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

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Message-ID: <20180219122046.kap53tbui6av5irl@flea.lan>
Date:   Mon, 19 Feb 2018 13:20:46 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Philipp Rossak <embed3d@...il.com>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        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 Sun, Feb 18, 2018 at 06:55:58PM +0100, Philipp Rossak wrote:
> On 16.02.2018 14:15, Chen-Yu Tsai wrote:
> > On Fri, Feb 16, 2018 at 9:07 PM, Maxime Ripard
> > <maxime.ripard@...tlin.com> wrote:
> > > 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?
> > 
> > There is no placeholder for missing parents, unlike the regulator
> > subsystem that has a dummy regulator for this purpose.
> > 
> > > That's pretty bad :/
> > 
> > Yeah. I didn't expect this to happen. But to be fair, I should
> > have done the check on clk_hw_get_parent_by_index.
> > 
> > > Is there a way to test before registering that all our parents are
> > > actually there? clk_get?
> > 
> > That's probably the way to do it. However in the AC100 RTC case,
> > I left it open to be missing on purpose, so we could use the RTC
> > without waiting for the codec to be supported.
> > 
> > ChenYu
> > 
> 
> So how should we proceed with this issue?
> 
> Should I send a new version with a fixed comment or should I implement the
> check in clk_get function?
> 
> For the second option I will need about 3 weeks to submit a proper patch
> since I have the next two weeks some other stuff to do.
> If a proper fix is required earlier, it might be better if someone else is
> taking care about a fix.

A better comment will do.

Maxime

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

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