linux-kernel - Re: [PATCH 04/19] clk: sunxi: Add TCON channel1 clock

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Message-ID: <20151119153516.GP32142@lukather>
Date:	Thu, 19 Nov 2015 16:35:16 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Chen-Yu Tsai <wens@...e.org>
Cc:	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	David Airlie <airlied@...ux.ie>,
	Thierry Reding <thierry.reding@...il.com>,
	devicetree <devicetree@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-clk <linux-clk@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	linux-sunxi <linux-sunxi@...glegroups.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Hans de Goede <hdegoede@...hat.com>,
	Alexander Kaplan <alex@...tthing.co>,
	Wynter Woods <wynter@...tthing.co>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Rob Clark <robdclark@...il.com>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH 04/19] clk: sunxi: Add TCON channel1 clock

On Mon, Nov 09, 2015 at 11:36:15AM +0800, Chen-Yu Tsai wrote:
> >> > +       sclk1_parents[0] = sclk2_name;
> >> > +       sclk1_parents[1] = sclk2d2_name;
> >>
> >> Is there any need to expose these 2 clocks via DT using of_clk_add_provider?
> >
> > No, as far as I'm aware, there's no user external to this clock
> > driver.
> >
> >> Note that these complex clock trees within a clock node breaks the
> >> assigned-clock-parents mechanism, as you can no longer specify the output
> >> clock's direct parents.
> >
> > There's no point of changing the parent either. Hardware blocks are
> > always connected to the leaf clock (sclk1). We could also model it as
> > an extra 1-bit divider, which would simplify a bit the logic though.
> 
> Probably not. You still have a gate to handle. It's just moving the
> divider from 1 clock to the other. I think the current approach of
> modeling it like the hardware is better.

Not really if you model it using sclk2 being a mux + gate, and sclk1
being a divider + gate. It works great using the composite clocks.

> About reparenting, what I meant was if sclk2 is not exposed through
> of_clk_add_provider, then we can't do assigned-clocks stuff on it,
> like setting a default parent or making each channel use a different
> source pll.

And we don't really want to. Using the divider allow us to simply set
the rate of sclk1, and the mux / divider will do the rest. Since only
sclk1 is exposed to the rest of the system, we do not really care
about the rate of sclk2 anyway.

> What I'm saying is if it is not expected to work with another core
> binding, we should probably note it somewhere.

Indeed.

Maxime

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

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