sunvnet: Reduce LDC message overhead.

This patchset has an updated version of the v1 changes to reduce the overhead from LDC messages.

Re-submitting both patches for completeness.

- Send the minimum number of start and stop messages between producer and consumer.
Changes since v1 to pull out the unused label again' and the unused retries' variable

- When reading the descriptor rings, don't give up as soon as a dring that is not VIO_DESC_READY is found.
Changes since v1: the `again' label and retries variable belong to this patch.

I understand the Puristic objection to the polling-delay from patch 2/2, but the practical reality is that when there is a burst of data, and performance is actually most critical, the overhead from hastily sending an LDC stop and then processing another LDC start message is far greater than the slight penalty of a small delay when the network traffic is light.

Note that I'm separately working on moving the data handling to a bottom-half or softirq context, where the poll-based delay will be even less critical. We could also consider adding a procfs or ethtool based knob to disable the fudge-factored delay, and avoid the minor polling penalty.

Sowmini Varadhan (2):
Avoid sending superfluous LDC messages.
Re-check for a VIO_DESC_READY data descriptor after short udelay()

 drivers/net/ethernet/sun/sunvnet.c | 87 ++++++++++++++++++++++++++++++++++----
 drivers/net/ethernet/sun/sunvnet.h |  4 ++
 2 files changed, 83 insertions(+), 8 deletions(-)

1.8.4.2