Re: [PATCH net-next 6/8] net: mdio: realtek-rtl9300: relocate topology setup

[Andrew Lunn <andrew@lunn.ch>]

> The hardware polling in these devices is very important for the DSA
> driver. Especially as a port can be either PHY or SerDes driven. Some
> MAC operations simply do not work if a wrong state is reported.

And you will get the wrong state if the timing of the poll is in the
middle of the PHY driver swapping the page to read/write something.

I've not looked at what PHYs you plan to support, but code like

https://elixir.bootlin.com/linux/v7.0.10/source/drivers/net/phy/realtek/realtek_main.c#L907

could be called every 50ms to blink the LEDs. That changes the page,
writes a register, and then changes back to the original page. If the
HW polling happens in the middle of that, it will read from the wrong
place. If that HW poll actually changes the page, without restoring
it, the write made in that function could go to the wrong page.

So, in order to be able to use the Linux PHY drivers, you need to
ensure the Linux PHY drivers has exclusive access to the bus when it
needs it.

All the phy_*_paged() methods call phy_lock_mdio_bus(), do what they
need to do, and then call phy_unlock_mdio_bus(). You will need to
extend those calls to disable the HW polling while the bus is locked,
and re-enable it when the bus is unlocked.

> There is no evidence about any issues in the OpenWrt ecosystem 

Try blinking the LEDs as fast as you can, and see if anything
breaks. Some PHYs also have hwmon sensors, and statistics. Read them
as well, as fast as you can, so you create contention on the bus. You
are then more likely to hit the race conditions.

	Andrew