linux-kernel - Re: Why do we need reset_control_get

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Date:	Thu, 28 Jul 2016 19:52:06 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Philipp Zabel <p.zabel@...gutronix.de>,
	Axel Lin <axel.lin@...ics.com>,
	Hans de Goede <hdegoede@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Lee Jones <lee.jones@...aro.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Why do we need reset_control_get_optional() ?

Hi Arnd,


2016-07-28 19:09 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
> On Thursday, July 28, 2016 11:43:00 AM CEST Philipp Zabel wrote:
>> > I want to deprecate _optional variants in the following steps:
>> >
>> > [1] Add "depends on RESET_CONTROLLER" to drivers
>> >     for which reset_control is mandatory.
>> >
>> >     We can find those driver easily by grepping
>> >     the reference to non-optional reset_control_get().
>>
>> Since we have the stubs, the RESET_CONTROLLER dependency is only at
>> runtime, not at build time.
>>
>> I think Arnd wanted to move this in the opposite direction and remove
>> the configurable RESET_CONTROLLER symbol. Maybe we should let all
>> drivers that currently request non-optional resets have:
>>         depends on (ARCH_HAS_)RESET_CONTROLLER || COMPILE_TEST
>> ?
>
> There are various ways to improve the current situation.
>
> I think it's important that a driver that has an optional
> reset line behaves in exactly the same way whether the reset
> subsystem is enabled or disabled when no reset line is
> provided for a machine.
>
> When a driver requires a reset line, we can either have a
> build-time failure when the reset subsystem is disabled
> (enforcing the Kconfig dependency), or cause a runtime
> failure if either there is no reset line or the subsystem
> is disabled.

Yes.  I am suggesting the "enforcing the Kconfig dependency".

"I will let you build this driver, but it would never work"
is not the right thing to do, I think.



> In my experimental patch, I make the _optional functions
> return NULL if no "resets" property is provided but return
> an error if there are reset lines but the subsystem is
> disabled, i.e. an optional reset must be used if it's in the
> DT, but can be ignored otherwise.

I do not like this idea.

reset_control_get() (or variants) should not return NULL, it is ambiguous.
It should return ERR_PTR(-ENOENT) if no "resets" property.

I only want two types for functions that return a pointer.

[1] return a valid pointer on success, or return NULL on failure
    (for example, kmalloc())
[2] return a valid pointer on success, or return error pointer on failure
   (many of _register() functions)

Mixing [1] and [2] will be a mess.



-- 
Best Regards
Masahiro Yamada