cns3xxx: ethernet - resolve SMP issue

The combination of r35942 and r35952 causes an issue where eth_schedule_poll()
can be called from a different CPU between the call to napi_complete() and the
setting of cur_index which can break the rx ring accounting and cause ethernet
latency and/or ethernet stalls.  The issue can be easilly created by adding
a couple of artificial delays such as:

@@ -715,6 +715,7 @@ static int eth_poll(struct napi_struct *napi, int budget)

 	if (!received) {
 		napi_complete(napi);
+udelay(1000);
 		enable_irq(IRQ_CNS3XXX_SW_R0RXC);
 	}

@@ -727,6 +728,7 @@ static int eth_poll(struct napi_struct *napi, int budget)
 	rx_ring->cur_index = i;

 	wmb();
+udelay(1000);
 	enable_rx_dma(sw);

 	return received;

This patch moves the setting of cur_index back up where it needs to be and
addresses the original corner case that r35942 was trying to catch in an
improved fashion by checking to see if the rx descriptor ring has become
full before interrupts were re-enabled so that a poll can be scheduled again
and avoid an rx stall caused by rx interrupts ceasing to fire again.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39761 3c298f89-4303-0410-b956-a3cf2f4a3e73
cns3xxx: ethernet - clean up tx descs only when needed

We already clean up tx descriptors in the napi eth_poll() function so it
would likely be rare to run out of available descriptors in eth_xmit. Thus
we can clean them up only when needed and return busy only when we
still don't have enough.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39762 3c298f89-4303-0410-b956-a3cf2f4a3e73