Re: [RFC] clk: add boot clock support

[Rob Herring <robh@kernel.org>]

+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@collabora.com/
> 
> Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
> ---
>  .../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.

Rob