Re: [PATCH 07/10] clk: amlogic: Support POWER_OF_TWO for PLL pre-divider

[Jian Hu <jian.hu@amlogic.com>]

On 5/26/2026 8:27 PM, Jerome Brunet wrote:
> [ EXTERNAL EMAIL ]
>
> On mar. 26 mai 2026 at 17:58, Jian Hu <jian.hu@amlogic.com> wrote:
>
>> On 5/20/2026 3:35 PM, Jerome Brunet wrote:
>>> [ EXTERNAL EMAIL ]
>>>
>>> On mer. 20 mai 2026 at 13:47, Jian Hu <jian.hu@amlogic.com> wrote:
>>>
>>>> On 5/14/2026 11:11 PM, Jerome Brunet wrote:
>>>>> [ EXTERNAL EMAIL ]
>>>>>
>>>>> On lun. 11 mai 2026 at 20:47, Jian Hu via B4 Relay <devnull+jian.hu.amlogic.com@kernel.org> wrote:
>>>>>
>>>>>> From: Jian Hu <jian.hu@amlogic.com>
>>>>>>
>>>>>> The A9 PLL pre-divider uses a division factor of 2^n to ensure a clock
>>>>>> duty cycle of 50% after predivision.
>>>>>>
>>>>>> Add flag 'CLK_MESON_PLL_N_POWER_OF_TWO' to indicate that the PLL
>>>>>> pre-divider division factor is 2^n.
>>>>> I understand what you are doing here but I have to ask why this can't be
>>>>> implemented with independent dividers that already supports power of 2 ?
>>>> If we use independent dividers, the n member would have to be removed from
>>>> meson_clk_pll_data.
>>>>
>>>> However, n is referenced 35 times in clk-pll.c, which means we would need
>>>> to modify all
>>>> related logic across the file. This would be a relatively large
>>>> change.
>>> Yes
>>>
>>>> Moreover, for all Amlogic chips, the n divider is an indispensable part of
>>>> the DCO clock.
>>> There is hardly a justification here
>>>
>>>> The difference between SoC generations is as follows:
>>>>       Previous SoCs PLL: n = 1, 2, 3, 4... (linear divider)
>>>>       A9 SoC PLL:            n = 2^0, 2^1, 2^2, 2^3, 2^4... (power-of-two
>>>> divider)
>>> Yes that was fairly obvious
>>>
>>>> Therefore, splitting out the n divider from the DCO clock might not be a
>>>> good design choice.
>>> I'm not sure I agree and you've only stated your point of view without
>>> providing any technical justification here.
>>>
>>>   From the datasheets of the different SoC we have, the documented
>>> limitation is always the DCO output rate range. Nothing related to n (or
>>> m, or the mult-range for that matter). This is a legacy problem, we
>>> started with monolithic driver and slowly simplified it.
>>>
>>> As far as I can see now, reworking the PLL driver to be a simple
>>> multiplier driver with range output rate constraint could actually be
>>> simpler than the current code. I would also make simpler to accomodate
>>> differences such as the one presented here.
>>>
>>> Unless you can provide technical reasons why going in this direction
>>> would be incorrect, that's where I'd prefer to go.
>>>
>>>> [...]
>>>>
>>>> Best regards,
>>>>
>>>> Jian
>>> --
>>> Jerome
>>
>> I agree that having an independent N divider would simplify the PLL rate
>> calculation.
>>
>> A separate pre-divider for N is technically possible, but there are some
>> hardware constraints that need to be considered:
>>
>> N = 1 is the preferred operating mode except a few fixed-frequency PLLs.
>> Larger N values reduce the PLL phase detector frequency, which may
>> negatively impact
>> jitter performance and overall PLL stability.
> Understood. You could really make a difference by going deeper and
> explaining what those constraints are, especially since you ask question
> internally at Amlogic.
>
> At the moment what is documented is a range regarding the output rate of
> the PLLs. A PLL is made of a pre-divider and fractional multiplier.
> and you are saying that for the multiplier to work and lock, there is
> actually a constraint the input rate too.
>
> If you can discuss with your HW team and clarify what the constraints
> really are, that would help to better model the PLL. In then more likely
> for us to figure out the best way to drive it.


I have discussed with the HW PLL team. And here is the discussion results:

When N increases by a factor of X, the PLL bandwidth decreases by a 
factor of X,
deviating from the default optimal bandwidth, which leads to a decrease in
clock performance and affects the stability of clock-dependent modules 
in the chip.

>
>> Because of this, we cannot guarantee stable system operation when arbitrary
>> larger
>> N values are used.
>>
>> Some PLLs require non-1 N values to generate specific fixed output
>> frequencies because
>> the target rate cannot be achieved with N = 1 while keeping the PLL while
>> keeping the
>> PLL within its valid operating range. So N is designed to have other values
>> ​​to
>> satisfy this requirement.
> Again it seems like the constraints we are using are not the real
> limitation, just by-products, which the situation unclear.


When N=3, this type of PLL is designed with the optimal bandwidth based 
on N=3.

>> For example, the AXG PCIe PLL uses N = 3 to generate the required 100 MHz
>> output frequency,
>> since the target frequency cannot be achieved with N = 1.
>>
> PCIe is a topic in itself. It uses different ops for historic reasons though
> I suspect, with proper constraints, it would not really need to.


PCIe has a strict protocol that must be followed. It is also a highly 
sensitive block
with hundreds of complex and stringent lab test requirements.

The PCIe PLL lock sequence needs to fully follow the Amlogic HW team's 
released demo code.
>> Additionally, is the refactored pre-divider N implemented as a separate
>> patchset,
>> independent from the A9 PLL changes?
> I could be seen as a pre-requisite.


Understood.

>>
>> Best regards,
>>
>>
>> Jian
> --
> Jerome


Best regards,


Jian