Re: [PATCH 0/4] clk: meson: Fix an issue with inaccurate hifi_pll frequency

[Jerome Brunet <jbrunet@baylibre.com>]

On Fri 06 Sep 2024 at 13:52, Chuan Liu via B4 Relay <devnull+chuan.liu.amlogic.com@kernel.org> wrote:

> Some PLLs with fractional multipliers have fractional denominators that
> are fixed to "100000" instead of the previous "(1 << pll->frac.width)".
>
> The hifi_pll for both C3 and S4 supports a fractional multiplier and has
> a fixed fractional denominator of "100000".
>
> Here are the results of the C3-based command tests (already defined
> CLOCK_ALLOW_WRITE_DEBUGFS):
> * echo 491520000 > /sys/kernel/debug/clk/hifi_pll/clk_rate
> * cat /sys/kernel/debug/clk/hifi_pll/clk_rate
> 491520000
> * echo 1 > /sys/kernel/debug/clk/hifi_pll/clk_prepare_enable
> * cat /sys/kernel/debug/meson-clk-msr/clks/hifi_pll_clk
> 491515625       +/-15625Hz
> * devmem 0xfe008100 32
> 0xD00304A3
> * devmem 0xfe008104 32
> 0x00014820
>
> Based on the register information read above, it can be obtained:
> m = 0xA3 = 0d163;
> n = 0x1 = 0d1
> frac = 0x14820 = 0d84000
> od = 0x3 = 0d3
>
> hifi_pll calculates the output frequency:
> calc_rate = xtal_rate / n * (m + (frac / frac_max)) >> od;
> calc_rate = 24000000 / 1 * (163 + (84000 / 100000)) >> 3;
> calc_rate = 491520000
>
> clk_rate, msr_rate, and calc_rate all match.

Thanks for the detailed description.

Is there a possibility this applies to the g12/sm1 as well ? HiFi PLL
has had trouble on these SoCs since support has been added. It sometimes
takes a long time to report a lock. So long we consider it a failure.

There was no such issue on AXG. If you check DT, it is the reason why
AXG use the HiFi PLL for the sound card and G12/SM1 does not.

>
> The test and calculation results of S4 are consistent with those of C3,
> which will not be repeated here.
>
> Signed-off-by: Chuan Liu <chuan.liu@amlogic.com>
> ---
> Chuan Liu (4):
>       clk: meson: Support PLL with fixed fractional denominators
>       clk: meson: c3: pll: hifi_pll frequency is not accurate
>       clk: meson: s4: pll: hifi_pll support fractional multiplier
>       clk: meson: s4: pll: hifi_pll frequency is not accurate
>
>  drivers/clk/meson/c3-pll.c  |  1 +
>  drivers/clk/meson/clk-pll.c | 22 +++++++++++++++++++---
>  drivers/clk/meson/clk-pll.h |  1 +
>  drivers/clk/meson/s4-pll.c  |  9 +++++++--
>  4 files changed, 28 insertions(+), 5 deletions(-)
> ---
> base-commit: adac147c6a32e2919cb04555387e12e738991a19
> change-id: 20240904-fix_clk-668f7a1a2b16
>
> Best regards,

-- 
Jerome