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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Date:	Fri, 13 Feb 2015 19:57:13 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Liu Ying <Ying.Liu@...escale.com>,
	dri-devel@...ts.freedesktop.org, stefan.wahren@...e.com,
	devicetree@...r.kernel.org, kernel@...gutronix.de,
	sboyd@...eaurora.org, linux-kernel@...r.kernel.org,
	a.hajda@...sung.com, andy.yan@...k-chips.com,
	mturquette@...aro.org, 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 13, 2015 at 04:35:36PM +0200, Tomi Valkeinen wrote:
> On 12/02/15 15:41, Sascha Hauer wrote:
> 
> > Tomis patch is based on the assumption that clk_set_rate(clk_round_rate(rate))
> > is equal to clk_round_rate(rate). So when this assumption is wrong then
> > it should simply be reverted.
> 
> When is it not equal?
> 
> I agree that doing clk_set_rate(clk, clk_round_rate(clk, rate)) is
> pointless, but shouldn't it still work?
> 
> And we can forget about clk_round_rate. Without my patch, this would
> behave oddly also:
> 
> rate = clk_get_rate(clk);
> clk_set_rate(clk, rate);
> 
> The end result could be something else than 'rate'.

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.

Sascha

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