Re: [PATCH v2] of: Fix of platform build on powerpc due to bad of disaply code

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 20.01.23 um 12:27 schrieb Michal Suchánek:
> Hello,
> 
> On Thu, Jan 19, 2023 at 04:20:57PM +0100, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 19.01.23 um 14:23 schrieb Michal Suchánek:
>>> On Thu, Jan 19, 2023 at 02:11:13PM +0100, Thomas Zimmermann wrote:
>>>> Hi
>>>>
>>>> Am 19.01.23 um 11:24 schrieb Christophe Leroy:
>>>>>
>>>>>
>>>>> Le 19/01/2023 à 10:53, Michal Suchanek a écrit :
>>>>>> The commit 2d681d6a23a1 ("of: Make of framebuffer devices unique")
>>>>>> breaks build because of wrong argument to snprintf. That certainly
>>>>>> avoids the runtime error but is not the intended outcome.
>>>>>>
>>>>>> Also use standard device name format of-display.N for all created
>>>>>> devices.
>>>>>>
>>>>>> Fixes: 2d681d6a23a1 ("of: Make of framebuffer devices unique")
>>>>>> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
>>>>>> ---
>>>>>> v2: Update the device name format
>>>>>> ---
>>>>>>      drivers/of/platform.c | 12 ++++++++----
>>>>>>      1 file changed, 8 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
>>>>>> index f2a5d679a324..8c1b1de22036 100644
>>>>>> --- a/drivers/of/platform.c
>>>>>> +++ b/drivers/of/platform.c
>>>>>> @@ -525,7 +525,9 @@ static int __init of_platform_default_populate_init(void)
>>>>>>      	if (IS_ENABLED(CONFIG_PPC)) {
>>>>>>      		struct device_node *boot_display = NULL;
>>>>>>      		struct platform_device *dev;
>>>>>> -		int display_number = 1;
>>>>>> +		int display_number = 0;
>>>>>> +		char buf[14];
>>>>>
>>>>> Can you declare that in the for block where it is used instead ?
>>>>>
>>>>>> +		char *of_display_format = "of-display.%d";
>>>>>
>>>>> Should be const ?
>>>>
>>>> That should be static const of_display_format[] = then
>>>
>>> Why? It sounds completely fine to have a const pointer to a string
>>> constatnt.
>>
>> Generally speaking:
>>
>> 'static' because your const pointer is then not a local variable, so it
>> takes pressure off the stack. For global variables, you don't want them to
>> show up in any linker symbol tables.
> 
> This sounds a lot like an exemplar case of premature optimization.
> A simplistic compiler might do exactly what you say, and allocate a slot
> for the variable on the stack the moment the function is entered.
> 
> However, in real compilers there is no stack pressure from having a
> local variable:
>   - the compiler can put the variable into a register
>   - it can completely omit the variable before and after it's actually
>     used which is that specific function call
> 
>> The string "of-display.%d" is stored as an array in the ELF data section.
>> And your char pointer is a reference to that array. For static pointers,
>> these indirections take CPU cycles to update when the loader has to relocate
> 
> Provided that the char pointer ever exists in the compiled code. Its
> address is not taken so it does not need to.
> 
>> sections. If you declare of_display_format[] directly as array, you avoid
>> the reference and work directly with the array.
>>
>> Of course, this is a kernel module and the string is self-contained within
>> the function. So the compiler can probably detect that and optimize the code
>> to be like the 'static const []' version. It's still good to follow best
>> practices, as someone might copy from this function.
> 
> If it could not detect it there would be a lot of trouble all around.

The issues definitely exist in userspace code. Kernel modules are 
simpler, so compiler optimization is easier.

But I'm not really trying to make a technical argument. My point here is 
that someone might read your code and duplicate the pattern. That's not 
unreasonable: it's core Linux code, so it can be assumed to be good (or 
at least not bad). But your current code teaches the reader a bad 
practices, which should be avoided. It is better to do the correct 
thing, even if it makes no difference to the compiled code.

Best regards
Thomas

> 
> Thanks
> 
> Michal

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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