netdev - RE: [PATCH v3] netpoll: Remove 4s sleep during carrier detection

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Date:   Thu, 26 Jan 2023 09:04:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Breno Leitao' <leitao@...ian.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "leit@...com" <leit@...com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael van der Westhuizen" <rmikey@...a.com>
Subject: RE: [PATCH v3] netpoll: Remove 4s sleep during carrier detection

From: Breno Leitao
> Sent: 25 January 2023 18:53
> This patch removes the msleep(4s) during netpoll_setup() if the carrier
> appears instantly.
> 
> Here are some scenarios where this workaround is counter-productive in
> modern ages:
> 
> Servers which have BMC communicating over NC-SI via the same NIC as gets
> used for netconsole. BMC will keep the PHY up, hence the carrier
> appearing instantly.
> 
> The link is fibre, SERDES getting sync could happen within 0.1Hz, and
> the carrier also appears instantly.
> 
> Other than that, if a driver is reporting instant carrier and then
> losing it, this is probably a driver bug.

I can't help feeling that this will break something.
The 4 second delay does look counter productive though.
Obvious alternatives are 'wait a bit before the first check'
and 'require carrier to be present for a few checks'.

It also has to be said that checking every ms seems over enthusiastic.

	David

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