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Re: [PATCH v6 0/9] Clock framework API

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v6 0/9] Clock framework API
Date: Wed, 4 Dec 2019 21:34:33 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 12/4/19 5:40 PM, Damien Hedde wrote:
On 12/2/19 5:15 PM, Peter Maydell wrote:

The one topic I think we could do with discussing is whether
a simple uint64_t giving the frequency of the clock in Hz is
the right representation. In particular in your patch 9 the
board has a clock frequency that's not a nice integer number
of Hz. I think Philippe also mentioned on irc some board where
the UART clock ends up at a weird frequency. Since the
representation of the frequency is baked into the migration
format it's going to be easier to get it right first rather
than trying to change it later.

Important precision for Damien, IIUC we can not migrate float/double types.

So what should the representation be? Some random thoughts:

1) ptimer internally uses a 'period plus fraction' representation:
  int64_t period is the integer part of the period in nanoseconds,
  uint32_t period_frac is the fractional part of the period
(if you like you can think of this as "96-bit integer
period measured in units of one-2^32nd of a nanosecond").
However its only public interfaces for setting the frequency
are (a) set the frequency in Hz (uint32_t) or (b) set
the period in nanoseconds (int64_t); the period_frac part
is used to handle frequencies which don't work out to
a nice whole number of nanoseconds per cycle.

This is very clear, thanks Peter!

The period+period_frac split allow us to migrate the 96 bits:

        VMSTATE_UINT32(period_frac, ptimer_state),
        VMSTATE_INT64(period, ptimer_state),

2) I hear that SystemC uses "value plus a time unit", with
the smallest unit being a picosecond. (I think SystemC
also lets you specify the duty cycle, but we definitely
don't want to get into that!)

The "value" is internally stored in a 64bits unsigned integer.

3) QEMUTimers are basically just nanosecond timers

Similarly to SystemC, the QEMUTimers macro use a 'scale' unit, of:

#define SCALE_MS 1000000
#define SCALE_US 1000
#define SCALE_NS 1

4) The MAME emulator seems to work with periods of
96-bit attoseconds (represented internally by a
32-bit count of seconds plus a 64-bit count of
attoseconds). One attosecond is 1e-18 seconds.

Does anybody else have experience with other modelling
or emulator technology and how it represents clocks ?

5) In linux, a clock rate is an "unsigned long" representing Hz.

I feel we should at least be able to represent clocks
with the same accuracy that ptimer has.

Then is a maybe a good idea to store the period and not the frequency in
clocks so that we don't loose anything when we switch from a clock to a
ptimer ?

I think storing the period as an integer type is a good idea.

However if we store the period in nanoseconds, we get at most 1GHz frequency.

The attosecond granularity feels overkill.

If we use a 96-bit integer to store picoseconds and use similar SCALE macros we get to 1THz.

Regardless the unit chosen, as long it is integer, we can migrate it.
If can migrate the period, we don't need to migrate the frequency.
We can then use the float type in with the timer API to pass frequencies (which in the modeled hardware are ratios, likely not integers).

So we could use set_freq(100e6 / 3), set_freq(40e6 / 5.5) directly.

Regarding the clock, I don't see any strong obstacle to switch
internally to a period based value.
The only things we have to choose is how to represent a disabled clock.
Since putting a "0" period to a ptimer will disable the timer in
ptimer_reload(). We can choose that (and it's a good value because we
can multiply or divide it, it stays the same).

We could use the same representation as a ptimer. But if we don't keep a
C number representation, then computation of frequencies/periods will be
complicated at best and error prone.

 From that point of view, if we could stick to a 64bits integer (or
floating point number) it would be great. Can we use a sub nanosecond
unit that fit our needs ?

I did some test with a unit of 2^-32 of nanoseconds on 64bits (is that
the unit of the ptimer fractional part ?) and if I'm not mistaken
+ we have a frequency range from ~0.2Hz up to 10^18Hz
+ the resolution is decreasing with the frequency (but at 100Mhz we have
a ~2.3mHz resolution, at 1GHz it's ~0.23Hz and at 10GHz ~23Hz
resolution). We hit 1Hz resolution around 2GHz.

So it sounds to me we have largely enough resolution to model clocks in
the range of frequencies we will have to handle. What do you think ?

Back to your series, I wonder why you want to store the frequency in ClockIn. ClockIn shouldn't be aware at what frequency it is clocked. What matters is ClockOut, and each device exposing ClockOuts has a (migrated) state of the output frequencies (rather in fields, or encoded in registers). Once migrated, after the state is loaded back into the device, we call post_load(). Isn't it a good place to call clock_set_frequency(ClockOut[]) which will correctly set each ClockIn frequency.

IOW I don't think ClockIn/ClockOut require to migrate a frequency field.

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