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From: Timur Tabi <timur@codeaurora.org>
To: David Laight <David.Laight@ACULAB.COM>,
	"David S. Miller" <davem@davemloft.net>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control autonegotiation by default
Date: Wed, 2 Aug 2017 10:08:35 -0500	[thread overview]
Message-ID: <115c4698-cfcb-83dc-e70b-89207ef326a8@codeaurora.org> (raw)
In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6DD00491F6@AcuExch.aculab.com>

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.

-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

  reply	other threads:[~2017-08-02 15:08 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-01 21:37 [PATCH 0/2] net: qcom/emac: fixes for pause frame floods Timur Tabi
2017-08-01 21:37 ` [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control autonegotiation by default Timur Tabi
2017-08-01 21:55   ` Florian Fainelli
2017-08-01 22:02     ` Timur Tabi
2017-08-01 22:08       ` Florian Fainelli
2017-08-01 23:15   ` Andrew Lunn
2017-08-02  0:56     ` Timur Tabi
2017-08-02  2:58       ` Andrew Lunn
2017-08-02  3:22         ` Timur Tabi
2017-08-02 13:48   ` David Laight
2017-08-02 14:21     ` Timur Tabi
2017-08-02 14:51       ` David Laight
2017-08-02 15:08         ` Timur Tabi [this message]
2017-08-02 15:38           ` David Laight
2017-08-02 17:54   ` David Miller
2017-08-02 18:23     ` Timur Tabi
2017-08-02 18:35       ` David Miller
2017-08-02 18:39         ` Timur Tabi
2017-08-02 23:15           ` David Miller
2017-08-03  1:00             ` Timur Tabi
2017-08-02 18:36       ` David Miller
2017-08-01 21:37 ` [PATCH 2/2] net: qcom/emac: add software control for pause frame mode Timur Tabi
2017-08-01 21:51   ` Florian Fainelli
2017-08-01 22:00     ` Timur Tabi
2017-08-01 22:06       ` Florian Fainelli
2017-09-12 22:07   ` Timur Tabi

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