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

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Message-ID: <20150220192043.GM8656@n2100.arm.linux.org.uk>
Date:	Fri, 20 Feb 2015 19:20:43 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mike Turquette <mturquette@...aro.org>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Tomi Valkeinen <tomi.valkeinen@...com>, stefan.wahren@...e.com,
	devicetree@...r.kernel.org, sboyd@...eaurora.org,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	a.hajda@...sung.com, kernel@...gutronix.de,
	andy.yan@...k-chips.com, Liu Ying <Ying.Liu@...escale.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate
 if no bestdiv is normally found

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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