linux-kernel - Re: [PATCH v3] clk: add MOXA ART SoCs clock driver

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Message-ID: <20130718110201.GB29153@e106331-lin.cambridge.arm.com>
Date:	Thu, 18 Jul 2013 12:02:01 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Jonas Jensen <jonas.jensen@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"arm@...nel.org" <arm@...nel.org>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] clk: add MOXA ART SoCs clock driver

On Thu, Jul 18, 2013 at 11:36:40AM +0100, Jonas Jensen wrote:
> Hi Mark, thanks for taking a look at this.
> 
> On 18 July 2013 11:50, Mark Rutland <mark.rutland@....com> wrote:
> > This confuses me. moxart_of_clk_init gets called because there was a
> > "moxa,moxart-core-clock", node in the dt, but the driver only seems to
> > use the information to figure out the configuration of another clock
> > ("moxa,moxart-apb-clock"), and never registers a clock specifically for
> > the core-clock.
> >
> > I couldn't find "moxa,moxart-apb-clock" described in mainline. COuld you
> > describe the relationship between core-clock and apb-clock?
> 
> It's true core-clock exist only so the register can be mapped.

Ok. I'm just concerned that the linkage isn't explicit or obvious.

> 
> apb-clock is part of a patch set that will add new device tree files for the
> MOXA ART SoC, but it's not in mainline:
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2013-July/181757.html
> 
> apb-clock could be a fixed rate 48MHz DT only clock, but because we can't
> be sure it's 48MHz on all platforms, reading it from a register with core-clock
> is more portable.

This does leave apb-clock completely dependent on core-clock, and unless
I've missed something there's no linkage between the two described in
the dt.

How does core-clock physically relate to apb-clock? Does it feed or is
it fed by apb-clock?

Are we always guaranteed to have core-clock if we have apb-clock, and is
it part of the same block in hardware? If so we could describe the
amalgamation as a provider with two clock outputs, with core-clock's
registers for configuration at probe-time.

Thanks,
Mark.

> 
> 
> Best regards,
> Jonas
> 
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