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

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Date:   Tue, 31 Mar 2020 22:10:14 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Lokesh Vutla <lokeshvutla@...com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Tony Lindgren <tony@...mide.com>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
        Sekhar Nori <nsekhar@...com>, Vignesh R <vigneshr@...com>
Subject: Re: [PATCH v3 4/5] pwm: omap-dmtimer: Do not disable pwm before
 changing period/duty_cycle

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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