linux-kernel - Re: [PATCH] RDMA/mlx5: Use DIV_ROUND_UP

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Date:   Thu, 23 May 2019 08:58:03 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, linux-rdma@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] RDMA/mlx5: Use DIV_ROUND_UP_ULL macro to allow 32 bit to
 build

On Wed, May 22, 2019 at 02:54:50PM -0400, Steven Rostedt wrote:
> 
> From: Steven Rostedt (VMware) <rostedt@...dmis.org>
> 
> When testing 32 bit x86, my build failed with:
> 
>   ERROR: "__udivdi3" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
> 
> It appears that a few non-ULL roundup() calls were made, which uses a
> normal division against a 64 bit number. This is fine for x86_64, but
> on 32 bit x86, it causes the compiler to look for a helper function
> __udivdi3, which we do not have in the kernel, and thus fails to build.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
...
> diff --git a/drivers/infiniband/hw/mlx5/main.c b/drivers/infiniband/hw/mlx5/main.c
> index abac70ad5c7c..40d4c5f7ea43 100644
> --- a/drivers/infiniband/hw/mlx5/main.c
> +++ b/drivers/infiniband/hw/mlx5/main.c
> @@ -2344,7 +2344,7 @@ static int handle_alloc_dm_sw_icm(struct ib_ucontext *ctx,
>  	/* Allocation size must a multiple of the basic block size
>  	 * and a power of 2.
>  	 */
> -	act_size = roundup(attr->length, MLX5_SW_ICM_BLOCK_SIZE(dm_db->dev));
> +	act_size = DIV_ROUND_UP_ULL(attr->length, MLX5_SW_ICM_BLOCK_SIZE(dm_db->dev));
>  	act_size = roundup_pow_of_two(act_size);
>  
>  	dm->size = act_size;

This seems wrong: roundup() rounds up to a multiple of second argument
but DIV_ROUND_UP_ULL() would divide with rounding up.

Michal Kubecek