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

["Michal Suchánek" <msuchanek@suse.de>]

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.

Thanks

Michal