netdev - Re: [PATCH v2] net: dsa: mv88e6xxx: Single chip mode detection for MV88E6*41

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Date:   Fri, 29 Apr 2022 01:17:11 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Nathan Rossi <nathan@...hanrossi.com>
Cc:     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: Single chip mode detection for
 MV88E6*41

On Wed, Apr 27, 2022 at 01:09:28PM +0000, Nathan Rossi wrote:
> The mv88e6xxx driver expects switches that are configured in single chip
> addressing mode to have the MDIO address configured as 0. This is due to
> the switch ADDR pins representing the single chip addressing mode as 0.
> However depending on the device (e.g. MV88E6*41) the switch does not
> respond on address 0 or any other address below 16 (the first port
> address) in single chip addressing mode. This allows for other devices
> to be on the same shared MDIO bus despite the switch being in single
> chip addressing mode.
> 
> When using a switch that works this way it is not possible to configure
> switch driver as single chip addressing via device tree, along with
> another MDIO device on the same bus with address 0, as both devices
> would have the same address of 0 resulting in mdiobus_register_device
> -EBUSY errors for one of the devices with address 0.
> 
> In order to support this configuration the switch node can have its MDIO
> address configured as 16 (the first address that the device responds
> to). During initialization the driver will treat this address similar to
> how address 0 is, however because this address is also a valid
> multi-chip address (in certain switch models, but not all) the driver
> will configure the SMI in single chip addressing mode and attempt to
> detect the switch model. If the device is configured in single chip
> addressing mode this will succeed and the initialization process can
> continue. If it fails to detect a valid model this is because the switch
> model register is not a valid register when in multi-chip mode, it will
> then fall back to the existing SMI initialization process using the MDIO
> address as the multi-chip mode address.
> 
> This detection method is safe if the device is in either mode because
> the single chip addressing mode read is a direct SMI/MDIO read operation
> and has no side effects compared to the SMI writes required for the
> multi-chip addressing mode.
> 
> In order to implement this change, the reset gpio configuration is moved
> to occur before any SMI initialization. This ensures that the device has
> the same/correct reset gpio state for both mv88e6xxx_smi_init calls.
> 
> Signed-off-by: Nathan Rossi <nathan@...hanrossi.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew