linux-kernel - Re: [PATCH net-next v1 1/3] net: phy: dp83tg720: implement soft reset with asymmetric delay

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Message-ID: <aEgm25HcomOxE8oX@pengutronix.de>
Date: Tue, 10 Jun 2025 14:36:43 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Andrew Lunn <andrew@...n.ch>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	David Jander <david@...tonic.nl>, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v1 1/3] net: phy: dp83tg720: implement soft
 reset with asymmetric delay

On Tue, Jun 10, 2025 at 02:16:39PM +0200, Andrew Lunn wrote:
> On Tue, Jun 10, 2025 at 10:10:57AM +0200, Oleksij Rempel wrote:
> > From: David Jander <david@...tonic.nl>
> > 
> > Add a .soft_reset callback for the DP83TG720 PHY that issues a hardware
> > reset followed by an asymmetric post-reset delay. The delay differs
> > based on the PHY's master/slave role to avoid synchronized reset
> > deadlocks, which are known to occur when both link partners use
> > identical reset intervals.
> > 
> > The delay includes:
> > - a fixed 1ms wait to satisfy MDC access timing per datasheet, and
> > - an empirically chosen extra delay (97ms for master, 149ms for slave).
> > 
> > Co-developed-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > Signed-off-by: David Jander <david@...tonic.nl>
> 
> Hi Oleksij
> 
> Since you are submitting it, your Signed-off-by should come last. The
> order signifies the developers who passed it along towards merging.

Ack. checkpatch blamed it, so i changed the order.

> > ---
> >  drivers/net/phy/dp83tg720.c | 75 ++++++++++++++++++++++++++++++++-----
> >  1 file changed, 65 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/net/phy/dp83tg720.c b/drivers/net/phy/dp83tg720.c
> > index 7e76323409c4..2c86d05bf857 100644
> > --- a/drivers/net/phy/dp83tg720.c
> > +++ b/drivers/net/phy/dp83tg720.c
> > @@ -12,6 +12,42 @@
> >  
> >  #include "open_alliance_helpers.h"
> >  
> > +/*
> > + * DP83TG720 PHY Limitations and Workarounds
> > + *
> > + * The DP83TG720 1000BASE-T1 PHY has several limitations that require
> > + * software-side mitigations. These workarounds are implemented throughout
> > + * this driver. This section documents the known issues and their corresponding
> > + * mitigation strategies.
> 
> Is there a public errata you can reference?

The PHY Reset Sequence on polling is described in the "DP83TC81x,
DP83TG72x Software Implementation Guide", A Appendix:
https://www.ti.com/lit/an/snla404/snla404.pdf

I do not have access to the errata sheet.

> > + *
> > + * 1. Unreliable Link Detection and Synchronized Reset Deadlock
> > + * ------------------------------------------------------------
> > + * After a link loss or during link establishment, the DP83TG720 PHY may fail
> > + * to detect or report link status correctly. To work around this, the PHY must
> > + * be reset periodically when no link is detected.
> > + *
> > + * However, in point-to-point setups where both link partners use the same
> > + * driver (e.g. Linux on both sides), a synchronized reset pattern may emerge.
> > + * This leads to a deadlock, where both PHYs reset at the same time and
> > + * continuously miss each other during auto-negotiation.
> > + *
> > + * To address this, the reset procedure includes two components:
> > + *
> > + * - A **fixed minimum delay of 1ms** after issuing a hardware reset, as
> > + *   required by the "DP83TG720S-Q1 1000BASE-T1 Automotive Ethernet PHY with
> > + *   SGMII and RGMII" datasheet. This ensures MDC access timing is respected
> > + *   before any further MDIO operations.
> > + *
> > + * - An **additional asymmetric delay**, empirically chosen based on
> > + *   master/slave role. This reduces the risk of synchronized resets on both
> > + *   link partners. Values are selected to avoid periodic overlap and ensure
> > + *   the link is re-established within a few cycles.
> 
> Maybe there is more about this in the following patches, i've not read
> them yet. Does autoneg get as far as determining master/slave role? Or
> are you assuming the link partners are somehow set as
> prefer_master/prefer_slave?

This PHY do not support autoneg (as required for automotive PHYs),
master/slave roles should be assigned by strapping or from software to
make the link functional.

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