linux-kernel - Re: [PATCH v2 06/14] ARM: sun8i: clk: Add clk-factor rate application method

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Message-ID: <20160726063253.GW7419@lukather>
Date:	Tue, 26 Jul 2016 08:32:53 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Ondřej Jirman <megous@...ous.com>
Cc:	dev@...ux-sunxi.org, linux-arm-kernel@...ts.infradead.org,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Chen-Yu Tsai <wens@...e.org>,
	Emilio López <emilio@...pez.com.ar>,
	"open list:COMMON CLK FRAMEWORK" <linux-clk@...r.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/14] ARM: sun8i: clk: Add clk-factor rate
 application method

On Thu, Jul 21, 2016 at 11:52:15AM +0200, Ondřej Jirman wrote:
> >>> If so, then yes, trying to switch to the 24MHz oscillator before
> >>> applying the factors, and then switching back when the PLL is stable
> >>> would be a nice solution.
> >>>
> >>> I just checked, and all the SoCs we've had so far have that
> >>> possibility, so if it works, for now, I'd like to stick to that.
> >>
> >> It would need to be tested. U-boot does the change only once, while the
> >> kernel would be doing it all the time and between various frequencies
> >> and PLL settings. So the issues may show up with this solution too.
> > 
> > That would have the benefit of being quite easy to document, not be a
> > huge amount of code and it would work on all the CPUs PLLs we have so
> > far, so still, a pretty big win. If it doesn't, of course, we don't
> > really have the choice.
> 
> It's probably more code though. It has to access different register from
> the one that is already defined in dts, which would add a lot of code
> and require dts changes. The original patch I sent is simpler than that.

Why?

You can use container_of to retrieve the parent structure of the clock
notifier, and then you get a ccu_common structure pointer, with the
CCU base address, the clock register, its lock, etc.

Look at what is done in drivers/clk/meson/clk-cpu.c. It's like 20 LoC.

I don't really get why anything should be changed in the DT, or why it
would add a lot of code. Or maybe we're not talking about the same
thing?

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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