Re: [PATCH 07/10] clk: amlogic: Support POWER_OF_TWO for PLL pre-divider
From: Jian Hu
Date: Tue May 26 2026 - 05:59:27 EST
On 5/20/2026 3:35 PM, Jerome Brunet wrote:
[ EXTERNAL EMAIL ]
On mer. 20 mai 2026 at 13:47, Jian Hu <jian.hu@xxxxxxxxxxx> wrote:
On 5/14/2026 11:11 PM, Jerome Brunet wrote:Yes
[ EXTERNAL EMAIL ]
On lun. 11 mai 2026 at 20:47, Jian Hu via B4 Relay <devnull+jian.hu.amlogic.com@xxxxxxxxxx> wrote:
From: Jian Hu <jian.hu@xxxxxxxxxxx>I understand what you are doing here but I have to ask why this can't be
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.
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.
There is hardly a justification here
Moreover, for all Amlogic chips, the n divider is an indispensable part of
the DCO clock.
The difference between SoC generations is as follows:Yes that was fairly obvious
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)
Therefore, splitting out the n divider from the DCO clock might not be aI'm not sure I agree and you've only stated your point of view without
good design choice.
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.
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.
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.
Additionally, is the refactored pre-divider N implemented as a separate patchset,
independent from the A9 PLL changes?
Best regards,
Jian