Re: [PATCH] igb: Record hardware RX overruns in net_stats

[David Miller <davem@davemloft.net>]

From: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Date: Tue, 5 May 2009 14:32:04 -0700

> the manual[1] for the hardware says:
> RNBC:
> This register counts the number of times that frames were received
> when there were no available buffers in host memory to store those
> frames (receive descriptor head and tail pointers were equal). The
> packet is still received if there is space in the FIFO. This register
> only increments if receives are enabled. This register does not
> increment when flow control packets are received.
> 
> The critical bit "The packet is still received if there is space in
> the FIFO" (AND a host memory buffer becomes available) So the reason
> we don't want to put it in the net_stats stats for drops is that the
> packet
> *wasn't* necessarily dropped.
> 
> The rx_missed errors is for packets that were definitely dropped, and
> is already stored in the net_stats structure.

While not an "rx_missed" because we do eventually take the
packet, conceptually it is a "fifo overflow" in the sense
that we exceeded available receive resources at the time that
the packet arrived.