linux-kernel - Re: [PATCH] PM: EM: do not allow pd creation prior to debugfs initialization

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Date:   Wed, 20 Oct 2021 13:54:04 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Chandrasekhar L <clingutla@...eaurora.org>
Cc:     rafael.j.wysocki@...el.com, qperret@...gle.com,
        daniel.lezcano@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] PM: EM: do not allow pd creation prior to debugfs
 initialization

On 10/20/21 1:03 PM, Chandrasekhar L wrote:
> Thanks Lukasz for comment.
> For any reason (ex: HW dependency, etc), if  init_call level of cpufreq/devfreq driver changed
> prior to fs_init call, we would land there right?

It's not the same triggering point, so we should be safe.

> 
> One of such example is, 'drivers/cpufreq/qcom-cpufreq-hw.c' uses postcore_initcall().

It uses the postcore_initcall to probe and register a driver into
the cpufreq framework. Then the cpufreq framework later constructs the
'policy' and calls your cpufreq_driver::init() function that your
driver provided during registration. Thus, these are two different
phases. It used to be true that if a driver required to use an
'advanced' EM registration with custom private 'em_data_callback',
we put the registration call into that .init() code [1] (old [2]).
Recently Viresh added a dedicated callback for this, which IMO
is good and avoids confusion where to put that custom registration
code.

In your driver code, there is also this callback but using a
generic function [3]. It's a 'simple' EM, which is based on OPP
framework helper. A few drivers use that option, if their platform
doesn't need the 'advanced' EM (but that's not in $subject).

Regards,
Lukasz

[1] 
https://elixir.bootlin.com/linux/v5.15-rc1/source/drivers/cpufreq/scmi-cpufreq.c#L249
[2] 
https://elixir.bootlin.com/linux/v5.14/source/drivers/cpufreq/scmi-cpufreq.c#L192
[3] 
https://elixir.bootlin.com/linux/v5.15-rc6/source/drivers/cpufreq/qcom-cpufreq-hw.c#L561