Re: [PATCHv2] net: mvpp2: fix dma unmapping of TX buffers for fragments

From: Thomas Petazzoni <redacted>
Date: Wed, 21 Dec 2016 11:28:49 +0100

Since commit 71ce391dfb784 ("net: mvpp2: enable proper per-CPU TX
buffers unmapping"), we are not correctly DMA unmapping TX buffers for
fragments.

Indeed, the mvpp2_txq_inc_put() function only stores in the
txq_cpu->tx_buffs[] array the physical address of the buffer to be
DMA-unmapped when skb != NULL. In addition, when DMA-unmapping, we use
skb_headlen(skb) to get the size to be unmapped. Both of this works fine
for TX descriptors that are associated directly to a SKB, but not the
ones that are used for fragments, with a NULL pointer as skb:

 - We have a NULL physical address when calling DMA unmap
 - skb_headlen(skb) crashes because skb is NULL

This causes random crashes when fragments are used.

To solve this problem, we need to:

 - Store the physical address of the buffer to be unmapped
   unconditionally, regardless of whether it is tied to a SKB or not.

 - Store the length of the buffer to be unmapped, which requires a new
   field.

Instead of adding a third array to store the length of the buffer to be
unmapped, and as suggested by David Miller, this commit refactors the
tx_buffs[] and tx_skb[] arrays of 'struct mvpp2_txq_pcpu' into a
separate structure 'mvpp2_txq_pcpu_buf', to which a 'size' field is
added. Therefore, instead of having three arrays to allocate/free, we
have a single one, which also improve data locality, reducing the
impact on the CPU cache.

Fixes: 71ce391dfb784 ("net: mvpp2: enable proper per-CPU TX buffers unmapping")
Reported-by: Raphael G <redacted>
Cc: Raphael G <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Petazzoni <redacted>
---
Changes since v1:
 - As requested by David Miller, move the per TX buffer fields into a
   separate structure, 'struct mvpp2_txq_pcpu_buf', so that instead of
   allocating three arrays, we allocate a single array.

Indeed, this looks a lot better.

Applied, thanks.