Date: Fri, 20 Feb 2015 19:20:43 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Mike Turquette <mturquette@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>,
        Tomi Valkeinen <tomi.valkeinen@ti.com>, stefan.wahren@i2se.com,
        devicetree@vger.kernel.org, sboyd@codeaurora.org,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        a.hajda@samsung.com, kernel@pengutronix.de, andy.yan@rock-chips.com,
        Liu Ying <Ying.Liu@freescale.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate
 if no bestdiv is normally found
Message-ID: <20150220192043.GM8656@n2100.arm.linux.org.uk>
References: <1423720903-24806-2-git-send-email-Ying.Liu@freescale.com>
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 <20150212103944.GA1290@victor>
 <20150212122405.GW12209@pengutronix.de>
 <20150212125646.GT8656@n2100.arm.linux.org.uk>
 <20150212134131.GX12209@pengutronix.de>
 <54DE0BB8.7070004@ti.com>
 <20150213185713.GA12209@pengutronix.de>
 <20150216112724.GN8656@n2100.arm.linux.org.uk>
 <20150220191306.421.58746@quantum>
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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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