Re: [PATCH] drm: tegra: shut up harmless warning on NOMMU

[Arnd Bergmann]

On Thursday, January 19, 2017 12:00:58 PM CET Thierry Reding wrote:
> On Thu, Jan 12, 2017 at 12:13:51PM +0100, Arnd Bergmann wrote:
> > The tegra DRM driver is almost ok without an MMU, but there
> > is one small warning that I get:
> > 
> > drivers/gpu/drm/tegra/gem.c: In function 'tegra_drm_mmap':
> > drivers/gpu/drm/tegra/gem.c:508:12: unused variable 'prot'
> > 
> > This marks the variable as __maybe_unused instead.
> > 
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >  drivers/gpu/drm/tegra/gem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/tegra/gem.c b/drivers/gpu/drm/tegra/gem.c
> > index 7d853e6b5ff0..63f14b7a59a0 100644
> > --- a/drivers/gpu/drm/tegra/gem.c
> > +++ b/drivers/gpu/drm/tegra/gem.c
> > @@ -505,7 +505,7 @@ int tegra_drm_mmap(struct file *file, struct vm_area_struct *vma)
> >  
> >  		vma->vm_pgoff = vm_pgoff;
> >  	} else {
> > -		pgprot_t prot = vm_get_page_prot(vma->vm_flags);
> > +		pgprot_t prot __maybe_unused = vm_get_page_prot(vma->vm_flags);
> 
> This seems to me like a suboptimal solution. The reason why this fails
> is because pgprot_writecombine(prot) for NOMMU translates to __pgprot(0)
> via a macro. This also means that we need to potentially add a
> __maybe_unused annotation to every local variable that stores a value
> that gets passed to pgprot_writecombine().
> 
> There fortunately aren't very many of those cases, but I still think
> that a better solution would be to turn pgprot_writecombine() into a
> static inline function, so that the parameter would get silently
> ignored. Or perhaps if it must remain a macro, then doing the following
> should still avoid the need to modify every call site:
> 
> 	#define pgprot_writecombine(prot) ({ (void)prot; __pgprot(0); })
> 
> Thierry
> 

Makes sense. How about this version?

	Arnd
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