Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate if no bestdiv is normally found

From: Russell King - ARM Linux
Date: Fri Feb 20 2015 - 14:21:04 EST

On Fri, Feb 20, 2015 at 11:13:06AM -0800, Mike Turquette wrote:
> Quoting Russell King - ARM Linux (2015-02-16 03:27:24)
> > On Fri, Feb 13, 2015 at 07:57:13PM +0100, Sascha Hauer wrote:
> > > I agree that it's a bit odd, but I think it has to be like this.
> > > Consider that you request a rate of 100Hz, but the clock can only
> > > produce 99.5Hz, so due to rounding clk_round_rate() returns 99Hz.
> > > Now when you request 99Hz from clk_set_rate() the 99.5Hz value
> > > can't be used because it's too high.
> >
> > Math rounding rules normally state that anything of .5 and greater
> > should be rounded up, not rounded down. So, for 99.5Hz, you really
> > ought to be returning 100Hz, not 99Hz.
> >
> > However, you do have a point for 99.4Hz, which would be returned as
> > 99Hz, and when set, it would result in something which isn't 99.4Hz.
> More practically, this again raises the issue of whether or not unsigned
> long rate should be in millihertz or something other than hertz.

You still get the same issue if it's millihertz. Take 10 2/3rds Hz.
Is 10.666 Hz less than 10 2/3rds Hz?

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