Hi,

On Mon, Mar 29, 2021 at 01:03:20PM -0700, Saravana Kannan wrote:

On Fri, Mar 26, 2021 at 2:52 AM Sebastian Reichel
[off-list ref] wrote:

quoted

On Thu, Mar 25, 2021 at 06:55:52PM -0700, Saravana Kannan wrote:

quoted

On Thu, Mar 25, 2021 at 6:27 PM Rob Herring [off-list ref] wrote:

quoted

On Thu, Mar 18, 2021 at 10:03:18PM +0100, Sebastian Reichel wrote:

quoted

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@collabora.com/ (local)

Signed-off-by: Sebastian Reichel <redacted>
---
 .../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@collabora.com/ (local)

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";
};
------------------------------------

quoted

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.

Thanks. I skimmed through the RTC driver code and
imx6q_obtain_fixed_clk_hw() and the DT files.

quoted

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).

But the RTC (which is a proper I2C device) will never probe before
CLK_OF_DECLARE() initializes the core clock controller. So, it's not
clear how "protected-clocks" helps here since it doesn't change
whether you get -EPROBE_DEFER from imx6q_obtain_fixed_clk_hw() (which
is called from the CLK_OF_DECLARE() callback). Oof... I see what you
are doing with of_clk_register_boot_clk(). You are having the consumer
register its own clock and then use it. Kinda beats the whole point of
describing the link in the first place.

I agree, that it does not make sense from a code point of view for
this platform. All of this is just to make the DT look correct.
From a platform point of view the most logical way is to handle the
RTC clock as do-not-touch always enabled fixed-clock.

quoted

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'll explain how it'd help. Let's assume "fsl,imx6q-ccm" was
implemented as an actual platform device driver and not using
CLK_OF_DECLARE() to initialize ALL the clocks. I'll get to this
assumption later.

In that case, fw_devlink will notice this cycle:
syntax: consumer -(reason)-> supplier
clks -(clocks property)-> rtc -(parent)-> i2c3  -(clocks property)-> clks.

It'll then reason that it doesn't make sense for a device (clks) to
have a supplier (rtc) whose parent (i2c3) in turn depends on the
device (clks). It'll then drop the clks -> rtc dependency because
that's the most illogical one in terms of probing.

So all you'd need to do is delete any -EPROBE defer you might do in
"fsl,imx6q-ccm" driver for "ckil". For cases where there's no cycle,
fw_devlink will make sure the supplier of ckil has probed first. For
cases where there's a cycle like this, it'll be smart enough to drop
this dependency during probe ordering.

What do you mean drop? Anything using ckil will not be registered?
That will basically kill the system within a few seconds, since the
watchdog hardware uses ckil.

I don't know enough about the clocks in imx6q to comment if you can
get away without using CLK_OF_DECLARE() at all. The only clock that
really needs to use CLK_OF_DECLARE() is any clock that's needed for
the scheduler timer. Other than that, everything else can be
initialized by a normal driver. Including UART clocks. I can get into
more specifics if you go down this path.

So, that's how fw_devlink could help here if you massage
drivers/clk/imx/clk-imx6q.c to be a proper platform driver. You'll
have to set fw_devlink=on in the kernel commandline though (it's work
in progress to set this by default). There are some additional details
here about keeping clocks on, but we can discuss the solution for that
if it becomes an issue.

quoted

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)

Even if you completely ignore fw_devlink, why not just model this
clock as a fixed-clock in DT for this specific machine?

It's clearly expecting the clock to be an always on fixed clock.

Yes. SoC runs unreliably with this. Downstream vendor kernel does
not contain a clock driver for the squarewave pin of the RTC (i.e.
their driver does not yet contain 1373e77b4f10) and just works.
Upstream kernel disables the RTC's squarewave and then goes into
reboot loop because of watchdog going crazy.

This will also remove the need for adding "boot-clock-frequencies"
binding.  "fixed-clocks" devices are initialized very early on
(they use CLK_OF_DECLARE too) even without their parents probing
(not sure I agree with this, but this is how it works now).

Something like:

rtc: m41t62@68 {
compatible = "st,m41t62";
reg = <0x68>;

    clock-ckil {
                    compatible = "fixed-clock";
                    #clock-cells = <0>;
                    clock-frequency = <32768>;
            };
};

I hope this helps.

This looks like a complex way of my initial patchset with
'protected-clocks' property replaced by a fixed-clock
node. RTC driver needs to check if that exists and avoid
registering its own clock.

-- Sebastian