clk: Set req_rate on reparenting

If a non-rate clock started by default with a parent that never
registered, core->req_rate will be 0. The expectation is that whenever
the parent will be registered, req_rate will be updated with the new
value that has just been computed.

However, if that clock is a mux, clk_set_parent() can also make that
clock no longer orphan. In this case however, we never update req_rate.

The natural solution to this would be to update core->rate and
core->req_rate in clk_reparent() by calling clk_recalc().

However, this doesn't work in all cases. Indeed, clk_recalc() is called
by __clk_set_parent_before(), __clk_set_parent() and
clk_core_reparent(). Both __clk_set_parent_before() and __clk_set_parent
will call clk_recalc() with the enable_lock taken through a call to
clk_enable_lock(), the underlying locking primitive being a spinlock.

clk_recalc() calls the backing driver .recalc_rate hook, and that
implementation might sleep if the underlying device uses a bus with
accesses that might sleep, such as i2c.

In such a situation, we would end up sleeping while holding a spinlock,
and thus in an atomic section.

In order to work around this, we can move the core->rate and
core->req_rate update to the clk_recalc() calling sites, after the
enable_lock has been released if it was taken.

The only situation that could still be problematic is the
clk_core_reparent() -> clk_reparent() case that doesn't have any
locking. clk_core_reparent() is itself called by clk_hw_reparent(),
which is then called by 4 drivers:

  * clk-stm32mp1.c, stm32/clk-stm32-core.c and tegra/clk-tegra210-emc.c
    use it in their set_parent implementation. The set_parent hook is
    only called by __clk_set_parent() and clk_change_rate(), both of
    them calling it without the enable_lock taken.

  * clk/tegra/clk-tegra124-emc.c calls it as part of its set_rate
    implementation. set_rate is only called by clk_change_rate(), again
    without the enable_lock taken.

In both cases we can't end up in a situation where the clk_hw_reparent()
caller would hold a spinlock, so it seems like this is a good
workaround.

Let's also add some unit tests to make sure we cover the original bug.

Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> # imx8mp
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> # exynos4210, meson g12b
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220816112530.1837489-14-maxime@cerno.tech
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>