The RX handler of the driver has two paths switched between, depending on
the size of the frame received, as determined by SKBUFF_RX_COPYBREAK.
When a small frame is received, a new skb allocated has data space large
enough to hold the incoming frame only, and data is copied there from the
original skb whose buffer is returned to the DMA RX ring; in that case
`rx_in_place' is 0. When a large frame is received, a new skb allocated
has data space large enough to hold the largest frame possible, including
the overhead for alignment, the receive status and padding, over 4.5kiB
overall, and its buffer is placed on the DMA RX ring while the original
buffer is passed up to the network stack avoiding the need to copy data;
in that case `rx_in_place' is 1.
However the latter scenario is only possible when dynamic buffers are
used, as determined by DYNAMIC_BUFFERS, because otherwise the buffers used
for the DMA RX ring are fixed at the time the interface is brought up.
That leads to an observation that the preprocessor conditional around the
`rx_in_place' check is inverted, the check only really matters when
dynamic buffers are in use. It has gone unnoticed for many years since
support for using dynamic buffers on the DMA RX ring was introduced in
2.1.40 -- because the only problem that results is in the case where
`rx_in_place' is 1 frame data received is unnecessarily copied to the
newly-allocated buffer, before the buffer placed on the the DMA receive RX
and its contents ignored. Therefore the only symptom is some performance
loss.
Rather than flipping the condition though I decided to discard the
conditional altogether -- in the case of static buffers `rx_in_place' is
always 0 so GCC will optimise the C conditional away instead.
Tested on a few DEFPA and DEFTA boards successfully using both small and
large frames, both with DYNAMIC_BUFFERS defined and with the macro
undefined.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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