linux-kernel - Re: [PATCH 1/2] clk: Add clk_get_first_to_set

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Date:   Sat, 13 Mar 2021 14:28:45 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Michael Turquette <mturquette@...libre.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Russell King <linux@...linux.org.uk>,
        Ulf Hansson <ulf.hansson@...aro.org>
Cc:     od@...c.me, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-mmc@...r.kernel.org, Paul Cercueil <paul@...pouillou.net>
Subject: Re: [PATCH 1/2] clk: Add clk_get_first_to_set_rate

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@...pouillou.net>