Re: [PATCH v3 1/8] dt-bindings: clock: imx8m-clock: support spread spectrum clocking

Krzysztof Kozlowski
2024-11-09T10:22:21Z

On 09/11/2024 11:05, Krzysztof Kozlowski wrote:
> On 09/11/2024 01:37, Peng Fan wrote:
>>> Subject: Re: [PATCH v3 1/8] dt-bindings: clock: imx8m-clock: support
>>> spread spectrum clocking
>>>
>>> On 08/11/2024 13:50, Peng Fan wrote:
>>>>> Subject: Re: [PATCH v3 1/8] dt-bindings: clock: imx8m-clock:
>>> support
>>>>> spread spectrum clocking
>>>>>
>>>>> On 07/11/2024 15:57, Dario Binacchi wrote:
>>>>>> clocks = <&osc_32k>, <&osc_24m>, <&clk_ext1>, <&clk_ext2>,
>>>>>> <&clk_ext3>, <&clk_ext4>;
>>>>>> clock-names = "osc_32k", "osc_24m", "clk_ext1", "clk_ext2",
>>>>>> "clk_ext3", "clk_ext4";
>>>>>> assigned-clocks = <&clk IMX8MN_CLK_A53_SRC>,
>>>>>> <&clk IMX8MN_CLK_A53_CORE>,
>>>>>> <&clk IMX8MN_CLK_NOC>,
>>>>>> <&clk IMX8MN_CLK_AUDIO_AHB>,
>>>>>> <&clk IMX8MN_CLK_IPG_AUDIO_ROOT>,
>>>>>> <&clk IMX8MN_SYS_PLL3>,
>>>>>> <&clk IMX8MN_AUDIO_PLL1>,
>>>>>> <&clk IMX8MN_AUDIO_PLL2>;
>>>>>> assigned-clock-parents = <&clk IMX8MN_SYS_PLL1_800M>,
>>>>>> <&clk IMX8MN_ARM_PLL_OUT>,
>>>>>> <&clk IMX8MN_SYS_PLL3_OUT>,
>>>>>> <&clk IMX8MN_SYS_PLL1_800M>;
>>>>>> assigned-clock-rates = <0>, <0>, <0>,
>>>>>> <400000000>,
>>>>>> <400000000>,
>>>>>> <600000000>,
>>>>>> <393216000>,
>>>>>> <361267200>; };
>>>>>>
>>>>>> The spread spectrum is not configurable on these clocks or, more
>>>>>> generally, may not be configurable (only 4 PLLs have this
>>> capability).
>>>>>> Therefore, I need the "fsl,ssc-clocks"
>>>>>
>>>>> No. That's not true. You do not need it.
>>>>>
>>>>
>>>> i.MX8M clock hardware is similar as:
>>>>
>>>> OSC->ANATOP->CCM
>>>>
>>>> ANATOP will produce PLLs.
>>>> CCM use PLLs as input source.
>>>>
>>>> Currently there is no dedicated ANATOP driver in linux.
>>>> The CCM linux driver will parse the ANATOP node and register clk_hw
>>>> for the PLLs.
>>>
>>> I do not know what is CCM and how does it fit here. What's more, I
>>> don't get driver context here. We talk about bindings.
>>
>>
>> CCM: Clock Control Module, it accepts PLL from anatop as inputs,
>> and outputs clocks to various modules, I2C, CAN, NET, SAI and ...
>>
>>>
>>>
>>>>
>>>>
>>>>> First, the clock inputs for this device are listed in clocks *only*.
>>>>> What is no there, is not an input to the device. Including also Linux
>>>>> aspect (missing devlinks etc). Therefore how can you configure
>>> spread
>>>>> spectrum on clocks which are not connected to this device?
>>>>
>>>> I not understand this well, you mean
>>>> add clocks = <xx CLK_IMX8MM_VIDEO_PLL> in the ccm dtb node?
>>>
>>> Yes. Let me re-iterate and please respond to this exactly comment
>>> instead of ignoring it.
>>>
>>> How a device can care about spread spectrum of a clock which is not
>>> supplied to this device?
>>
>> I hope we are on same page of what spread spectrum means.
>> spread spectrum of a clock is the clock could produce freq in a range,
>> saying [500MHz - 100KHz, 500MHz + 100KHz]. software only need
>> to configure the middle frequency and choose the up/down border
>> range(100KHz here) and enable spread spectrum.
>>
>> device: I suppose you mean the Clock Control Module(CCM) here.
>
> I mean the device we discuss here, in this binding. Whatever this device
> is. CCM or CCX
>
>> CCM does not care, it just accepts the PLL as input, and output
>
> Takes PLL as input but you refuse to add it as clocks? Are you really
> responding to my question?
>
> I asked how can you set spread spectrum on clock which you do not
> receive. Why you do not receive? Because you refuse to add it to clocks
> even though I speak about this since months.
>
>> divided clock to various IPs(Video here). The video IPs care about
>> the spread spectrum of the clock.
>
> So which device do we talk about? I am not a NXP clock developer and I
> care zero about NXP, so keep it simple. We discuss this one specific
> binding for specific device which is called "imx8m-clock" - see subject
> prefix.
>
>>
>> The clock hardware path is as below:
>>
>> OSC(24M) --> Anatop(produce PLL with spread spectrum) ->
>> Clock Control Module(output clock to modules) -> Video IP
>>
>> From hardware perspective, Clock Control Module does not
>> care spread spectrum. Video IP cares spread spectrum.
>>
>>
>>>
>>> Why would you care about spread spectrum of some clock which is not
>>> coming to this device?
>>
>> device, I suppose you mean clock control module(CCM).
>>
>> There is no 'clocks = <&ccm CLK_IMX8M_VIDEO_PLL>' under ccm node.
>> Because in current design, ccm is taken as producer of
>> CLK_IMX8M_VIDEO_PLL, not consumer.
>
> I don't understand now even more. Or I understand even less now. Why
> binding references its own clocks via phandle? This makes no sense at
> all, except of course assigned clocks, but that's because we have one
> property for multiple cases.

And BTW if that was the point then the example is confusing because the
&clk phandle is not the device node in the example but it should.
Neither description says which device's clocks are these.

This is expressed very poorly in the binding, look:
"Phandles of the PLL" - it clearly suggests some other clocks, not its
own, that's so obvious I did not even think of asking. Patchset goes
slow also because of poor explanation, lack of diagrams and expecting me
to remember your clock hierarchy.

Best regards,
Krzysztof