netdev - Re: [PATCH net-next] phy: avoid unnecessary link-up delay in polling mode

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Message-Id: <20200219.161748.303311549500621079.davem@davemloft.net>
Date:   Wed, 19 Feb 2020 16:17:48 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     poros@...hat.com
Cc:     netdev@...r.kernel.org, andrew@...n.ch, f.fainelli@...il.com,
        hkallweit1@...il.com, ivecera@...hat.com
Subject: Re: [PATCH net-next] phy: avoid unnecessary link-up delay in
 polling mode

From: Petr Oros <poros@...hat.com>
Date: Tue, 18 Feb 2020 10:35:55 +0100

> commit 93c0970493c71f ("net: phy: consider latched link-down status in
> polling mode") removed double-read of latched link-state register for
> polling mode from genphy_update_link(). This added extra ~1s delay into
> sequence link down->up.
> Following scenario:
>  - After boot link goes up
>  - phy_start() is called triggering an aneg restart, hence link goes
>    down and link-down info is latched.
>  - After aneg has finished link goes up. In phy_state_machine is checked
>    link state but it is latched "link is down". The state machine is
>    scheduled after one second and there is detected "link is up". This
>    extra delay can be avoided when we keep link-state register double read
>    in case when link was down previously.
> 
> With this solution we don't miss a link-down event in polling mode and
> link-up is faster.
> 
> Details about this quirky behavior on Realtek phy:
> Without patch:
> T0:    aneg is started, link goes down, link-down status is latched
> T0+3s: state machine runs, up-to-date link-down is read
> T0+4s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
>        here i read link-down (BMSR_LSTATUS==0),
> T0+5s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
>        up-to-date link-up is read (BMSR_LSTATUS==1),
>        phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING
> 
> With patch:
> T0:    aneg is started, link goes down, link-down status is latched
> T0+3s: state machine runs, up-to-date link-down is read
> T0+4s: state machine runs, aneg is finished (BMSR_ANEGCOMPLETE==1),
>        first BMSR read: BMSR_ANEGCOMPLETE==1 and BMSR_LSTATUS==0,
>        second BMSR read: BMSR_ANEGCOMPLETE==1 and BMSR_LSTATUS==1,
>        phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING
> 
> Signed-off-by: Petr Oros <poros@...hat.com>

Applied, thank you.