[RFC v1 0/7] Meson8b: make the CPU clock mutable

[Martin Blumenstingl <martin.blumenstingl@googlemail.com>]

This allows changing the CPU clock on the 32-bit Amlogic Meson SoCs
(Meson8, Meson8b and Meson8m2).
CPU frequency scaling will be enabled with a separate series by adding
the CPU clock and the OPP tables to meson8.dtsi and meson8b.dtsi.

While changing the CPU frequency (sys_pll or any of it's post-dividers)
we need to run the CPU clock off the XTAL clock. Otherwise the system
will lock up because we need to disable the sys_pll to change it's
rate.

This also makes the clk-pll's .enable hook a no-op if the clock is
already enabled. Otherwise we're getting lockups when calling the
first clk_{prepare_}enable on the sys_pll or any of it's children (as
the CCF propagates the enable event up to the sys_pll). This is because
the .enable hook unconditionally disables and enables the clock.
However, we can't disable that clock (not even temporarily) if the CPU
is running off sys_pll.

Additionally this adds support for more M/N combinations in sys_pll to
achieve all of the OPPs on Meson8b and all OPPs <= 1608 MHz on Meson8
and Meson8m2.

Compared to Amlogic's 3.10 kernel there's one notable difference: we
are actually allowing changes to the sys_pll. Amlogic's kernel sets
sys_pll to a fixed rate during boot and then uses a timer generate a
"virtual clock rate" by toggling between various dividers (for example:
sys_pll is set to 1536MHz. to achieve 1008MHz they are toggling every
2500us between 1536MHZ and 768MHz so the average over <period, for
example one second> is 1008MHz).
I could reproduce any situation where changing sys_pll failed (for
example due to high temperature). To prove that I ran "stress --cpu 4"
for multiple hours and then cycled through all available CPU
frequencies (while keeping "stress" running in the background). This
worked fine on my Meson8b Odroid-C1 and EC-100 boards as well as my
Meson8m2 board.


Martin Blumenstingl (7):
  clk: meson: meson8b: run from the XTAL when changing the CPU frequency
  clk: meson: meson8b: do not use cpu_div3 for cpu_scale_out_sel
  clk: meson: clk-pll: check if the clock is already enabled
  clk: meson: clk-pll: add the is_enabled function in the clk_ops
  clk: meson: meson8b: mark the CPU clock as CLK_IS_CRITICAL
  clk: meson: meson8b: add support for more M/N values in sys_pll
  clk: meson: meson8b: allow changing the CPU clock tree

 drivers/clk/meson/clk-pll.c | 23 +++++++++
 drivers/clk/meson/meson8b.c | 94 +++++++++++++++++++++++++++++++++----
 2 files changed, 108 insertions(+), 9 deletions(-)

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2.19.1