netdev - RE: [PATCH v2 2/2] dpaa_eth: support all modes with rate adapting PHYs

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Date:   Wed, 29 Jan 2020 10:58:20 +0000
From:   "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>
CC:     Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        netdev <netdev@...r.kernel.org>,
        "ykaukab@...e.de" <ykaukab@...e.de>
Subject: RE: [PATCH v2 2/2] dpaa_eth: support all modes with rate adapting
 PHYs

> -----Original Message-----
> From: Vladimir Oltean <olteanv@...il.com>
> Sent: Wednesday, January 29, 2020 12:19 PM
> To: Madalin Bucur (OSS) <madalin.bucur@....nxp.com>
> Cc: Andrew Lunn <andrew@...n.ch>; David S. Miller <davem@...emloft.net>;
> Florian Fainelli <f.fainelli@...il.com>; Heiner Kallweit
> <hkallweit1@...il.com>; netdev <netdev@...r.kernel.org>; ykaukab@...e.de
> Subject: Re: [PATCH v2 2/2] dpaa_eth: support all modes with rate
> adapting PHYs
> 
> Hi Madalin,
> 
> On Wed, 29 Jan 2020 at 11:09, Madalin Bucur (OSS)
> <madalin.bucur@....nxp.com> wrote:
> >
> > > -----Original Message-----
> > > From: Vladimir Oltean <olteanv@...il.com>
> > > Sent: Tuesday, January 28, 2020 5:42 PM
> > > To: Andrew Lunn <andrew@...n.ch>
> > > Cc: Madalin Bucur (OSS) <madalin.bucur@....nxp.com>; David S. Miller
> > > <davem@...emloft.net>; Florian Fainelli <f.fainelli@...il.com>;
> Heiner
> > > Kallweit <hkallweit1@...il.com>; netdev <netdev@...r.kernel.org>;
> > > ykaukab@...e.de
> > > Subject: Re: [PATCH v2 2/2] dpaa_eth: support all modes with rate
> > > adapting PHYs
> > >
> > > Hi Andrew,
> > >
> > > On Mon, 27 Jan 2020 at 18:04, Andrew Lunn <andrew@...n.ch> wrote:
> > > >
> > > > > Is this sufficient?
> > > > > I suppose this works because you have flow control enabled by
> > > default?
> > > > > What would happen if the user would disable flow control with
> > > ethtool?
> > > >
> > > > It will still work. Network protocols expect packets to be dropped,
> > > > there are bottlenecks on the network, and those bottlenecks change
> > > > dynamically. TCP will still be able to determine how much traffic
> it
> > > > can send without too much packet loss, independent of if the
> > > > bottleneck is here between the MAC and the PHY, or later when it
> hits
> > > > an RFC 1149 link.
> > >
> > > Following this logic, this patch isn't needed at all, right? The PHY
> > > will drop frames that it can't hold in its small FIFOs when adapting
> a
> > > link speed to another, and higher-level protocols will cope. And flow
> > > control at large isn't needed.
> >
> > I'm afraid you missed the patch description that explains there will be
> > no link with a 1G partner without this change:
> >
> 
> So why not just remove that linkmode_and() at all, then?
> What is it trying to accomplish, anyway? Avoiding the user from
> shooting themselves in the foot maybe?

If you would take the time to read the patch set, I think it would be clear
that no, I do not intend to remove that altogether, but only when the PHY
can make the different modes work by performing rate adaptation.

> > << After this
> > commit, the modes removed by the dpaa_eth driver were no longer
> > advertised thus autonegotiation with 1G link partners failed.>>
> >
> > > What I was trying to see Madalin's opinion on was whether in fact we
> > > want to keep the RX flow control as 'fixed on' if the MAC supports it
> > > and the PHY needs it, _as a function of the current phy_mode and
> maybe
> > > link speed_ (the underlined part is important IMO).
> >
> > That's a separate concern, by default all is fine, should the user want
> to
> > shoot himself in the foot, we may need to allow him to do it.
> >> > >
> > > >     Andrew
> > > >
> > >
> > > Thanks,
> > > -Vladimir
> 
> -Vladimir