Re: [PATCH net-next v15 08/13] net: mdio: realtek-rtl9300: Add page tracking

[Andrew Lunn <andrew@lunn.ch>]

On Mon, Aug 31, 2026 at 04:34:34PM +0200, Markus Stockhausen wrote:
61;8001;1c> The hardware polling unit of the Realtek switches has a very special
> handling for c22 PHY register 31 (aka Realtek page register) in place.
> 
> - On the RTL838x it is permanently reset to zero.
> - On other devices there is some magic saving/restoring (aka parking)
>   in the background in place.
> 
> This makes access to PHYs a gamble.
> 
> It is vital to keep the polling alive so the MAC layer can rely on
> consistent data. Intercept access to c22 register 31 and handle it
> internally. Store the desired value for each port in the driver. When
> issuing hardware access to other registers add the page to the command
> towards the controller. Given this, the hardware will run two c22
> commands that are not interrupted by polling.
> 
>   ... hardware poll ...
>   phy_write(phy, 31, page)
>   phy_write(phy, reg, value)
>   ... hardware poll ...
> 
> Looking at this implementation one might argue that disabling/enabling
> polling might be a cleaner solution. But one must remember that
> 
> - This driver differentiates clearly between C22 and C45 buses. During
>   probing it enables only one of the protocols for a bus.
> - All known devices run RTL8218 (B/D/E) or RTL8214FC on 1G
> - RTL839x gives link flapping when deactivating polling for a port
> 
> So a solution for a Realtek-only ecosystem is required. This commit
> copies the downstream-proven driver-only page handling patch without
> any new MDIO callbacks and is the lowest common denominator. If a
> non-Realtek PHY is identified on a c22 bus the attachment aborts. It
> should be noted that bus scan runs with the page handling already in
> place before the check in notify_phy_attach(). This is accepted for
> now.
> 
> Remark: To keep this simple, writes to register 31 are only accepted
> if they are lower than the device specific raw page - 0..4094/8190.
> Otherwise -EINVAL is returned. Under the above assumption (Only 1G
> Realtek PHYs on a c22 bus) this is no limitation.
> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

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

    Andrew