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

[Paul Cercueil <paul@crapouillou.net>]

Le mar. 13 août 2019 à 7:27, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= 
<u.kleine-koenig@pengutronix.de> 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
> 
> --
> Pengutronix e.K.                           | Uwe Kleine-König        
>     |
> Industrial Linux Solutions                 | 
> http://www.pengutronix.de/  |