netdev - Re: mv88e6240 configuration broken for B850v3

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Date:   Mon, 6 Dec 2021 22:23:08 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Martyn Welch <martyn.welch@...labora.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, kernel@...labora.com
Subject: Re: mv88e6240 configuration broken for B850v3

On Mon, Dec 06, 2021 at 08:07:45PM +0000, Russell King (Oracle) wrote:
> My conclusion from having read this thread is the CPU port is using PPU
> polling, meaning that in mac_link_up():
> 
>         if ((!mv88e6xxx_phy_is_internal(ds, port) &&
>              !mv88e6xxx_port_ppu_updates(chip, port)) ||
>             mode == MLO_AN_FIXED) {
> 
> is false - because mv88e6xxx_port_ppu_updates() returns true, and
> consequently we never undo this force-down.

We know that
1. A == mv88e6xxx_phy_is_internal(ds, port), B == mv88e6xxx_port_ppu_updates(chip, port), C == mode == MLO_AN_FIXED
2. (!A && !B) || C == false. This is due to the effect we observe: link is not forced up
2. C == false. This is due to the device tree.
3. !A && !B == false. This is due to statement (2), plus the rule that if X || Y == false and Y == false, then X must also be false.
4. We know that A is true, again due to device tree: port 4 < .num_internal_phys for MV88E6240 which is 5.
5. !A is false, due to 4.

So we have:

false && !B == false.

Therefore "!B" is "don't care". In other words we don't know whether
mv88e6xxx_port_ppu_updates() is true or not.