[PATCH 0/3] clk: divider: three exactness fixes (and a rant)
Philipp Zabel
p.zabel at pengutronix.de
Fri Mar 13 01:13:52 PDT 2015
Am Freitag, den 13.03.2015, 08:50 +0100 schrieb Uwe Kleine-König:
> On Thu, Mar 12, 2015 at 09:57:53AM +0100, Philipp Zabel wrote:
> > Am Mittwoch, den 11.03.2015, 18:21 -0700 schrieb Stephen Boyd:
> > [...]
> > > Why does Philipp like 110Hz the most? Where is the desire for that rate
> > > coming from?
> > >
> > > > And the lower
> > > > abs(1 / 110 - 1 / r) the better.
> > >
> > > Similarly, where is this requirement coming from? Some datasheet? Or is
> > > it just some arbitrary decision we've made that may not hold true for
> > > all consumers?
> It's not comming from a datasheet. But that's what I guess is the right
> metric for quite some cases. E.g. an UART sample rate and I also
> wouldn't be surprised if Philipp's panel example would call for this
> metric, too.
>
> For an UART running with say 38400 Bd you want to sample with a freqency
> of 38400 Hz (not considering oversampling, but that is only a factor
> that doesn't makes my reasoning wrong). If you now consider 38401 Hz and
> 38399 Hz the respective deltas are 1 Hz. But if you look at the time
> between two samples we have:
>
> 38401 Hz -> 26.04098852 us -> delta: 0.6781507 ns
> 38400 Hz -> 26.04166667 us
> 38399 Hz -> 26.04234485 us -> delta: 0.6781861 ns
>
> So with 38401 it takes a little longer until the slightly deviating rate
> results in sampling the wrong bit.
>
> > In this use case, the driver doesn't want the pixel clock to stay below
> > a hard frequency limit, but to get as close as possible to the target
> > frequency, either above or below, so the relative error to the nominal
> > panel refresh rate stays as small as possible. Thus for a fictional
> > target rate of 110 Hz, I'd like to minimize abs((round_rate / 110) - 1).
> Note that minimizing
>
> abs((round_rate / 110) - 1)
>
> is equivalent to minimizing
>
> abs(round_rate - 110)
Of course, and you're right, I should want to minimize the delta of the
interval time, not of the rate so that if playing back a video stream at
exactly the nominal frequency, it takes as long as possible until I have
to drop or duplicate a frame to stay in sync.
regards
Philipp
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