netdev - Re: [PATCH net] at803x: insure minimum delay for SGMII link AN completion ckeck

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Date:   Mon, 13 Feb 2017 11:15:58 +0100
From:   Zefir Kurtisi <zefir.kurtisi@...atec.com>
To:     Claudiu Manoil <claudiu.manoil@....com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH net] at803x: insure minimum delay for SGMII link AN
 completion ckeck

On 02/10/2017 05:42 PM, Claudiu Manoil wrote:
> Commit: f62265b "at803x: double check SGMII side autoneg"
> introduced a regression for the p1010rdb board which has
> two of the ethernet controllers (eTSEC) connected through
> SGMII links to external Atheros SGMII AR8033 PHYs.
> The issue consists in a dead link for these ports, and is
> 100% reproducible on kernel 4.9 (and later):
> 
> root@...10rdb-pb:~# ifconfig eth2 172.16.1.1
> [  203.274263] IPv6: ADDRCONF(NETDEV_UP): eth2: link is not ready
> root@...10rdb-pb:~# [  206.408255] 803x_aneg_done: SGMII link is not ok
> 
> root@...10rdb-pb:~# ethtool eth2
> Settings for eth2:
>         Supported ports: [ MII ]
>         Supported link modes:   10baseT/Half 10baseT/Full
>                                 100baseT/Half 100baseT/Full
>                                 1000baseT/Full
>         Supported pause frame use: Symmetric Receive-only
>         Supports auto-negotiation: Yes
>         Advertised link modes:  10baseT/Half 10baseT/Full
>                                 100baseT/Half 100baseT/Full
>                                 1000baseT/Full
>         Advertised pause frame use: No
>         Advertised auto-negotiation: Yes
>         Link partner advertised link modes:  10baseT/Half 10baseT/Full
>                                              100baseT/Half 100baseT/Full
>                                              1000baseT/Half 1000baseT/Full
>         Link partner advertised pause frame use: Symmetric Receive-only
>         Link partner advertised auto-negotiation: Yes
>         Speed: 1000Mb/s
>         Duplex: Full
>         Port: MII
>         PHYAD: 2
>         Transceiver: internal
>         Auto-negotiation: on
>         Supports Wake-on: g
>         Wake-on: d
>         Current message level: 0x0000003f (63)
>                                drv probe link timer ifdown ifup
>         Link detected: no
> 
> Insuring up to 100 usecs for the SGMII link side AN to complete
> proves to be enough to have a working SGMII link, for this board.
> The need for a delay for the SGMII link side may be explained by
> the fact that there are two levels of auto-negotiation (AN) for a
> SGMII link.  First the PHY autonegotiates the link parameters w/
> its link partner over the copper link. In the second stage, the
> AN results are then passed to the eTSEC MAC over the SGMII link
> using the Clause 37 auto-negotiation functionality.  While the
> aneg_done() hook is called by the phylib state machine to check
> for the completion of the 1st stage AN of the external PHY,
> there's no mechanism to insure proper AN completion of the internal
> SGMII link (which is actually handled on the eTSEC side by a
> "internal PHY", called TBI).
> 
> Fixes: f62265b "at803x: double check SGMII side autoneg"
> 
> Signed-off-by: Claudiu Manoil <claudiu.manoil@....com>
> ---
>  drivers/net/phy/at803x.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/phy/at803x.c b/drivers/net/phy/at803x.c
> index a52b560..55fa7c4 100644
> --- a/drivers/net/phy/at803x.c
> +++ b/drivers/net/phy/at803x.c
> @@ -366,6 +366,7 @@ static void at803x_link_change_notify(struct phy_device *phydev)
>  static int at803x_aneg_done(struct phy_device *phydev)
>  {
>  	int ccr;
> +	int timeout = 100; /* usecs */
>  
>  	int aneg_done = genphy_aneg_done(phydev);
>  	if (aneg_done != BMSR_ANEGCOMPLETE)
> @@ -383,7 +384,13 @@ static int at803x_aneg_done(struct phy_device *phydev)
>  	phy_write(phydev, AT803X_REG_CHIP_CONFIG, ccr & ~AT803X_BT_BX_REG_SEL);
>  
>  	/* check if the SGMII link is OK. */
> -	if (!(phy_read(phydev, AT803X_PSSR) & AT803X_PSSR_MR_AN_COMPLETE)) {
> +	do {
> +		if (phy_read(phydev, AT803X_PSSR) & AT803X_PSSR_MR_AN_COMPLETE)
> +			break;
> +		udelay(1);
> +	} while (--timeout);
> +
> +	if (!timeout) {
>  		pr_warn("803x_aneg_done: SGMII link is not ok\n");
>  		aneg_done = 0;
>  	}
> 

Could you confirm that you are using PHY_HAS_INTERRUPT? In polling mode the effect
would be very unlikely to happen, since you'd need to run the state machine
exactly between the two AN stages.

As for the 100us delay proposed, is this something Atheros suggested or is it
based on empirical considerations? Since ending up in a situation where the
double-check fails might left you with a permanent link loss, having a reliable
minimum required delay between first and second stage AN is essential - to me
100us look quite short.

Same goes for the readout polling proposed: given that reading an MDIO register
takes ~16us (at assumed 2.5MHz MDC), delaying for 1us between reads is kind of
useless and ends up in storm-reading the register (and also extends the wait-time
by the same factor). Imo, it won't hurt to sleep for milliseconds between reads
here (see phy_poll_reset() for reference).


Cheers,
Zefir