linux-kernel - Re: [PATCH v2] net: dsa: mv88e6xxx: Fix port_hidden_wait to account for port_base

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Date:   Mon, 25 Apr 2022 00:33:15 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Marek Behún <kabel@...nel.org>
Cc:     Nathan Rossi <nathan@...hanrossi.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v2] net: dsa: mv88e6xxx: Fix port_hidden_wait to account
 for port_base_addr

On Sun, Apr 24, 2022 at 09:33:59PM +0200, Marek Behún wrote:
> On Sun, 24 Apr 2022 21:26:58 +0200
> Andrew Lunn <andrew@...n.ch> wrote:
> 
> > On Sun, Apr 24, 2022 at 03:31:43PM +0000, Nathan Rossi wrote:
> > > The other port_hidden functions rely on the port_read/port_write
> > > functions to access the hidden control port. These functions apply the
> > > offset for port_base_addr where applicable. Update port_hidden_wait to
> > > use the port_wait_bit so that port_base_addr offsets are accounted for
> > > when waiting for the busy bit to change.
> > > 
> > > Without the offset the port_hidden_wait function would timeout on
> > > devices that have a non-zero port_base_addr (e.g. MV88E6141), however
> > > devices that have a zero port_base_addr would operate correctly (e.g.
> > > MV88E6390).
> > > 
> > > Fixes: ea89098ef9a5 ("net: dsa: mv88x6xxx: mv88e6390 errata")  
> > 
> > That is further back than needed. And due to the code moving around
> > and getting renamed, you are added extra burden on those doing the
> > back port for no actual gain.
> > 
> > Please verify what i suggested, 609070133aff1 is better and then
> > repost.
> 
> The bug was introduced by ea89098ef9a5.

I have to disagree with that. ea89098ef9a5 adds:

mv88e6390_hidden_wait()

The mv88e6390_ means it should be used with the mv88e6390 family. And
all members of that family have port offset 0. There is no bug here.

609070133aff1 renames it to mv88e6xxx_port_hidden_wait(). It now has
the generic mv88e6xxx_ prefix, so we can expect it to work with any
device. But it does not. This is where the bug has introduced.

But what i think is more important, is i doubt git cherry-pick is
clever enough to be able to follow 609070133aff1 and know where to
make the change in revisions before then. So it is going to need a
human to figure out the backport. And that effort is a waist of time,
because there is no bug before then.

	Andrew