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Subject: Re: [PATCH v2 02/18] clk: intel: Add clock driver for Intel MIPS SoCs
To:     Stephen Boyd <sboyd@kernel.org>,
        Songjun Wu <songjun.wu@linux.intel.com>,
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        Michael Turquette <mturquette@baylibre.com>,
        linux-kernel@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
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From:   "Zhu, Yi Xin" <yixin.zhu@linux.intel.com>
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Date:   Wed, 29 Aug 2018 18:34:26 +0800
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>>>> +}
>>>> +
>>>> +CLK_OF_DECLARE(intel_grx500_cgu, "intel,grx500-cgu", grx500_clk_init);
>>> Any reason a platform driver can't be used instead of CLK_OF_DECLARE()?
>> It provides CPU clock which is used in early boot stage.
>>
> Ok. What is the CPU clock doing in early boot stage? Some sort of timer
> frequency? If the driver can be split into two pieces, one to handle the
> really early stuff that must be in place to get timers up and running
> and the other to register the rest of the clks that aren't critical from
> a regular platform driver it would be good. That's preferred model if
> something is super critical.
>
Just to make sure my approach is same as you think.

In the driver, there's two clock registrations.

- One through CLK_OF_DECLARE for early stage clocks.

- The other via platform driver for the non-critical clocks.

In the device tree,  two clock device nodes are required.

e.g. device tree:

cgu: cgu@16200000 {
                 compatible = "intel,grx500-clk", "syscon";
                 reg = <0x16200000 0x200>;
                 #clock-cells = <1>;
};

clk: clk {
                 compatible = "intel,grx500-cgu";
                 #clock-cells = <1>;
                 intel,cgu-syscon = <&cgu>;
};

source code:

CLK_OF_DECLARE(intel_grx500_cgu, "intel,grx500-cgu", grx500_clk_init);

static const struct of_device_id of_intel_grx500_cgu_match[] = {
         { .compatible = "intel,grx500-clk" },
         {}
};

static struct platform_driver intel_grx500_clk_driver = {
         .probe  = intel_grx500_clk_probe,
         .driver = {
                 .name = "grx500-cgu",
                 .of_match_table = of_match_ptr(of_intel_grx500_cgu_match),
         },
};

static int __init intel_grx500_cgu_init(void)
{
         return platform_driver_register(&intel_grx500_clk_driver);
}
arch_initcall(intel_grx500_cgu_init);