linux-kernel - Re: [PATCH v7 04/20] kernel: Add combined power-off+restart handler call chain API

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Date:   Fri, 6 May 2022 17:10:24 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
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        Geert Uytterhoeven <geert@...ux-m68k.org>,
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Subject: Re: [PATCH v7 04/20] kernel: Add combined power-off+restart handler
 call chain API

On 4/20/22 21:47, Rafael J. Wysocki wrote:
>>>> +       POWEROFF_PREPARE,
>>>> +};
>>>> +
>>>> +/**
>>>> + * struct power_off_data - Power-off callback argument
>>>> + *
>>>> + * @cb_data: Callback data.
>>>> + */
>>>> +struct power_off_data {
>>>> +       void *cb_data;
>>>> +};
>>>> +
>>>> +/**
>>>> + * struct power_off_prep_data - Power-off preparation callback argument
>>>> + *
>>>> + * @cb_data: Callback data.
>>>> + */
>>>> +struct power_off_prep_data {
>>>> +       void *cb_data;
>>>> +};
>>> Why does this need to be a separate data type?
>> To allow us extend the "struct power_off_prep_data" with more parameters
>> later on without a need to update each driver with the new arguments.

> I'm not really sure what you mean here.  Can you give an example?
> 

The restart callbacks use more than the cb_data and we have:

struct restart_data {
	void *cb_data;
	const char *cmd;
	bool stop_chain;
	enum reboot_mode mode;
};

If we'll ever need to extended struct power_off_data similarly to the
restart_data, then we will need to update all the power-off callbacks
instead of adding a new field to the power_off_data.

Hence, for example, if you'll want to extend power_off_data with "enum
poweroff_mode mode", then for each driver you'll need to do this change:

-power_off(void *cb_data)
+power_off(void *cb_data, enum poweroff_mode mode)

and you won't need to do that using struct power_off_data.

Why do we need this? Because I saw in the past people changing kernel
APIs that way when they wanted to add new arguments and then needed to
update every call site around the kernel.

-- 
Best regards,
Dmitry