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

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Date:   Wed, 29 Jan 2020 09:09:44 +0000
From:   "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>
To:     Vladimir Oltean <olteanv@...il.com>, 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" <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: 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:

<< 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