Subject: Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control autonegotiation by default

From: Timur Tabi <[email-masked]>
Date: Wed, 2 Aug 2017 10:08:35 -0500

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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