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

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Date:   Sat, 13 Mar 2021 23:09:23 +0000
From:   Paul Cercueil <paul@...pouillou.net>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Russell King <linux@...linux.org.uk>,
        Ulf Hansson <ulf.hansson@...aro.org>, od@...c.me,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org
Subject: Re: [PATCH 1/2] clk: Add clk_get_first_to_set_rate

Hi Stephen,


Le sam. 13 mars 2021 à 14:28, Stephen Boyd <sboyd@...nel.org> a écrit 
:
> 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?

You mean with clk_rate_exclusive_get()? That sounds like a good idea, 
but what would happen when a different driver calls the non-exclusive 
clk_set_rate() on this clock (or the parent), would it return -EBUSY, 
lock on a mutex? ...

Cheers,
-Paul

> 
>> 
>>  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>