Re: [PATCH v3 4/5] pwm: omap-dmtimer: Do not disable pwm before changing period/duty_cycle

[Thierry Reding <thierry.reding@gmail.com>]

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On Wed, Apr 01, 2020 at 01:47:32PM +0200, Uwe Kleine-König wrote:
> On Wed, Apr 01, 2020 at 03:52:21PM +0530, Lokesh Vutla wrote:
> > Hi Uwe,
> > 
> > On 01/04/20 1:52 PM, Uwe Kleine-König wrote:
> > > Hello Thierry,
> > > 
> > > On Tue, Mar 31, 2020 at 10:45:59PM +0200, Thierry Reding wrote:
> > >> On Mon, Mar 30, 2020 at 09:16:54PM +0200, Uwe Kleine-König wrote:
> > >>> On Mon, Mar 30, 2020 at 04:14:36PM +0200, Thierry Reding wrote:
> > >>>> On Thu, Mar 12, 2020 at 07:40:42AM +0100, Uwe Kleine-König wrote:
> > >>>>> On Thu, Mar 12, 2020 at 09:52:09AM +0530, Lokesh Vutla wrote:
> > >>>>>> Only the Timer control register(TCLR) cannot be updated when the timer
> > >>>>>> is running. Registers like Counter register(TCRR), loader register(TLDR),
> > >>>>>> match register(TMAR) can be updated when the counter is running. Since
> > >>>>>> TCLR is not updated in pwm_omap_dmtimer_config(), do not stop the
> > >>>>>> timer for period/duty_cycle update.
> > >>>>>
> > >>>>> I'm not sure what is sensible here. Stopping the PWM for a short period
> > >>>>> is bad, but maybe emitting a wrong period isn't better. You can however
> > >>>>> optimise it if only one of period or duty_cycle changes.
> > >>>>>
> > >>>>> @Thierry, what is your position here? I tend to say a short stop is
> > >>>>> preferable.
> > >>>>
> > >>>> It's not clear to me from the above description how exactly the device
> > >>>> behaves, but I suspect that it may latch the values in those registers
> > >>>> and only update the actual signal output once a period has finished. I
> > >>>> know of a couple of other devices that do that, so it wouldn't be
> > >>>> surprising.
> > >>>>
> > >>>> Even if that was not the case, I think this is just the kind of thing
> > >>>> that we have to live with. Sometimes it just isn't possible to have all
> > >>>> supported devices adhere strictly to an API. So I think the best we can
> > >>>> do is have an API that loosely defines what's supposed to happen and
> > >>>> make a best effort to implement those semantics. If a device deviates
> > >>>> slightly from those expectations, we can always cross fingers and hope
> > >>>> that things still work. And it looks like they are.
> > >>>>
> > >>>> So I think if Lokesh and Tony agree that this is the right thing to do
> > >>>> and have verified that things still work after this, that's about as
> > >>>> good as it's going to get.
> > >>>
> > >>> I'd say this isn't for the platform people to decide. My position here
> > >>> is that the PWM drivers should behave as uniform as possible to minimize
> > >>> surprises for consumers. And so it's a "PWM decision" that is to be made
> > >>> here, not an "omap decision".
> > >>
> > >> I think there's a fine line to be walked here. I agree that we should
> > >> aim to have as much consistency between drivers as possible. At the same
> > >> time I think we need to be pragmatic. As Lokesh said, the particular use
> > >> case here requires this type of on-the-fly adjustment of the PWM period
> > >> without stopping and restarting the PWM. It doesn't work otherwise. So
> > >> th alternative that you're proposing is to say that we don't support
> > >> that use-case, even though it works just fine given this particular
> > >> hardware. That's not really an option.
> > > 
> > > I understand your opinion here. The situation now is that in current
> > > mainline the driver stops the hardware for reconfiguration and it
> > > doesn't fit Lokesh's use case so he changed to on-the-fly update
> > > (accepting that maybe a wrong period is emitted). What if someone relies
> > > on the old behaviour? What if in a year someone comes and claims the
> > > wrong period is bad for their usecase and changes back to
> > > stop-to-update?
> > > 
> > > When I write a consumer driver, do I have a chance to know how the PWM,
> > > that I happen to use, behaves? To be able to get my consumer driver
> > > reliable I might need to know that however.
> > > 
> > >>>> I know this is perhaps cheating a little, or turning a blind eye, but I
> > >>>> don't know what the alternative would be. Do we want to tell people that
> > >>>> a given PWM controller can't be used if it doesn't work according to our
> > >>>> expectations? That's hard to argue if that controller works just fine
> > >>>> for all known use-cases.
> > >>>
> > >>> I'd like have some official policy here which of the alternatives is the
> > >>> preferred cheat.
> > >>>
> > >>> The situation here is that period and duty_cycle cannot be updated
> > >>> atomically. So the two options are:
> > >>>
> > >>>  - stop shortly
> > >>>  - update with hardware running and maybe emit a broken period
> > >>
> > >> I think we can already support both of those with the existing API. If
> > >> a consumer wants to stop the PWM while reconfiguring, they should be
> > >> able to do pwm_enable(), pwm_config(), pwm_enable() (or the atomic
> > >> equivalent) and for the second case they can just do pwm_config() (or
> > >> the atomic equivalent).
> > > 
> > > Yes, the consumer can force the stop and update. But assume I'm "only" a
> > > consumer driver author and I want: atomic update and if this is not
> > > possible I prefer "stop-to-update" over "on-the-fly-and-maybe-faulty".
> > > So I cannot benefit from a good driver/hardware that can do atomic
> > > updates? Or I have to patch each driver that I actually use to use
> > > stop-to-update?
> > > 
> > >> Some hardware may actually require the PWM to be disabled before
> > >> reconfiguring, so they won't be able to strictly adhere to the second
> > >> use-case.
> > >>
> > >> But as discussed above, I don't want to strive for a lowest common
> > >> denominator that would preclude some more specific use-cases from
> > >> working if the hardware supports it.
> > >>
> > >> So I think we should aim for drivers to implement the semantics as
> > >> closely as possible. If the hardware doesn't support some of these
> > >> requirements strictly while a particular use-case depends on that, then
> > >> that just means that the hardware isn't compatible with that use-case.
> > >> Chances are that the system just isn't going to be designed to support
> > >> that use-case in the first place if the hardware can't do it.
> > >>
> > >> The sysfs interface is a bit of a special case here because it isn't
> > >> possible to know what use-cases people are going to come up with.
> > > 
> > > In my eyes the sysfs interface isn't special here. You also don't know
> > > what the OMAP PWM hardware is used for.
> > > 
> > >> It's most likely that they'll try something and if it doesn't work
> > >> they can see if a driver patch can improve things.
> > > 
> > > So either the group who prefers "stop-to-update" or the group who
> > > prefers "on-the-fly-and-maybe-faulty" has to carry a system specific
> > > driver patch?
> > > 
> > >> One possible extension that I can imagine would be to introduce some
> > >> sort of capability structure that drivers can fill in to describe the
> > >> behaviour of the hardware. Drivers like pwm-omap-dmtimer, for example,
> > >> could describe that they are able to change the period and/or duty cycle
> > >> while the PWM is on. There could be another capability bit that says
> > >> that the current period will finish before new settings are applied. Yet
> > >> another capability could describe that duty-cycle and period can be
> > >> applied atomically. Consumers could then check those capabilities to see
> > >> if they match their requirements.
> > >>
> > >> But then again, I think that would just make things overly complicated.
> > >> None of the existing consumers need that, so it doesn't seem like there
> > >> is much demand for that feature. In practice I suspect most consumers
> > >> work fine despite potentially small deviations in how the PWM behaves.
> > > 
> > > I think the status quo is what I asked about above: People use sysfs and
> > > if the PWM behaves different than needed, the driver is patched and most
> > > of the time not mainlined. If your focus is to support a certain
> > > industrial system with a defined use case, this is fine. If however you
> > > target for an universal framework that works for any combination of
> > > consumer + lowlevel driver without patching (that at least is able to
> > > diagnose: This PWM cannot provide what my consumer needs), this is bad.
> > > Also this means that whenever a system designer changes something on
> > > their machine (kernel update, different hardware, an new usecase for a
> > > PWM) they might have to reverify if the given PWM driver behaves as
> > > needed.
> > > 
> > > My suggestion for now is to start documenting how the drivers behave
> > > expanding how limitations are documented in some drivers. So maybe
> > > change from "Limitations" to "Implementation and Hardware Details"?
> > 
> > Does it help if a new DT property is introduced across PWM subsystem,
> > representing dynamic period/duty-cycle updates. Based on this property driver
> > can handle the updates. If the property is not present existing behaviour can be
> > restored. This way based on the use-case things can be changed and need not
> > patch the driver :). Does this sound good or you have other thoughts?
> 
> That's something that I'd rather see in the pwm API. (Either by a rule
> that drivers should prefer one or the other, or by making it
> configurable.) IMHO this property doesn't belong into the hardware
> description as it is a software property.
> 
> That's not constructive though as I don't have an idea how to map this
> into the API.

We can already enforce disable/config/enable with the existing API. The
only think that we can't enforce is that a configuration will always be
applied atomically or without disabling and reenabling the PWM.

One possible solution would be to extend struct pwm_state with a set of
flags that can be set. For that PTP kind of applications, consumers
could set some pwm_state.strict (or whatever) flag and then a driver
could fail ->apply() if it doesn't support changing the period/duty-
cycle atomically and without disabling the PWM first. Or it could be
more fine-grained, like:

	state.on_the_fly = true;
	state.consistent = true;

To specify that the PWM needs to be changed on the fly (i.e. without
disabling and reenabling) and duty-cycle and period must be consistent
(i.e. be applied to the signal at the same time).

Some driver may be able to only respect state.on_the_fly == true but not
state.consistent == true.

But then again, I don't think we'll see those cases in practice, since
no hardware designer is going to make a board for a PTP use-case with a
PWM that doesn't support it.

That said, if somebody sees value in that and can come up with a good
series of patches and concrete use-cases to show how this would be
useful, I'd be willing to take those patches.

Thierry

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