Re: [PATCH 1/2] clk: Add clk_get_first_to_set_rate

[Stephen Boyd <sboyd@kernel.org>]

Quoting Paul Cercueil (2021-03-07 09:07:41)
> The purpose of this function is to be used along with the notifier
> mechanism.
> 
> When a parent clock can see its rate externally changed at any moment,
> and a driver needs a specific clock rate to function, it can register a
> notifier on the parent clock, and call clk_set_rate() on the base clock
> to adjust its frequency according to the new parent clock.

Can the driver use the rate locking mechanism to get a certain rate
instead of registering for notifiers and trying to react to changes?

> 
> This works fine, until the base clock has the CLK_SET_RATE_PARENT flag
> set. In that case, calling clk_set_rate() on the base clock will call
> clk_set_rate() on the parent clock, which will trigger the notifier
> again, and we're in a loop.
> 
> For that reason, we need to register the notifier on the parent clock of
> the first ancestor of the base clock that will effectively modify its
> rate when clk_set_rate() is called, which we can now obtain with
> clk_get_first_to_set_rate().
> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>