netdev - errors in alignment changes..

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Date:	Wed, 10 Dec 2014 15:52:03 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mitsuhiro.kimura.kc@...esas.com
CC:	netdev@...r.kernel.org
Subject: errors in alignment changes..

I just re-reviewed this change:

commit 4d6a949c62f123569fb355b6ec7f314b76f93735
Author: Mitsuhiro Kimura <mitsuhiro.kimura.kc@...esas.com>
Date:   Thu Nov 27 20:34:00 2014 +0900

    sh_eth: Fix skb alloc size and alignment adjust rule.

and it has serious problems.  Well, actually the code has
always been broken in this area.

+		/* The size of the buffer is a multiple of 16 bytes. */
+		rxdesc->buffer_length = ALIGN(mdp->rx_buf_sz, 16);
+		dma_map_single(&ndev->dev, skb->data, rxdesc->buffer_length,
+			       DMA_FROM_DEVICE);
 		rxdesc->addr = virt_to_phys(PTR_ALIGN(skb->data, 4));
 		rxdesc->status = cpu_to_edmac(mdp, RD_RACT | RD_RFP);

It doesn't make any sense to call dma_map_single() if you aren't
even going to use the return value.  The DMA mapping created is
the whole point of calling this function.

And then later we pass:

 			dma_sync_single_for_cpu(&ndev->dev, rxdesc->addr,
-						mdp->rx_buf_sz,
+						ALIGN(mdp->rx_buf_sz, 16),
 						DMA_FROM_DEVICE);

rxdesc->addr as the "DMA address", but that must be the return value
from dma_map_single() not what you've actually stored there which is
virt_to_phys() run on the skb->data.

This code must be fixed to:

1) Put the return value from dma_map_single() into a local variable,
   and check for mapping errors.

2) On success put that return value into rxdesc->addr

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