linux-kernel - Re: [PATCH v2 08/11] net: phylink: Adjust advertisement based on rate adaptation

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Date:   Thu, 21 Jul 2022 22:06:32 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Sean Anderson <sean.anderson@...o.com>, netdev@...r.kernel.org,
        Heiner Kallweit <hkallweit1@...il.com>,
        Alexandru Marginean <alexandru.marginean@....com>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH v2 08/11] net: phylink: Adjust advertisement based on
 rate adaptation

On Thu, Jul 21, 2022 at 08:36:03PM +0200, Andrew Lunn wrote:
> > I guess it would depend on the structure of the PHY - whether the PHY
> > is structured similar to a two port switch internally, having a MAC
> > facing the host and another MAC facing the media side. (I believe this
> > is exactly how the MACSEC versions of the 88x3310 are structured.)
> > 
> > If you don't have that kind of structure, then I would guess that doing
> > duplex adaption could be problematical.
> 
> If you don't have that sort of structure, i think rate adaptation
> would have problems in general. Pause is not very fine grained. You
> need to somehow buffer packets because what comes from the MAC is
> likely to be bursty. And when that buffer overflows, you want to be
> selective about what you throw away. You want ARP, OSPF and other
> signalling packets to have priority, and user data gets
> tossed. Otherwise your network collapses.

I don't think rate adaption is that inteligent - it's all about slowing
the MAC down to the speed of the media. From what I remember looking at
pause frames, they can specify how long to delay further transmission
by the receiver, and I would expect this to be set according to the
media speed for setups that use pause packets.

For those which don't, then that's a whole different ball game, because
they tend not to have MACs, and then you're probably down to the
capabilities of nothing more than a FIFO in the PHY.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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