clk: meson: g12a: add notifiers to handle cpu clock change

clk: meson: g12a: add notifiers to handle cpu clock change

In order to implement clock switching for the CLKID_CPU_CLK and
CLKID_CPUB_CLK, notifiers are added on specific points of the
clock tree :

cpu_clk / cpub_clk
|   \- cpu_clk_dyn
|      |  \- cpu_clk_premux0
|      |        |- cpu_clk_postmux0
|      |        |    |- cpu_clk_dyn0_div
|      |        |    \- xtal/fclk_div2/fclk_div3
|      |        \- xtal/fclk_div2/fclk_div3
|      \- cpu_clk_premux1
|            |- cpu_clk_postmux1
|            |    |- cpu_clk_dyn1_div
|            |    \- xtal/fclk_div2/fclk_div3
|            \- xtal/fclk_div2/fclk_div3
\ sys_pll / sys1_pll

This for each cluster, a single one for G12A, two for G12B.

Each cpu_clk_premux1 tree is marked as read-only and CLK_SET_RATE_NO_REPARENT,
to be used as "parking" clock in a safe clock frequency.

A notifier is added on each cpu_clk_premux0 to detech when CCF want to
change the frequency of the cpu_clk_dyn tree.
In this notifier, the cpu_clk_premux1 tree is configured to use the xtal
clock and then the cpu_clk_dyn is switch to cpu_clk_premux1 while CCF
updates the cpu_clk_premux0 tree.

A notifier is added on each sys_pll/sys1_pll to detect when CCF wants to
change the PLL clock source of the cpu_clk.
In this notifier, the cpu_clk is switched to cpu_clk_dyn while CCF
updates the sys_pll/sys1_pll frequency.

A third small notifier is added on each cpu_clk / cpub_clk and cpu_clk_dyn,
add a small delay at PRE_RATE_CHANGE/POST_RATE_CHANGE to let the other
notofiers change propagate before changing the cpu_clk_premux0 and sys_pll
clock trees.

This notifier set permits switching the cpu_clk / cpub_clk without any
glitches and using a safe parking clock while switching between sub-GHz
clocks using the cpu_clk_dyn tree.

This setup has been tested and validated on the Amlogic G12A and G12B
SoCs running the arm64 cpuburn at [1] and cycling between all the possible
cpufreq translations of each cluster and checking the final frequency using
the clock-measurer, script at [2].

[1] https://github.com/ssvb/cpuburn-arm/blob/master/cpuburn-a53.S
[2] https://gist.github.com/superna9999/d4de964dbc0f84b7d527e1df2ddea25f

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>