linux-kernel - [query] skb alignment in tx path of driver

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Message-ID: <cee4bbc90907230955s558123dpe021fa1e7b384068@mail.gmail.com>
Date:	Thu, 23 Jul 2009 22:25:30 +0530
From:	Kalpesh Rathod <kalpeshrathod@...il.com>
To:	linux-net@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [query] skb alignment in tx path of driver

Hi,

I am writing n/w device driver for sdio wifi card. I am testing it on
embedded platform.
DMA controller on the platform is able to xfer data to sdio only if
source data buf address is 4 byte aligned.

The firmware on the card understands 802.3 frames with snap headers.
So, I convert it to the required format in my driver.
4 byte additional header is also attached to this frame and sent over sdio.
This all conversion effectively adds 12 byes to the frame received in
hard_xmit method.

Problem is, frame received in hard_xmit method is 2 byte aligned.
(skb->data is 2 byte aligned). As a result,
after addition of 12 bytes header, skb->data remains 2 byte aligned
instead of 4 byte aligned. And DMA controller refuses
to transfer it.

So, how can I request IP stack so that frame received in hard_xmit is
on 4 byte aligned boundry?

I believe setting NET_IP_ALIGN will only matter in rx path. is this correct?

Thanks in advance.

Regards,
Kalpesh
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