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:	Mon, 16 Feb 2015 13:18:13 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
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 13/02/15 20:57, Sascha Hauer wrote:
> 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.

Would that problem better be fixed by changing the clock driver so that
when asked for 99Hz, it would look for rates less than 100Hz?

I think the old behavior was so odd that I would call it broken, so I
hope the current problems can be fixed via some other ways than breaking
it again.

 Tomi



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