linux-kernel - RE: [PATCH v2 1/4] ARM: OMAP2+: dpll: round rate to closest value

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Message-ID: <alpine.DEB.2.00.1301280721390.23108@utopia.booyaka.com>
Date:	Mon, 28 Jan 2013 07:24:30 +0000 (UTC)
From:	Paul Walmsley <paul@...an.com>
To:	"Mohammed, Afzal" <afzal@...com>
cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Mike Turquette <mturquette@...aro.org>,
	Russell King <linux@....linux.org.uk>
Subject: RE: [PATCH v2 1/4] ARM: OMAP2+: dpll: round rate to closest value

Hi

On Fri, 25 Jan 2013, Mohammed, Afzal wrote:

> On Fri, Jan 25, 2013 at 13:48:11, Paul Walmsley wrote:
> > On Wed, 23 Jan 2013, Afzal Mohammed wrote:
> 
> > > Currently round rate function would return proper rate iff requested
> > > rate exactly matches the PLL lockable rate. This causes set_rate to
> > > fail if exact rate could not be set. Instead round rate may return
> > > closest rate possible (less than the requested). And if any user is
> > > badly in need of exact rate, then return value of round rate could
> > > be used to decide whether to invoke set rate or not.
> > > 
> > > Modify round rate so that it return closest possible rate.
> > 
> > This doesn't look like the right approach to me.  For some PLLs, an exact 
> > rate is desired.
> 
> If exact rate is required, there is a way to achieve it as mentioned
> in the commit message, i.e. by first invoking round rate over reqd. rate
> and if it doesn't match, bail out w/o invoking set_rate.
> 
> And it seems requirement of CCF w.r.t to round rate is to return closest
> possible rate.

Hmm.  Maybe I need to take a closer look.  I'm a little worried that, 
since __clk_round_rate() can be called from omap3_noncore_dpll_set_rate(), 
we might wind up with inconsistent behavior.  Effectively we'd need to 
mandate that clk_round_rate() would have to be called first for any DPLL 
where we'd expect to set an exact rate.


- Paul
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