lkml.org 
[lkml]   [2019]   [Oct]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/7] pwm: jz4740: Improve algorithm of clock calculation
Hi,


Le mer., août 14, 2019 at 19:32, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> a écrit :
> Hello Paul,
>
> On Wed, Aug 14, 2019 at 06:10:35PM +0200, Paul Cercueil wrote:
>> Le mar. 13 août 2019 à 16:09, Uwe =?iso-8859-1?q?Kleine-K=F6nig?=
>> a écrit :
>> > On Tue, Aug 13, 2019 at 02:47:28PM +0200, Paul Cercueil wrote:
>> > > Le mar. 13 août 2019 à 14:33, Uwe Kleine-König a écrit :
>> > > > On Tue, Aug 13, 2019 at 01:01:06PM +0200, Paul Cercueil wrote:
>> > > > > 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.
>> > > >
>> > > > Using clk_round_rate correctly without additional knowledge
>> is hard. If
>> > > > you assume at least some sane behaviour you'd still have to
>> call it
>> > > > multiple times. Assuming maxrate is the maximal rate you can
>> handle
>> > > > without overflowing your PWM registers you have to do:
>> > > >
>> > > > rate = maxrate;
>> > > > rounded_rate = clk_round_rate(clk, rate);
>> > > > while (rounded_rate > rate) {
>> > > > if (rate < rounded_rate - rate) {
>> > > > /*
>> > > > * clk doesn't support a rate smaller than
>> > > > * maxrate (or the round_rate callback doesn't
>> > > > * round consistently).
>> > > > */
>> > > > return -ESOMETHING;
>> > > > }
>> > > > rate = rate - (rounded_rate - rate)
>> > > > rounded_rate = clk_round_rate(clk, rate);
>> > > > }
>> > > >
>> > > > return rate;
>> > > >
>> > > > Probably it would be sensible to put that in a function
>> provided by the
>> > > > clk framework (maybe call it clk_round_rate_down and maybe
>> with
>> > > > additional checks).
>> > >
>> > > clk_round_rate_down() has been refused multiple times in the
>> past for
>> > > reasons that Stephen can explain.
>> >
>> > I'd be really interested in these reasons as I think the clk
>> framework
>> > should make it easy to solve common tasks related to clocks. And
>> finding
>> > out the biggest supported rate not bigger than a given maxrate is
>> > something I consider such a common task.
>> >
>> > The first hit I found when searching was
>> > https://lkml.org/lkml/2010/7/14/260 . In there Stephen suggested
>> that
>> > clk_round_rate with the current semantic is hardly useful and
>> suggested
>> > clk_round_rate_up() and clk_round_rate_down() himself.
>>
>> That's from 2010, though.
>
> If you have a better link please tell me.
>
>> I agree that clk_round_rate_up() and clk_round_rate_down() should
>> exist.
>> Even if they return -ENOSYS if it's not implemented for a given
>> clock
>> controller.
>
> ack.
>
>> > > > > 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.
>> > > >
>> > > > Either that or Thierry failed to see the downside. The
>> obvious downside
>> > > > is that once you set the period to something long (and so the
>> clk was
>> > > > limited to a small frequency) you never make the clock any
>> faster
>> > > > afterwards.
>> > >
>> > > Read the algorithm again.
>> >
>> > I indeed missed a call to clk_set_rate(clk, parent_rate). I
>> thought I
>> > grepped for clk_set_rate before claiming the code was broken.
>> Sorry.
>> >
>> > So I think the code works indeed, but it feels like abusing
>> > clk_set_max_rate. So I'd like to see some words from Stephen
>> about this
>> > procedure.
>> >
>> > Also I think this is kind of inelegant to set the maximal rate
>> twice. At
>> > least call clk_set_max_rate only once please.
>>
>> Ok. I can do that.
>
> I would still prefer to hear from Stephen about this approach. It
> seems
> wrong to have two different ways to achieve the same goal and my
> impression is that clk_round_rate is the function designed for this
> use
> case.

Stephen, any feedback?
I'm still stuck here.


>> > > > > > > > > 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.
>> > > >
>> > > > But you hide it behind clk API functions that don't guarantee
>> this
>> > > > behaviour. And even if it works for you it might not for the
>> next person
>> > > > who copies your code to support another hardware.
>> > >
>> > > Again, I'm not *trying* to guarantee this behaviour.
>> >
>> > I didn't request you should guarantee this behaviour. I want you
>> to make
>> > it obvious for readers of your code that you rely on something
>> that
>> > isn't guaranteed. That your code works today isn't a good enough
>> excuse.
>> > There are various examples like these. If you want a few:
>> >
>> > - printf("string: %s\n", NULL); works fine with glibc, but
>> segfaults on
>> > other libcs.
>> > - setenv("MYVAR", NULL) used to work (and was equivalent to
>> > setenv("MYVAR", "")) but that was never guaranteed. Then at
>> some
>> > point of time it started to segfault.
>> > - Look into commits like
>> a4435febd4c0f14b25159dca249ecf91301c7c76. This
>> > used to work fine until compilers were changed to optimize more
>> > aggressively.
>> >
>> > Now if you use a clk and know that all rates smaller than the
>> requested
>> > one are divisors of the fast one and your code only works (here:
>> is
>> > optimal) when this condition is given, you're walking on thin ice
>> just
>> > because this fact it's not guaranteed.
>> > The least you can do is to add a code comment to make people
>> aware who
>> > debug the breakage or copy your code.
>>
>> If I was assuming something, it's not that the requested clock
>> rates are
>> always integer dividers of the parent rate - but rather that the
>> difference
>> in precision between two possible clock rates (even
>> non-integer-dividers) is
>> so tiny that we just don't care.
>
> I'm more exacting here. If you are asked for X and can provide X - 2
> you
> shouldn't provide X - 12. Depending on the use case the consumer is
> happy
> about every bit of accuracy they can get. So if you deliberately
> provide
> X - 12 because it is easier to do and good enough for you, at least
> document this laziness to not waste other people's time more than
> necessary.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K. | Uwe Kleine-König
> |
> Industrial Linux Solutions |
> http://www.pengutronix.de/ |


\
 
 \ /
  Last update: 2019-10-21 14:48    [W:0.106 / U:0.340 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site