Re: [PATCH 4/7] pwm: jz4740: Improve algorithm of clock calculation

From: Paul Cercueil
Date: Tue Aug 13 2019 - 07:01:15 EST



Le mar. 13 août 2019 à 7:27, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= <u.kleine-koenig@xxxxxxxxxxxxxx> a écrit :
Hello Paul,

[adding Stephen Boyd to Cc]

On Tue, Aug 13, 2019 at 12:16:23AM +0200, Paul Cercueil wrote:
Le lun. 12 août 2019 à 23:48, Uwe Kleine-König a écrit :
> On Mon, Aug 12, 2019 at 10:43:10PM +0200, Paul Cercueil wrote:
> > Le lun. 12 août 2019 à 8:15, Uwe Kleine-König a écrit :
> > > On Fri, Aug 09, 2019 at 07:14:45PM +0200, Paul Cercueil wrote:
> > > > Le ven. 9 août 2019 à 19:05, Uwe Kleine-König a écrit :
> > > > > On Fri, Aug 09, 2019 at 02:30:28PM +0200, Paul Cercueil wrote:
> > > > > > [...]
> > > > > > + /* Reset the clock to the maximum rate, and we'll reduce it if needed */
> > > > > > + ret = clk_set_max_rate(clk, parent_rate);
> > > > >
> > > > > What is the purpose of this call? IIUC this limits the allowed range of
> > > > > rates for clk. I assume the idea is to prevent other consumers to change
> > > > > the rate in a way that makes it unsuitable for this pwm. But this only
> > > > > makes sense if you had a notifier for clk changes, doesn't it? I'm
> > > > > confused.
> > > >
> > > > Nothing like that. The second call to clk_set_max_rate() might have set
> > > > a maximum clock rate that's lower than the parent's rate, and we want to
> > > > undo that.
> > >
> > > I still don't get the purpose of this call. Why do you limit the clock
> > > rate at all?
> >
> > As it says below, we "limit the clock to a maximum rate that still gives
> > us a period value which fits in 16 bits". So that the computed hardware
> > values won't overflow.
>
> But why not just using clk_set_rate? You want to have the clock running
> at a certain rate, not any rate below that certain rate, don't you?

I'll let yourself answer yourself:
https://patchwork.ozlabs.org/patch/1018969/

In that thread I claimed that you used clk_round_rate wrongly, not that
you should use clk_set_max_rate(). (The claim was somewhat weakend by
Stephen, but still I think that clk_round_rate is the right approach.)

Well, you said that I shouln't rely on the fact that clk_round_rate() will round down. That completely defeats the previous algorithm. So please tell me how to use it correctly, because I don't see it.

I came up with a much smarter alternative, that doesn't rely on the rounding method of clk_round_rate, and which is better overall (no loop needed). It sounds to me like you're bashing the code without making the effort to understand what it does.

Thierry called it a "neat trick" (https://patchwork.kernel.org/patch/10836879/) so it cannot be as bad as you say.



The upside of clk_round_rate is that it allows you to test for the
capabilities of the clock without actually changing it before you found
a setting you consider to be good.

I know what clk_round_rate() is for. But here we don't do trial-and-error to find the first highest clock rate that works, we compute the maximum clock we can use and limit the clock rate to that.



It's enough to run it below a certain rate, yes. The actual rate doesn't
actually matter that much.

1 Hz would be fine? I doubt it.

We use the highest possible clock rate. We wouldn't use 1 Hz unless it's the highest clock rate available.



> > E.g. if at a rate of 12 MHz your computed hardware value for the period
> > is 0xf000, then at a rate of 24 MHz it won't fit in 16 bits. So the clock
> > rate must be reduced to the highest possible that will still give you a
> > < 16-bit value.
> >
> > We always want the highest possible clock rate that works, for the sake of
> > precision.
>
> This is dubious; but ok to keep the driver simple. (Consider a PWM that
> can run at i MHz for i in [1, .. 30]. If a period of 120 ns and a duty
> cycle of 40 ns is requested you can get an exact match with 25 MHz, but
> not with 30 MHz.)

The clock rate is actually (parent_rate >> (2 * x) )
for x = 0, 1, 2, ...

So if your parent_rate is 30 MHz the next valid one is 7.5 MHz, and the
next one is 1.875 MHz. It'd be very unlikely that you get a better match at
a lower clock.

If the smaller freqs are all dividers of the fastest that's fine. Please
note in a code comment that you're assuming this.

No, I am not assuming this. The current driver just picks the highest clock rate that works. We're not changing the behaviour here.



> > > > Basically, we start from the maximum clock rate we can get for that PWM
> > > > - which is the rate of the parent clk - and from that compute the maximum
> > > > clock rate that we can support that still gives us < 16-bits hardware
> > > > values for the period and duty.
> > > >
> > > > We then pass that computed maximum clock rate to clk_set_max_rate(), which
> > > > may or may not update the current PWM clock's rate to match the new limits.
> > > > Finally we read back the PWM clock's rate and compute the period and duty
> > > > from that.
> > >
> > > If you change the clk rate, is this externally visible on the PWM
> > > output? Does this affect other PWM instances?
> >
> > The clock rate doesn't change the PWM output because the hardware values for
> > the period and duty are adapted accordingly to reflect the change.
>
> It doesn't change it in the end. But in the (short) time frame between
> the call to change the clock and the update of the PWM registers there
> is a glitch, right?

The PWM is disabled, so the line is in inactive state, and will be in that state
until the PWM is enabled again. No glitch to fear.

ok, please note in the commit log that the reordering doesn't affect the
output because the PWM is off and are done to make it more obvious what
happens.

> You didn't answer to the question about other PWM instances. Does that
> mean others are not affected?

Sorry. Yes, they are not affected - all PWM channels are independent.

ok.

> PS: It would be great if you could fix your mailer to not damage the
> quoted mail. Also it doesn't seem to understand how my name is encoded
> in the From line. I fixed up the quotes in my reply.

I switched Geary to "rich text". Is that better?

No. It looks exactly like the copy you bounced to the list. See
https://patchwork.ozlabs.org/comment/2236355/ for how it looks.

Best regards
Uwe

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