netdev - Re: [PATCH net-next v3 10/47] net: phylink: Adjust link settings based on rate adaptation

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Date:   Mon, 18 Jul 2022 17:14:42 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Sean Anderson <sean.anderson@...o.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Alexandru Marginean <alexandru.marginean@....com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH net-next v3 10/47] net: phylink: Adjust link settings
 based on rate adaptation

On Sat, Jul 16, 2022 at 06:37:22PM -0400, Sean Anderson wrote:
> On 7/16/22 4:17 PM, Andrew Lunn wrote:
> > On Fri, Jul 15, 2022 at 05:59:17PM -0400, Sean Anderson wrote:
> > > If the phy is configured to use pause-based rate adaptation, ensure that
> > > the link is full duplex with pause frame reception enabled. Note that these
> > > settings may be overridden by ethtool.
> > > 
> > > Signed-off-by: Sean Anderson <sean.anderson@...o.com>
> > > ---
> > > 
> > > Changes in v3:
> > > - New
> > > 
> > >   drivers/net/phy/phylink.c | 4 ++++
> > >   1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
> > > index 7fa21941878e..7f65413aa778 100644
> > > --- a/drivers/net/phy/phylink.c
> > > +++ b/drivers/net/phy/phylink.c
> > > @@ -1445,6 +1445,10 @@ static void phylink_phy_change(struct phy_device *phydev, bool up)
> > >   	pl->phy_state.speed = phy_interface_speed(phydev->interface,
> > >   						  phydev->speed);
> > >   	pl->phy_state.duplex = phydev->duplex;
> > > +	if (phydev->rate_adaptation == RATE_ADAPT_PAUSE) {
> > > +		pl->phy_state.duplex = DUPLEX_FULL;
> > > +		rx_pause = true;
> > > +	}
> > 
> > I would not do this. If the requirements for rate adaptation are not
> > fulfilled, you should turn off rate adaptation.
> > 
> > A MAC which knows rate adaptation is going on can help out, by not
> > advertising 10Half, 100Half etc. Autoneg will then fail for modes
> > where rate adaptation does not work.
> 
> OK, so maybe it is better to phylink_warn here. Something along the
> lines of "phy using pause-based rate adaptation, but duplex is %s".
> 
> > The MAC should also be declaring what sort of pause it supports, so
> > disable rate adaptation if it does not have async pause.
> 
> That's what we do in the previous patch.
> 
> The problem is that rx_pause and tx_pause are resolved based on our
> advertisement and the link partner's advertisement. However, the link
> partner may not support pause frames at all. In that case, we will get
> rx_pause and tx_pause as false. However, we still want to enable rx_pause,
> because we know that the phy will be emitting pause frames. And of course
> the user can always force disable pause frames anyway through ethtool.

If you want the MAC to enable rx_pause, that ought to be handled
separately in the mac_link_up() method, IMHO.

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