Re: [RFC] clk: add boot clock support

[Sebastian Reichel]

Hi Saravana,

On Thu, Mar 25, 2021 at 06:55:52PM -0700, Saravana Kannan wrote:
> On Thu, Mar 25, 2021 at 6:27 PM Rob Herring <robh@xxxxxxxxxx> wrote:
> >
> > +Saravana
> >
> > On Thu, Mar 18, 2021 at 10:03:18PM +0100, Sebastian Reichel wrote:
> > > On Congatec's QMX6 system on module one of the i.MX6 fixed clocks
> > > is provided by an I2C RTC. Specifying this properly results in a
> > > circular dependency, since the I2C RTC (and thus its clock) cannot
> > > be initialized without the i.MX6 clock controller being initialized.
> > >
> > > With current code the following path is executed when i.MX6 clock
> > > controller is probed (and ckil clock is specified to be the I2C RTC
> > > via DT):
> > >
> > > 1. imx6q_obtain_fixed_clk_hw(ccm_node, "ckil", 0);
> > > 2. of_clk_get_by_name(ccm_node, "ckil");
> > > 3. __of_clk_get(ccm_node, 0, ccm_node->full_name, "ckil");
> > > 4. of_clk_get_hw(ccm_node, 0, "ckil")
> > > 5. spec = of_parse_clkspec(ccm_node, 0, "ckil"); // get phandle
> > > 6. of_clk_get_hw_from_clkspec(&spec); // returns -EPROBE_DEFER
> > > 7. error is propagated back, i.MX6q clock controller is probe deferred
> > > 8. I2C controller is never initialized without clock controller
> > >    I2C RTC is never initialized without I2C controller
> > >    CKIL clock is never initialized without I2C RTC
> > >    clock controller is never initialized without CKIL
> > >
> > > To fix the circular dependency this registers a dummy clock when
> > > the RTC clock is tried to be acquired. The dummy clock will later
> > > be unregistered when the proper clock is registered for the RTC
> > > DT node. IIUIC clk_core_reparent_orphans() will take care of
> > > fixing up the clock tree.
> > >
> > > NOTE: For now the patch is compile tested only. If this approach
> > > is the correct one I will do some testing and properly submit this.
> > > You can find all the details about the hardware in the following
> > > patchset:
> > >
> > > https://lore.kernel.org/linux-devicetree/20210222171247.97609-1-sebastian.reichel@xxxxxxxxxxxxx/
> > >
> > > Signed-off-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>
> > > ---
> > >  .../bindings/clock/clock-bindings.txt         |   7 +
> > >  drivers/clk/clk.c                             | 146 ++++++++++++++++++
> > >  2 files changed, 153 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/clock/clock-bindings.txt b/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > > index f2ea53832ac6..66d67ff4aa0f 100644
> > > --- a/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > > +++ b/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > > @@ -32,6 +32,13 @@ clock-output-names: Recommended to be a list of strings of clock output signal
> > >                   Clock consumer nodes must never directly reference
> > >                   the provider's clock-output-names property.
> > >
> > > +boot-clock-frequencies: This property is used to specify that a clock is enabled
> > > +                     by default with the provided frequency at boot time. This
> > > +                     is required to break circular clock dependencies. For clock
> > > +                     providers with #clock-cells = 0 this is a single u32
> > > +                     with the frequency in Hz. Otherwise it's a list of
> > > +                     clock cell specifier + frequency in Hz.
> >
> > Seems alright to me. I hadn't thought about the aspect of needing to
> > know the frequency. Other cases probably don't as you only need the
> > clocks once both components have registered.
> >
> > Note this could be lost being threaded in the other series.
> 
> I read this thread and tried to understand it. But my head isn't right
> today (lack of sleep) so I couldn't wrap my head around it. I'll look
> at it again after the weekend. In the meantime, Sebastian can you
> please point me to the DT file and the specific device nodes (names or
> line number) where this cycle is present?

I have not yet sent an updated DT file, but if you look at this
submission:

https://lore.kernel.org/linux-devicetree/20210222171247.97609-7-sebastian.reichel@xxxxxxxxxxxxx/

There is a node

rtc: m41t62@68 { compatible = "st,m41t62"; };

That is an I2C RTC, which provides a 32.768 kHz clock by default
(i.e. after power loss). This clock signal is used to provide the
i.MX6 CKIL:

------------------------------------
&clks {
    clocks = <&rtc>;
    clock-names = "ckil";
};
------------------------------------

> Keeping a clock on until all its consumers probe is part of my TODO
> list (next item after fw_devlink=on lands). I already have it working
> in AOSP, but need to clean it up for upstream. fw_devlink can also
> break *some* cycles (not all). So I'm wondering if the kernel will
> solve this automatically soon(ish). If it can solve it automatically,
> I'd rather not add new DT bindings because it'll make it more work for
> fw_devlink.

As written above on Congatec QMX6 an I2C RTC provides one of the
SoC's input frequencies. The SoC basically expects that frequency
to be always enabled and this is what it works like before clock
support had been added to the RTC driver.

With the link properly being described the Kernel tries to probe 
the SoC's clock controller during early boot. Then it tries to get a
reference to the linked clock, using imx6q_obtain_fixed_clk_hw()
and that returns -EPROBE_DEFER (because the RTC driver has not
yet been probed). Without the clock controller basically none of
the i.MX6 SoC drivers can probe including the I2C driver. Without
the I2C bus being registered, the RTC driver never probes and the
boot process is stuck.

I'm not sure how fw_devlink can help here. I see exactly two
options to solve this:

a) do not describe the link and keep RTC clock enabled somehow.
   (my initial patchset)
b) describe the link, but ignore it during boot.
   (what I'm trying to do here)

Thanks,

-- Sebastian

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