linux-kernel - Re: [PATCH 3/3] dma-contiguous: Type cast MAX

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Message-ID: <20210211080417.GC14448@lst.de>
Date:   Thu, 11 Feb 2021 09:04:17 +0100
From:   Christoph Hellwig <hch@....de>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, akpm@...ux-foundation.org,
        will@...nel.org, Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] dma-contiguous: Type cast MAX_ORDER as unsigned int

On Thu, Feb 11, 2021 at 11:52:11AM +0530, Anshuman Khandual wrote:
> Type cast MAX_ORDER as unsigned int to fix the following build warning.
> 
> In file included from ./include/linux/kernel.h:14,
>                  from ./include/asm-generic/bug.h:20,
>                  from ./arch/arm64/include/asm/bug.h:26,
>                  from ./include/linux/bug.h:5,
>                  from ./include/linux/mmdebug.h:5,
>                  from ./arch/arm64/include/asm/memory.h:166,
>                  from ./arch/arm64/include/asm/page.h:42,
>                  from kernel/dma/contiguous.c:46:
> kernel/dma/contiguous.c: In function ‘rmem_cma_setup’:
> ./include/linux/minmax.h:18:28: warning: comparison of distinct pointer
> types lacks a cast
>   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
>                             ^~
> ./include/linux/minmax.h:32:4: note: in expansion of macro ‘__typecheck’
>    (__typecheck(x, y) && __no_side_effects(x, y))
>     ^~~~~~~~~~~
> ./include/linux/minmax.h:42:24: note: in expansion of macro ‘__safe_cmp’
>   __builtin_choose_expr(__safe_cmp(x, y), \
>                         ^~~~~~~~~~
> ./include/linux/minmax.h:58:19: note: in expansion of macro
> ‘__careful_cmp’
>  #define max(x, y) __careful_cmp(x, y, >)
>                    ^~~~~~~~~~~~~
> kernel/dma/contiguous.c:402:35: note: in expansion of macro ‘max’
>   phys_addr_t align = PAGE_SIZE << max(MAX_ORDER - 1, pageblock_order);
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: iommu@...ts.linux-foundation.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  kernel/dma/contiguous.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 3d63d91cba5c..1c2782349d71 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -399,7 +399,7 @@ static const struct reserved_mem_ops rmem_cma_ops = {
>  
>  static int __init rmem_cma_setup(struct reserved_mem *rmem)
>  {
> -	phys_addr_t align = PAGE_SIZE << max(MAX_ORDER - 1, pageblock_order);
> +	phys_addr_t align = PAGE_SIZE << max((unsigned int)MAX_ORDER - 1, pageblock_order);

MAX_ORDER and pageblock_order should be the same type.  So either fix
MAX_ORDER to be an unsigned constant, which would be fundamentally
the right thing to do but might cause some fallout, or turn
pageblock_order into an int, which is probably much either as the stub
define of it already has an integer type derived from MAX_ORDER as well.