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Message-ID: <115c4698-cfcb-83dc-e70b-89207ef326a8@codeaurora.org>
Date: Wed, 2 Aug 2017 10:08:35 -0500
From: Timur Tabi <timur@...eaurora.org>
To: David Laight <David.Laight@...LAB.COM>,
"David S. Miller" <davem@...emloft.net>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control
autonegotiation by default
On 08/02/2017 09:51 AM, David Laight wrote:
> Sending pause frames just tells the adjacent switch not to send you packets
> (because you'll discard them).
> Since the idea is to avoid the discards, the switch will buffer the
> packets it would have sent.
> The buffers in the switch then fill up with packets it isn't sending you.
I was under the impression that the switch forwards the pause frames to
other devices, so that the transmitting NIC can stop sending the data,
but your explanation makes a lot more sense. If the EMAC never stops
sending pause frames, then the switch's buffers will fill up, disabling
all other devices. If the switch does not have per-port buffers, then
it makes sense when the buffer is full, it blocks all ports.
> The switch then runs out of buffers, it has 2 choices:
> 1) Throw the packets away.
> 2) Send 'pause' frames to the sources.
> If it sends 'pause' frames the entire network will very quickly lock up.
> If it discards the packets they might as well have been discarded by the
> receiving MAC.
>
> Doesn't this mean that pause frames are 99.999% useless??
Pause frames are intended for situations where the receiving CPU is
temporarily overwhelmed and just needs a second or two to resume
processing incoming packets. That makes sense on a dinky single-core
32-bit CPU.
--
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