linux-kernel - Re: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed functions

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Message-ID: <a61eb838-4167-3032-bde8-3a8845b780f8@roeck-us.net>
Date:   Wed, 11 Jan 2017 06:25:35 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Marc Gonzalez <marc_gonzalez@...madesigns.com>
Cc:     Wim Van Sebroeck <wim@...ana.be>, linux-watchdog@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Mans Rullgard <mans@...sr.com>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Mark Rutland <mark.rutland@....com>,
        Uwe Kleine-Konig <u.kleine-koenig@...gutronix.de>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed
 functions

On 01/11/2017 04:31 AM, Marc Gonzalez wrote:
> On 11/01/2017 11:52, Guenter Roeck wrote:
>
>> On 01/11/2017 01:07 AM, Marc Gonzalez wrote:
>>
>>>> @@ -134,12 +134,15 @@ static int tangox_wdt_probe(struct platform_device *pdev)
>>>>  	err = clk_prepare_enable(dev->clk);
>>>>  	if (err)
>>>>  		return err;
>>>> +	err = devm_add_action_or_reset(&pdev->dev,
>>>> +				       (void(*)(void *))clk_disable_unprepare,
>>>> +				       dev->clk);
>>>> +	if (err)
>>>> +		return err;
>>>
>>> Hello Guenter,
>>>
>>> I would rather avoid the function pointer cast.
>>> How about defining an auxiliary function for the cleanup action?
>>>
>>> clk_disable_unprepare() is static inline, so gcc will have to
>>> define an auxiliary function either way. What do you think?
>>
>> Not really. It would just make it more complicated to replace the
>> call with devm_clk_prepare_enable(), should it ever find its way
>> into the light of day.
>
> More complicated, because the cleanup function will have to be deleted later?
> The compiler will warn if someone forgets to do that.
>
> In my opinion, it's not a good idea to rely on the fact that casting
> void(*)(struct clk *clk) to void(*)(void *) is likely to work as expected
> on most platforms. (It has undefined behavior, strictly speaking.)
>
I do hear that you object to this code.

However, I must admit that you completely lost me here. It is a cast from
one function pointer to another, passed as argument to another function,
with a secondary cast of its argument from a typed pointer to a void pointer.
I don't think C permits for "undefined behavior, strictly speaking".
Besides, that same mechanism is already used elsewhere, which is how I
got the idea. Are you claiming that there are situations where it won't
work ?

> Do you really dislike the portable solution I suggested? :-(
>
It is not more portable than the above. It is more expensive and adds more
code.

Thanks,
Guenter