ __ __
> > PWMA __/ \______________/ \______________/ \______
> > __ __
> > PWMB ___________/ \______________/ \______________/
> > ^ ^ ^ ^ ^ ^
> Ok.
>
> > That is PWMA and PWMB are active only every 2nd period taking alternate
> > turns, right?
>
> Yes.
>
> >
> >
> >> If polarity is inversed:
> >> __ ________ ________
> >> PWMH |__| |__|
> >> ________ ________ __
> >> PWML |__| |__|
> >> <--T-->
> >
> > That's again with duty cycle 1/3:
> >
> > __ ______________ ______________ ______
> > PWMA \__/ \__/ \__/
> > ___________ ______________ ______________
> > PWMB \__/ \__/ \
> > ^ ^ ^ ^ ^ ^
> >
>
> Ok.
>
> > Given that the start of period isn't externally visible this is
> > equivalent to using a duty cycle 2/3 and not inverting which results in:
> >
> > __ ______________ ______________ ______
> > PWMA \__/ \__/ \__/
> > ___________ ______________ ______________
> > PWMB \__/ \__/ \
> > ^ ^ ^ ^ ^
> >
> > I would really like if a more detailed description of the modes would be
> > created as part of this series.
>
> Sure, I will try to document it better.

Note here I just leaked my belief that the PWM framework shouldn't
necessarily expose inversion to users which is part of my discussion
with Thierry. I think it would be sensible to drop it here.

> > Currently there are a few implied
> > properties hidden in the PWM API (unrelated to this series) which I try
> > to resolve together with Thierry. Getting this documented right from the
> > start would be great here.
>
> Could you tell me if you want something specific to be touch as part of
> documentation process for these PWM modes?

At least having something about the expectations in Documenation/ would
be great.

> > I didn't look in detail into the driver implementation, but from the
> > PWMs implemented in the STM32F4 family I would have chosen a different
> > model which makes me wonder if we should stick to a more general way to
> > describe two outputs from a single PWM channel.
> >
> > I would use four values with nanosecond resolution to describe these:
> >
> > .period
> > .duty_cycle
> > .alt_duty_cycle
> > .alt_offset
> >
> > period and duty_cycle is as before for the primary output and then the
> > alt_* values describe offset and duty cycle of the secondary output.
> >
> > What you called "normal mode" would then be specified using
> >
> > .period = $period
> > .duty_cycle = $duty_cycle
> > .alt_duty_cycle = 0
> > .alt_offset = dontcare
> >
> > Your "push pull mode" would be:
> >
> > .period = 2 * $period
> > .duty_cycle = $duty_cycle
> > .alt_duty_cycle = $duty_cycle
> > .alt_offset = $period
> >
> > and complementary mode would be specified using:
> >
> > .period = $period
> > .duty_cycle = $duty_cycle
> > .alt_duty_cycle = $period - $duty_cycle
> > .alt_offset = $duty_cycle
> >
>
> On Atmel PWM controller the push-pull mode is hardware generated based on
> period and duty cycles that are setup for only one channel. The hardware
> will take care of the synchronization b/w the outputs so that the push-pull
> characteristic to be generated.
>
> Having different configuration for every output part of the push-pull
> waveform will allow users to generate every kind of outputs. But for IPs
> that are capable of push-pull or complementary modes the generation of the
> 2 outputs are done in hardware (true in case of Atmel PWM controller). In
> case of STM32F4 as far as I can see from [1] "The advanced-control timers
> (TIM1 and TIM8 ) can output two complementary signals and
> manage the switching-off and the switching-on instants of the outputs."
> Maybe, in this case, if there are 2 hardware blocks that could be synced to
> work together, e.g. in complementary mode, the setting of these two timers
> should be done in driver so that the hardware blocks to be configured
> together, atomically, so that the complementary characteristics to be obtained.

The upside I see in my approach with .alt_duty_cycle and .alt_offset
over your .mode is that it allows to describe more use-cases. If I
wanted to support complementary with dead-time I'd need another member
in pwm_state to specify the dead time. Then the next controller can only
add dead time on one end of secondary output needing yet another mode
enum. With my approach I think you can specify all sensible(TM)
waveforms already now. Then a driver must not be adapted again when
someone adds support for another mode.

The downside is that if your PWM only supports complementary mode with
no dead-time you have to find out from .alt_duty_cycle and .alt_offset
that the specified wave form is indeed matching complementary mode. From
from the framework's POV this is more elegant though (but YMMV).

> > With this abstraction stuff like "complementary output with dead-time
> > insertion" (something like:
> >
> > __ __ __
> > PWMA __/ \______________/ \______________/ \______
> > __________ __________ __
> > PWMB _______/ \______/ \______/
> > ^ ^ ^
> >
> > ) could be modelled.
>
> Same for this, my opinion is that we should implement generic things in
> core and drivers should configure properly the IP so that it generates the
> proper signals.

This is common to both our approaches. Just the way the PWM user
specifies his/her wishes (and so the input for hw drivers) is different.

> >> The PWM working modes are per PWM channel registered as PWM's capabilities.
> >> The driver registers itself to PWM core a get_caps() function, in
> >> struct pwm_ops, that will be used by PWM core to retrieve PWM capabilities.
> >> If this function is not registered in driver's probe, a default function
> >> will be used to retrieve PWM capabilities. Currently, the default
> >> capabilities includes only PWM normal mode.
> >
> > In the i2c framework this is a function, too, and I wonder if simplicity
> > is better served when this is just a flags member in the pwm_ops
> > structure.
>
> Thierry proposed this so that we could retrieve capabilities per PWM channel.

I don't have a complete overview over the different hardware
implementations, but I'd expect that if two different implementations
share the operations then the return value of .get_caps is shared by
both. As long this is the case not introducing a callback for that is
the easier path. Adding a callback later when (and if) this is required
is trivial then.

Best regards
Uwe

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

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