linux-kernel - Re: [PATCH] IB/lmx4: silence GCC warning

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Date:	Wed, 10 Oct 2012 09:23:43 +0200
From:	Jack Morgenstein <jackm@....mellanox.co.il>
To:	Paul Bolle <pebolle@...cali.nl>
Cc:	Roland Dreier <roland@...nel.org>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/lmx4: silence GCC warning

You could use:

   u16 uninitialized_var(vlan);

instead.

Although this in the special QP data flow, I still prefer to avoid adding extra code (even setting
initial values at procedure entry).  The line above will also do the job.  "uninitialized_var"
is used elsewhere in the driver.  See, for example, mlx4_ib_post_send() in the same file (qp.c).

-Jack

On Friday 28 September 2012 14:48, Paul Bolle wrote:
> Building qp.o (part of the "Mellanox ConnectX HCA support" driver)
> triggers this GCC warning:
>     drivers/infiniband/hw/mlx4/qp.c: In function ‘mlx4_ib_post_send’:
>     drivers/infiniband/hw/mlx4/qp.c:1828:30: warning: ‘vlan’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>     drivers/infiniband/hw/mlx4/qp.c:1718:6: note: ‘vlan’ was declared here
> 
> Looking at the code it is clear 'vlan' is only set and used if 'is_eth'
> is non-zero. But there's no harm in initializing 'vlan' to 0 (which
> matches ib_get_cached_gid()'s default return) to silence GCC.
> 
> Signed-off-by: Paul Bolle <pebolle@...cali.nl>
> ---
> 0) I noticed this warning while building v3.6-rc7 on current Fedora 17,
> using Fedora's default config.
> 
> 1) Compile tested only. I tested against v3.6-rc7, with commit
> a41262bb5721f2b708ee8b23f67be2f2e16a2fab ("IB/mlx4: SR-IOV IB context
> objects and proxy/tunnel SQP") from linux-next cherry-picked, to take
> into account a trivial context change in linux-next.
> 
>  drivers/infiniband/hw/mlx4/qp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/hw/mlx4/qp.c b/drivers/infiniband/hw/mlx4/qp.c
> index a862251..71fdda6 100644
> --- a/drivers/infiniband/hw/mlx4/qp.c
> +++ b/drivers/infiniband/hw/mlx4/qp.c
> @@ -1715,7 +1715,7 @@ static int build_mlx_header(struct mlx4_ib_sqp *sqp, struct ib_send_wr *wr,
>  	int is_eth;
>  	int is_vlan = 0;
>  	int is_grh;
> -	u16 vlan;
> +	u16 vlan = 0;
>  	int err = 0;
>  
>  	send_size = 0;
--
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