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Date: Fri, 13 Mar 2015 08:50:58 +0100 From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de> To: Philipp Zabel <p.zabel@...gutronix.de>, Stephen Boyd <sboyd@...eaurora.org> Cc: Mike Turquette <mturquette@...aro.org>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Sören Brinkmann <soren.brinkmann@...inx.com>, kernel@...gutronix.de Subject: Re: [PATCH 0/3] clk: divider: three exactness fixes (and a rant) 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) . Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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