On Tue, Mar 31, 2020 at 08:59:47PM +0530, Lokesh Vutla wrote: > Hi Thierry, > > On 30/03/20 7:44 PM, 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. > > Yes this is needed especially in the use-case[0] that I am trying to enable > using PWM. In this case PWM cannot be stopped in between and needs to be updated > dynamically. Also hardware doesn't provide any restrictions on updating the > period. So IMHO, this might be the right thing to do. > > Tony did provide tested-by and I measured PWM signals on scope with these > changes. Let me know if any thing else is required? > > [0] https://sourceforge.net/p/linuxptp/mailman/message/36943248/ From you measurements, can you tell whether or not the signal actually gets updated in the middle of a period, or does it only get updated at the end of a full period? Does the reference manual document this? Thierry
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