netdev - Re: [PATCH] igb: Record hardware RX overruns in net

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Date:	Tue, 05 May 2009 14:35:29 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jeffrey.t.kirsher@...el.com
Cc:	hawk@...u.dk, hawk@...x.dk, netdev@...r.kernel.org,
	e1000-devel@...ts.sourceforge.net, jesse.brandeburg@...el.com,
	bruce.w.allan@...el.com, peter.p.waskiewicz.jr@...el.com,
	john.ronciak@...el.com
Subject: Re: [PATCH] igb: Record hardware RX overruns in net_stats

From: Jeff Kirsher <jeffrey.t.kirsher@...el.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.
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