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

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

On Thu, Jan 19, 2023 at 11:34:46AM +0100, Michal Suchánek wrote:
> Hello,
> 
> On Thu, Jan 19, 2023 at 10:24:07AM +0000, Christophe Leroy wrote:
> > 
> > 
> > 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 ?
> 
> No, there are two for blocks.
> 
> > 
> > > +		char *of_display_format = "of-display.%d";
> > 
> > Should be const ?
> 
> Yes, could be.
> 
> > 
> > >   		int ret;
> > >   
> > >   		/* Check if we have a MacOS display without a node spec */
> > > @@ -556,7 +558,10 @@ static int __init of_platform_default_populate_init(void)
> > >   			if (!of_get_property(node, "linux,opened", NULL) ||
> > >   			    !of_get_property(node, "linux,boot-display", NULL))
> > >   				continue;
> > > -			dev = of_platform_device_create(node, "of-display", NULL);
> > > +			ret = snprintf(buf, sizeof(buf), of_display_format, display_number++);
> > > +			if (ret >= sizeof(buf))
> > > +				continue;
> > 
> > 
> > Can you make buf big enough to avoid that ?
> 
> It would be a bit fragile that way.
> 
> The buffer would have to theoretically accomodate
> "of-display.-9223372036854775808", and any change to the format requires
> recalculating the length, by hand.
> 
> Of course, the memory would run out way before allocating that many
> devices so it's kind of pointless to try and accomodate all possible
> device numbers.
> 
> > 
> > And by the way could it be called something else than 'buf' ?
> > 
> > See exemple here : 
> > https://elixir.bootlin.com/linux/v6.1/source/drivers/fsi/fsi-occ.c#L690
> 
> Yes, that is quite possible. Nonetheless, just like 'ret' generic
> variable names also work.

And in fact judicious use of short generic variable names is more
readeable than naming all variables foobar_* as far as I am concerned.
Of course, YMMV.

Thanks

Michal