linux-kernel - Re: [PATCH v2 4/6] dt: bindings: add mt7621-pll dt binding documentation

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Date:   Sat, 17 Aug 2019 17:39:52 +0200
From:   Oleksij Rempel <fishor@....net>
To:     Chuanhong Guo <gch981213@...il.com>, Rob Herring <robh@...nel.org>
Cc:     "open list:COMMON CLK FRAMEWORK" <linux-clk@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        John Crispin <john@...ozen.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Weijie Gao <hackpascal@...il.com>, NeilBrown <neil@...wn.name>
Subject: Re: [PATCH v2 4/6] dt: bindings: add mt7621-pll dt binding
 documentation

Hi,

Am 17.08.19 um 16:42 schrieb Chuanhong Guo:
> Hi!
>
> On Tue, Aug 13, 2019 at 11:51 PM Rob Herring <robh@...nel.org> wrote:
>> [...]
>>> +Example:
>>> +     pll {
>>> +             compatible = "mediatek,mt7621-pll";
>>
>> You didn't answer Stephen's question on v1.
>
> I thought he was asking why there's a syscon in compatible string. I
> noticed that the syscon in my previous patch is a copy-paste error
> from elsewhere and dropped it.
>
>>
>> Based on this binding, there is no way to control/program the PLL. Is
>> this part of some IP block?
>
> The entire section is called "system control" in datasheet and is
> occupied in arch/mips/ralink/mt7621.c [0]
> Two clocks provided here is determined by reading some read-only
> registers in this part.
> There's another register in this section providing clock gates for
> every peripherals, but MTK doesn't provide a clock plan in their
> datasheet. I can't determine corresponding clock frequencies for every
> peripherals, thus unable to write a working clock driver.

In provided link [0] the  ralink_clk_init function is reading SYSC_REG_CPLL_CLKCFG0 R/W register.
This register is used to determine clock source,  clock freq and CPU or bus clocks.
SYSC_REG_CPLL_CLKCFG1 register is a clock gate controller. It is used to enable or disable clocks.
Jist wild assumption. All peripheral devices are suing bus clock.

IMO - this information is enough to create full blown drivers/clk/mediatek/clk-mt7621.c

>>> +
>>> +             #clock-cells = <1>;
>>> +             clock-output-names = "cpu", "bus";
>>> +     };
>>> --
>>> 2.21.0
>>>
>
> Regards,
> Chuanhong Guo
>
> [0] https://elixir.bootlin.com/linux/latest/source/arch/mips/ralink/mt7621.c#L156
>