netdev - RE: [PATCH 3/5] net: dsa: ksz: Factor out common tag code

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Date:   Mon, 10 Dec 2018 20:32:50 +0000
From:   <Tristram.Ha@...rochip.com>
To:     <andrew@...n.ch>
CC:     <netdev@...r.kernel.org>, <f.fainelli@...il.com>,
        <vivien.didelot@...oirfairelinux.com>, <Woojung.Huh@...rochip.com>,
        <UNGLinuxDriver@...rochip.com>, <davem@...emloft.net>,
        <marex@...x.de>
Subject: RE: [PATCH 3/5] net: dsa: ksz: Factor out common tag code

> On Fri, Dec 07, 2018 at 07:18:43PM +0100, Marek Vasut wrote:
> > From: Tristram Ha <Tristram.Ha@...rochip.com>
> >
> > Factor out common code from the tag_ksz , so that the code can be used
> > with other KSZ family switches which use differenly sized tags.
> 
> I prefer this implementation over what Tristram recently submitted. It
> is also what we suggested a while back. However, since then we have
> had Spectra/meltdown, and we now know a function call through a
> pointer is expensive. This is the hot path, every frame comes through
> here, so it is worth taking the time to optimize this. Could you try
> to remove the ksz_tag_ops structure. xmit looks simple, since it is a
> tail call, so you can do that in ksz9477_xmit. Receive looks a bit
> more complex.
> 
> I also think for the moment we need it ignore PTP until we have the
> big picture sorted out. If need be, the code can be refactored yet
> again. But i don't want PTP holding up getting more switches
> supported.

Yes, as you may already know, what Marek submitted was my previous
attempt to support different switches in tag_ksz.c.

So this ksz_tag_ops is still not acceptable?  As I understand the kernel is
using this mechanism all over the places.

What is left is a direct copying of the transmit and receive functions for each
new switch tail tag format.