linux-kernel - Re: [PATCH] iommu-common: Do not use 64 bit constant 0xffffffffffffffffl for computing align

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Date:	Sun, 19 Jul 2015 08:27:24 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	benh@...nel.crashing.org, davem@...emloft.net
Subject: Re: [PATCH] iommu-common: Do not use 64 bit  constant
 0xffffffffffffffffl for computing align_mask

On Sun, Jul 19, 2015 at 02:20:14PM +0200, Sowmini Varadhan wrote:
> 
> Using a 64 bit constant generates "warning: integer constant is too
> large for 'long' type" on 32 bit platforms. Instead use ~0l to get
> the desired effect.
> 
> Detected by Andrew Morton who has confirmed that this patch
> fixes the warning on i386/gcc-4.4.3, i386/gcc-4.4.0 and arm/gcc-4.4.4.
> 
> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
> ---
>  lib/iommu-common.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/lib/iommu-common.c b/lib/iommu-common.c
> index df30632..fd1297d 100644
> --- a/lib/iommu-common.c
> +++ b/lib/iommu-common.c
> @@ -119,7 +119,7 @@ unsigned long iommu_tbl_range_alloc(struct device *dev,
>  	unsigned long align_mask = 0;
>  
>  	if (align_order > 0)
> -		align_mask = 0xffffffffffffffffl >> (64 - align_order);
> +		align_mask = ~0l >> (64 - align_order);
>  
Wonder if this just hides the real problem. Unless align_order
is very large, the resulting mask on 32 bit systems may be 0.
Is this really the idea ?

Guenter
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