netdev - Re: mlx4_en: fix endianness with blue frame support

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Message-ID: <CAJZOPZLAQf8K_+naaGEiNzMN5RtRiupcm+gZSHf87ez2Dp7cYA@mail.gmail.com>
Date:	Thu, 20 Sep 2012 16:46:51 +0300
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	cascardo@...ux.vnet.ibm.com, netdev@...r.kernel.org,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Eli Cohen <eli@...lanox.co.il>
Subject: Re: mlx4_en: fix endianness with blue frame support

On Tue, Sep 18, 2012 at 10:34 AM, Dan Carpenter
<dan.carpenter@...cle.com> wrote:
> Hello Thadeu Lima de Souza Cascardo,
>
> The patch c5d6136e10d6: "mlx4_en: fix endianness with blue frame
> support" from Oct 10, 2011, leads to the following warning:
> drivers/net/ethernet/mellanox/mlx4/en_tx.c:720 mlx4_en_xmit()
>          warn: potential memory corrupting cast. 4 vs 2 bytes
>
> That patch introduced a call to cpu_to_be32() and added some endian notation.
>         *(__be32 *) (&tx_desc->ctrl.vlan_tag) |= cpu_to_be32(ring->doorbell_qpn);
> But it doesn't make sense because the data type is declared as u16 in
> the header and we would be corrupting the next elements in the struct
> which are ins_vlan and fence_size.
>
> struct mlx4_wqe_ctrl_seg {
>         __be32                  owner_opcode;
>         __be16                  vlan_tag;
>         u8                      ins_vlan;
>         u8                      fence_size;
>
> I guess the reason we get away with it is that the ->doorbell_qpn is
> normally less that 65k. But doorbell_qpn is a u32 type so I think there is a risk here.

Dan,

QP numbers are 24 bit in size, under blue-flame setting the QP number
is written
over the "vlan_tag" field and potentially also the "ins_vlan" field of
the control segment,
we can do a little cleanup here with introducing a modified version of
the mlx4_wqe_ctrl_seg
structure over which the cast is made  under the blue-flame flow.

Or.
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