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

[Lokesh Vutla]

Hi Uwe,

On 01/04/20 1:52 PM, Uwe Kleine-KÃnig 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-KÃnig 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-KÃnig 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?

Thanks and regards,
Lokesh

> 
> Best regards
> Uwe
>