Message-ID: <7a01e398-c8fb-43fd-9b47-7fefb7a692cb@roeck-us.net>
Date: Thu, 14 Nov 2024 06:40:03 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (core) Avoid ifdef in C source file

On 11/13/24 23:27, Thomas Weißschuh wrote:
> On 2024-11-13 22:51:37-0800, Guenter Roeck wrote:
>> On 11/13/24 20:40, Thomas Weißschuh wrote:
>>> On 2024-11-12 22:52:36-0800, Guenter Roeck wrote:
>>>> On 11/12/24 20:39, Thomas Weißschuh wrote:
>>>>> Using an #ifdef in a C source files to have different definitions
>>>>> of the same symbol makes the code harder to read and understand.
>>>>> Furthermore it makes it harder to test compilation of the different
>>>>> branches.
>>>>>
>>>>> Replace the ifdeffery with IS_ENABLED() which is just a normal
>>>>> conditional.
>>>>> The resulting binary is still the same as before as the compiler
>>>>> optimizes away all the unused code and definitions.
>>>>>
>>>>> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
>>>>> ---
>>>>> This confused me a bit while looking at the implementation of
>>>>> HWMON_C_REGISTER_TZ.
>>>>> ---
>>>>>     drivers/hwmon/hwmon.c | 21 ++++++---------------
>>>>>     1 file changed, 6 insertions(+), 15 deletions(-)
>>>>>
>>>>> diff --git a/drivers/hwmon/hwmon.c b/drivers/hwmon/hwmon.c
>>>>> index 9c35c4d0369d7aad7ea61ccd25f4f63fc98b9e02..86fb674c85d3f54d475be014c3fd3dd74c815c57 100644
>>>>> --- a/drivers/hwmon/hwmon.c
>>>>> +++ b/drivers/hwmon/hwmon.c
>>>>> @@ -147,11 +147,6 @@ static DEFINE_IDA(hwmon_ida);
>>>>>     /* Thermal zone handling */
>>>>> -/*
>>>>> - * The complex conditional is necessary to avoid a cyclic dependency
>>>>> - * between hwmon and thermal_sys modules.
>>>>> - */
>>>>> -#ifdef CONFIG_THERMAL_OF
>>>>>     static int hwmon_thermal_get_temp(struct thermal_zone_device *tz, int *temp)
>>>>>     {
>>>>>     	struct hwmon_thermal_data *tdata = thermal_zone_device_priv(tz);
>>>>> @@ -257,6 +252,9 @@ static int hwmon_thermal_register_sensors(struct device *dev)
>>>>>     	void *drvdata = dev_get_drvdata(dev);
>>>>>     	int i;
>>>>> +	if (!IS_ENABLED(CONFIG_THERMAL_OF))
>>>>> +		return 0;
>>>>> +
>>>>>     	for (i = 1; info[i]; i++) {
>>>>>     		int j;
>>>>> @@ -285,6 +283,9 @@ static void hwmon_thermal_notify(struct device *dev, int index)
>>>>>     	struct hwmon_device *hwdev = to_hwmon_device(dev);
>>>>>     	struct hwmon_thermal_data *tzdata;
>>>>> +	if (!IS_ENABLED(CONFIG_THERMAL_OF))
>>>>> +		return;
>>>>> +
>>>>>     	list_for_each_entry(tzdata, &hwdev->tzdata, node) {
>>>>>     		if (tzdata->index == index) {
>>>>>     			thermal_zone_device_update(tzdata->tzd,
>>>>
>>>> There is no dummy function for thermal_zone_device_update().
>>>> I really don't want to trust the compiler/linker to remove that code
>>>> unless someone points me to a document explaining that it is guaranteed
>>>> to not cause any problems.
>>>
>>> I'm fairly sure that a declaration should be enough, and believe
>>> to remember seeing such advise somewhere.
>>> However there is not even a function declaration with !CONFIG_THERMAL.
>>> So I can add an actual stub for it for v2.
>>>
>>> What do you think?
>>>
>> You mean an extern declaration without the actual function ?
> 
> Stub as in empty inline function:
> 
> static inline void thermal_zone_device_update(struct thermal_zone_device *,
>                                               enum thermal_notify_event)
> { }
> 

Sure, that would work, but it would have to be declared in the thermal subsystem.

>> I'd really want to see that documented. It would seem rather unusual.
> 
>>>From Documentation/process/coding-style.rst
> 
> 	21) Conditional Compilation
> 	---------------------------
> 
> 	Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
> 	files; doing so makes code harder to read and logic harder to follow.  Instead,
> 	use such conditionals in a header file defining functions for use in those .c
> 	files, providing no-op stub versions in the #else case, and then call those
> 	functions unconditionally from .c files.  The compiler will avoid generating
> 	any code for the stub calls, producing identical results, but the logic will
> 	remain easy to follow.
> 
> 	[..]
> 
> 	Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
> 	symbol into a C boolean expression, and use it in a normal C conditional:
> 
> 	.. code-block:: c
> 
> 		if (IS_ENABLED(CONFIG_SOMETHING)) {
> 			...
> 		}
> 
> 	The compiler will constant-fold the conditional away, and include or exclude
> 	the block of code just as with an #ifdef, so this will not add any runtime
> 	overhead.
> 
> 	[..]
> 
> While this primarily talks about stubs, the fact that
> "the compiler will constant-fold the conditional away" can be understood
> that the linker will never see those function calls and therefore the
> functions don't have to be present during linking.

Yes, I am aware of that. However, that is not a formal language definition.
Yes, in normal builds with a modern compiler it will be optimized away.
However, I don't think that will happen if the kernel is built with -O0.

> So a declaration would be enough. But an actual stub doesn't hurt either.
> 

I disagree. You did not point to a formal language definition saying that dead code
shall be optimized away and that functions called by such dead code don't have
to actually exist.

Do we really have to argue about this ? Please provide examples from elsewhere
in the kernel which implement what you have suggested (not just the use of
IS_ENABLED(), but the call to functions without stub which don't exist
if the code is not enabled), and we can go from there.

Thanks,
Guenter

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