linux-kernel - Re: [PATCH net] net: dsa: mxl-gsw1xx: manually clear RANEG bit

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Message-ID: <aTL_0ZHa3vggLz6z@shell.armlinux.org.uk>
Date: Fri, 5 Dec 2025 15:52:49 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Andrew Lunn <andrew@...n.ch>, Hauke Mehrtens <hauke@...ke-m.de>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rasmus Villemoes <ravi@...vas.dk>,
	"Benny (Ying-Tsan) Weng" <yweng@...linear.com>,
	John Crispin <john@...ozen.org>
Subject: Re: [PATCH net] net: dsa: mxl-gsw1xx: manually clear RANEG bit

On Fri, Dec 05, 2025 at 03:40:35PM +0000, Daniel Golle wrote:
> On Fri, Dec 05, 2025 at 03:17:37PM +0000, Russell King (Oracle) wrote:
> > On Fri, Dec 05, 2025 at 01:56:39PM +0000, Daniel Golle wrote:
> > > On Fri, Dec 05, 2025 at 02:45:35PM +0100, Andrew Lunn wrote:
> > > > On Fri, Dec 05, 2025 at 01:32:20AM +0000, Daniel Golle wrote:
> > > > > Despite being documented as self-clearing, the RANEG bit sometimes
> > > > > remains set, preventing auto-negotiation from happening.
> > > > > 
> > > > > Manually clear the RANEG bit after 10ms as advised by MaxLinear, using
> > > > > delayed_work emulating the asynchronous self-clearing behavior.
> > > > 
> > > > Maybe add some text why the complexity of delayed work is used, rather
> > > > than just a msleep(10)?
> > > > 
> > > > Calling regmap_read_poll_timeout() to see if it clears itself could
> > > > optimise this, and still be simpler.
> > > 
> > > Is the restart_an() operation allowed to sleep? Looking at other
> > > drivers I only ever see that it sets a self-clearing AN RESTART bit,
> > > never waiting for that bit to clear. Hence I wanted to immitate
> > > that behavior by clearing the bit asynchronously. If that's not needed
> > > and msleep(10) or usleep_range(10000, 20000) can be used instead that'd
> > > be much easier, of course.
> > 
> > Sleeping is permitted in this code path, but bear in mind that it
> > will be called from ethtool ops, and thus the RTNL will be held,
> > please keep sleep durations to a minimum.
> 
> In that sense 10ms (on top of the MDIO operation) is not that little.
> Maybe it is worth to use delayed_work to clear the bit after all...

... in which case I think you need to do a better job.

The cancel_delayed_work() in the pcs_disable() method means if we
stopp using this PCS briefly while the AN restart bit is set, there's
nothing that will clear it.

There are other implementations that have this problem. mvneta has
the same problem, but there we can write the register to set the
MVNETA_GMAC_INBAND_RESTART_AN, and then immediately write it again
without delay to clear this bit. The bit is documented as self
clearing, but practical observation indicates it never does.

Are you sure you need to wait 10ms ? What happens if you set the
bit and then immediately clear it, like we do for mvneta?

-- 
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