Re: Buiild error in i915/xe

[David Laight <david.laight.linux@gmail.com>]

On Sat, 18 Jan 2025 10:36:11 -0800
Guenter Roeck <linux@roeck-us.net> wrote:

> On 1/18/25 10:09, David Laight wrote:
> > On Sat, 18 Jan 2025 09:49:21 -0800
> > Guenter Roeck <linux@roeck-us.net> wrote:
> >   
> >> On Sat, Jan 18, 2025 at 05:09:59PM +0000, David Laight wrote:  
> >>> On Sat, 18 Jan 2025 08:13:06 -0800
> >>> Guenter Roeck <linux@roeck-us.net> wrote:
> >>>      
> >>>> Hi,
> >>>>
> >>>> On Mon, Nov 18, 2024 at 07:13:31PM +0000, David Laight wrote:  
> >>>>> Use BUILD_BUG_ON_MSG(statically_true(ulo > uhi), ...) for the sanity
> >>>>> check of the bounds in clamp().
> >>>>> Gives better error coverage and one less expansion of the arguments.
> >>>>>
> >>>>> Signed-off-by: David Laight <david.laight@aculab.com>  
> >>>>
> >>>> This patch triggers a build error when trying to build parisc:allmodconfig.
> >>>> See error message and bisect log below.
> >>>>
> >>>> I don't think there is anything wrong with the patch. The underlying
> >>>> problem seems to be that parisc:allmodconfig enables CONFIG_DRM_XE which
> >>>> tries to build the affected file even though CONFIG_DRM_I915 is not
> >>>> enabled/supported on parisc.  
> >>>
> >>> This has appeared before.
> >>> Any idea which inlined copy of scale() is causing the problem.
> >>> On the face of it they all look ok.
> >>>
> >>> If you can reproduce it maybe try commenting out some of the calls.
> >>>      
> >>
> >> See diff below. All three changes are needed.
> >> No idea why the compiler would know that the values are invalid.  
> > 
> > Maybe it isn't even an inlining issue.
> > Perhaps that compiler just doesn't like the function ?
> > What happens without the 'static' (and an extra prototype)?
> >   
> 
> 
> You mean like that ?
> 
> -static u32 scale(u32 source_val,
> +
> +u32 scale(u32 source_val,
> +                u32 source_min, u32 source_max,
> +                u32 target_min, u32 target_max);
> +
> +u32 scale(u32 source_val,
>                   u32 source_min, u32 source_max,
>                   u32 target_min, u32 target_max)
> 
> It doesn't help. Worse, after that change the error is still reported
> even with the #if 0 elsewhere.

Yes - that means the compiler is 'objecting' to the scale() function itself.
(Without any regard for its callers.)
Which should make it easy to reproduce outside the kernel build.

I think Mat had a successful build with a different (older?) version of gcc for
parisc.

There must be something odd causing the problem - there will be other clamp()
calls in the build that don't generate the error.

Remember that lack of the error messages requires the compiler optimise away
some code - so if the optimisation is skipped the call could be generated
and the warning output (even if the call is optimised away later).

Perhaps there is some obscure interaction with the WARN() statements?

I don't have the required compiler (neither does godbolt).

	David 

> 
> Guenter
>