[PATCH net] net: dsa: mt7530: fix impossible MDIO address and issue warning

[Simon Horman]

On Tue, Apr 30, 2024 at 12:45:46AM +0100, Daniel Golle wrote:
> The MDIO address of the MT7530 and MT7531 switch ICs can be configured
> using bootstrap pins. However, there are only 4 possible options for the
> switch itself: 7, 15, 23 and 31 (ie. only 3 and 4 can be configured, bit
> 0~2 are always 111). Practically all boards known as of today use the
> default setting which is to have the switch respond to address 31, while
> the built-in switch PHYs respond to address 0~4 in this case.
> 
> However, even in MediaTek's SDK the address of the switch is wrongly
> stated in the device trees as 0 (while in reality it is 31), so warn the
> user about such broken device tree and make a good guess what was
> actually intended.
> 
> This is imporant to not break compatibility with older Device Trees as
> with commit 868ff5f4944a ("net: dsa: mt7530-mdio: read PHY address of
> switch from device tree") the address in device tree will be taken into
> account. Doing so instead of assuming the switch is always at
> address 31 which was previously hard-coded will obviously break things
> for many existing downstream device trees as they contain the wrong
> address (0) which previously didn't matter.
> 
> Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
> Signed-off-by: Daniel Golle <daniel at makrotopia.org>

The cited commit is present in net-next but not net.
So I think this patch should target net-next.

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