linux-kernel - Re: [PATCH v3] rtc: ac100: Fix ac100 determine rate bug

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Date:   Fri, 16 Feb 2018 14:07:41 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Philipp Rossak <embed3d@...il.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        alexandre.belloni@...tlin.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        linux-rtc@...r.kernel.org,
        Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH v3] rtc: ac100: Fix ac100 determine rate bug

On Fri, Feb 16, 2018 at 12:10:18PM +0800, Chen-Yu Tsai wrote:
> > diff --git a/arch/arm/boot/dts/sun8i-a83t-bananapi-m3.dts
> > b/arch/arm/boot/dts/sun8i-a83t-bananapi-m3.dts
> > index 6550bf0e594b..6f56d429f17e 100644
> > --- a/arch/arm/boot/dts/sun8i-a83t-bananapi-m3.dts
> > +++ b/arch/arm/boot/dts/sun8i-a83t-bananapi-m3.dts
> > @@ -175,11 +175,18 @@
> >                         compatible = "x-powers,ac100-rtc";
> >                         interrupt-parent = <&r_intc>;
> >                         interrupts = <0 IRQ_TYPE_LEVEL_LOW>;
> > -                       clocks = <&ac100_codec>;
> > +                       clocks = <&ac100_rtc_32k>;
> >                         #clock-cells = <1>;
> >                         clock-output-names = "cko1_rtc",
> >                                              "cko2_rtc",
> >                                              "cko3_rtc";
> > +
> > +                       ac100_rtc_32k: rtc-32k-oscillator {
> > +                               compatible = "fixed-clock";
> > +                               #clock-cells = <0>;
> > +                               clock-frequency  = <32768>;
> > +                               clock-output-names = "ac100-rtc-32k";
> > +                       };
> >                 };
> >         };
> >  };
> >
> > What do you think about that solution?
> 
> That's not quite right either. As I mentioned before, the
> RTC block has two clock inputs, one 4MHz signal from the
> codec block, and one 32.768 kHz signal from an external
> crystal. The original device tree binding describes the
> first one, and the 32.768 kHz clock was registered by
> the RTC driver internally.
> 
> If you're going to add the crystal clock, you still need
> to keep the codec one. Note that this does not fix what
> Maxime is asking you. I've already provided an explanation:
> 
> The clock core allows registering clocks with not-yet-existing
> clock parents. Parents are matches by string names. If no
> clock by that name is registered yet, the clock core simply
> orphans the new clock if the unregistered parent is its
> current parent or simply ignores that parent if its not the
> current parent. This is entirely valid and is what we are
> counting on here, as we haven't implemented the codec-side
> driver.

So, we end up in a situation where clk_hw_get_num_parents returns the
amount of clocks we can be parented to (orphans or not), but
clk_hw_get_parent_by_index will not return the orphan clocks?

That's pretty bad :/

Is there a way to test before registering that all our parents are
actually there? clk_get?

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com

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