linux-kernel - Re: [PATCH 1/3] clk: divider: fix calculation of maximal parent rate for a given divider

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Message-ID: <20150305083518.GR7865@pengutronix.de>
Date:	Thu, 5 Mar 2015 09:35:18 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Sören Brinkmann <soren.brinkmann@...inx.com>,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH 1/3] clk: divider: fix calculation of maximal parent rate
 for a given divider

Hello Mike, hello Stephen,

On Sat, Feb 21, 2015 at 11:40:23AM +0100, Uwe Kleine-König wrote:
> The rate provided at the output of a clk-divider is calculated as:
> 
> 	DIV_ROUND_UP(parent_rate, div)
> 
> since commit b11d282dbea2 (clk: divider: fix rate calculation for
> fractional rates). So to yield a rate not bigger than r parent_rate
> must be <= r * div.
> 
> The effect of choosing a parent rate that is too big as was done before
> this patch results in wrongly ruling out good dividers.
> 
> Note that this is not a complete fix as __clk_round_rate might return a
> value >= its 2nd parameter. Also for dividers with
> CLK_DIVIDER_ROUND_CLOSEST set the calculation is not accurate. But this
> fixes the test case by Sascha Hauer that uses a chain of three dividers
> under a fixed clock.
> 
> Fixes: b11d282dbea2 (clk: divider: fix rate calculation for fractional rates)
> Suggested-by: Sascha Hauer <s.hauer@...gutronix.de>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Gentle ping.

IMHO this is patch should go into 4.0.

Feedback for patches 2 and 3 (or the alternative to rip off specialised
round-closest) would be very welcome, too.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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