[PATCH v14 6/6] clk: meson: a1: add Amlogic A1 Peripherals clock controller driver

[Martin Blumenstingl]

Hi Dmitry,

On Wed, May 17, 2023 at 12:34 PM Dmitry Rokosov
<ddrokosov at sberdevices.ru> wrote:
[...]
> > > Additionally, the CCF determines the best ancestor based on how close
> > > its rate is to the given one, based on arithmetic calculations. However,
> > > we have independent knowledge that a certain clock would be better, with
> > > less jitter and fewer intermediaries, which will likely improve energy
> > > efficiency. Sadly, the CCF cannot take this into account.
> > I agree that the implementation in CCF is fairly simple. There's ways
> > to trick it though: IIRC if there are multiple equally suitable clocks
> > it picks the first one. For me all of this has worked so far which is
> > what makes me curious in this case (not saying that anything is wrong
> > with your approach).
> >
> > Do you have a (real world) example where the RTC clock should be
> > preferred over another clock?
> >
>
> Yes, a real-life example is the need for a 32Khz clock for an external
> wifi chip. There is one option to provide this clock with high
> precision, which is RTC + GENCLK.
>
> > I'm thinking about the following scenario.
> > PWM parents:
> > - XTAL: 24MHz
> > - sys: not sure - let's say 166.67MHz
> > - RTC: 32kHz
> >
> > Then after that there's a divider and a gate.
> >
> > Let's say the PWM controller needs a 1MHz clock: it can take that from
> > XTAL or sys. Since XTAL is evenly divisible to 1MHz CCF will pick that
> > and use the divider.
> > But let's say the PWM controller needs a 32kHz clock: CCF would
> > automatically pick the RTC clock.
> > So is your implementation there to cover let's say 1kHz where
> > mathematically 24MHz can be divided evenly to 1kHz (and thus should
> > not result in any jitter) but RTC gives better precision in the real
> > world (even though it's off by 24Hz)?
> >
>
> I don't think so. The highest precision that RTC can provide is from a
> 32KHz rate only. However, I believe that a 1kHz frequency can also be
> achieved by using xtal 24MHz with a divider, which can provide high
> precision as well.
Thank you again for the great discussion on IRC today.
Here's my short summary so I don't forget before you'll follow up on this.

In general there's two known cases where the RTC clock needs to be used:
a) When using the GENCLK output of the SoC to output the 32kHz RTC
clock and connect that to an SDIO WiFi chip clock input (this seems
useful in my understanding because the RTC clock provides high
precision)
b) When using the PWM controller to output a 32kHz clock signal. In
this case my understanding is that using the RTC clock as input to the
PWM controller results in the best possible signal

The second case won't be supported with Heiner's patches [0] that use
CCF (common clock framework) in the PWM controller driver.
In this series the parent clock is calculated using:
  freq = div64_u64(NSEC_PER_SEC * (u64)0xffff, period);

A 32kHz clock means a PWM period of 30518ns. So with the above
calculation the PWM driver is asking for a clock rate of >=2GHz.
We concluded that letting the common clock framework choose the best
possible parent (meaning: removing CLK_SET_RATE_NO_REPARENT here) can
be a way forward.
But this means that the PWM controller driver must try to find the
best possible parent somehow. The easiest way we came up with
(pseudo-code):
  freq = NSEC_PER_SEC / period;
  fin_freq = clk_round_rate(channel->clk, freq);
  if (fin_freq != freq) {
    freq = div64_u64(NSEC_PER_SEC * (u64)0xffff, period);
    fin_freq = clk_round_rate(channel->clk, freq);
  }

The idea is: for a requested 32kHz signal the PWM period is 30518ns.
The updated logic would find that there's a matching clock input and
use that directly. If not: use the original logic as suggested by
Heiner.


Best regards,
Martin


[0] https://lore.kernel.org/linux-amlogic/9faca2e6-b7a1-4748-7eb0-48f8064e323e@gmail.com/