linux-kernel - Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn

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Date:   Sun, 04 Mar 2018 21:55:17 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Mathieu Malaterre <malat@...ian.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jiri Slaby <jslaby@...e.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Mathieu Malaterre <malat@...ian.org>
Subject: Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid

Mathieu Malaterre <malat@...ian.org> writes:

> Rewrite comparison since all values compared are of type `unsigned long`.
>
> Fix a warning (treated as error in W=1):
>
>   CC      arch/powerpc/kernel/irq.o
> In file included from ./include/linux/bug.h:5:0,
>                  from ./include/linux/cpumask.h:13,
>                  from ./include/linux/smp.h:13,
>                  from ./include/linux/kernel_stat.h:5,
>                  from arch/powerpc/kernel/irq.c:35:
> ./include/linux/dma-mapping.h: In function ‘dma_map_resource’:
> ./arch/powerpc/include/asm/page.h:129:32: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
>  #define pfn_valid(pfn)  ((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
>                                 ^
> Suggested-by: Segher Boessenkool <segher@...nel.crashing.org>
> Signed-off-by: Mathieu Malaterre <malat@...ian.org>
> ---
>  arch/powerpc/include/asm/page.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
> index 8da5d4c1cab2..19dea64e7ed2 100644
> --- a/arch/powerpc/include/asm/page.h
> +++ b/arch/powerpc/include/asm/page.h
> @@ -126,7 +126,8 @@ extern long long virt_phys_offset;
>  
>  #ifdef CONFIG_FLATMEM
>  #define ARCH_PFN_OFFSET		((unsigned long)(MEMORY_START >> PAGE_SHIFT))
> -#define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
> +#define pfn_valid(pfn) \
> +		(((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET))

I'm not a big fan of this one, because the original code is *far* more
obvious as to what it's doing.

I'm not sure if we can make this one a static inline, or whether that
would help, but it would be worth investigating.

cheers