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? Thanks and regards, Lokesh > > Best regards > Uwe >
