8260 and skb implementation

[Paul Mackerras]

Dan Malek writes:

> We should make the change to the receive side so this happens.  The
> transmit doesn't copy.  I just copied the code from the 8xx long ago
> and never made the change.  We can't properly do zero copy receive
> on processors that don't have coherent caches due to alignment and
> other information potentially shared in a cache line.

Doing zero-copy receive requires intelligent adaptor firmware/hardware
that can look in the headers and put the packet in different places
depending on what it finds.  Very few adaptors can do this and I
assume the CPM can't.

Cache coherence shouldn't be a problem on transmission generally since
the adaptor is just reading the packet.  On reception we need to make
sure the data area of the skb is cacheline-aligned.  Since the driver
allocates the skbs this should be easy.

Paul.

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