linux-kernel - Re: [PATCH v2 2/2] clk: add accuracy support for fixed clock

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Message-ID: <20131202031558.GB29282@titan.lakedaemon.net>
Date:	Sun, 1 Dec 2013 22:15:58 -0500
From:	Jason Cooper <jason@...edaemon.net>
To:	boris brezillon <b.brezillon@...rkiz.com>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Rob Landley <rob@...dley.net>,
	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Russell King <linux@....linux.org.uk>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/2] clk: add accuracy support for fixed clock

On Thu, Nov 28, 2013 at 09:02:58AM +0100, boris brezillon wrote:
> On 27/11/2013 19:10, Mike Turquette wrote:
> >Quoting boris brezillon (2013-11-27 09:19:08)
> >>>On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote:
...
> >>>I would also prefer to see an unknown accuracy be -1.
> >>I decided to keep 0 as a default value for unimplemented recalc_accuracy
> >>(or unspecified fixed accuracy) to keep existing implementation coherent.
> >>
> >>0 means the clk is perfect, and I thought it would be easier to handle a
> >>perfect clk (even if this is not really the case) than handling an error
> >>case.
> >>
> >>Another aspect is the propagation of the clk accuracy accross the clk tree.
> >>Returning -1 in the middle of the clk chain will drop the previous clk
> >>accuracy
> >>calculation.
> >>
> >>Anyway, I can change this if you think this is more appropriate.
> >What about the absence of the property?
> >Instead of requiring a -1 value
> >can we simply detect that the property does not exist? This is nicer for
> >backwards compatibility with existing DTS.
> 
> I already handle the absence of the clock-accuracy property.
> In this case the clock is considered perfect (accuracy = 0 ppb).

Yeah, in order to calculate the theoretical accuracy of a leaf node, a
missing accuracy in the middle of the chain really derails things.
Since my previous concern (modelling the real accuracy of the clocks) is
not an issue here, I think assuming the absent accuracy is 0 is fine.
Especially since all Boris is trying to do is avoid the RC clocks.

thx,

Jason.
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