embassy_stm32/eth: support compliance testing

[Gerharddc]

This change adds the possibility to perform Ethernet compliance testing with STM32 systems when using a custom PHY struct to generate test patterns.

[Gerharddc]

It seems I've only modified eth v1, I still need to modify eth v2 but it would be great to already get some feedback on the PR if possible.

[Gerharddc]

On second thought, it is probably better to add a separate struct called EthernetTester (or similar) which consumes the Ethernet struct in order to enter test mode. It would then have a function to give back the Ethernet struct which would reset the PHY first. This would then leverage the type system to ensure you don't hand over Ethernet to the networking stack while the PHY is still in test mode.

[Dirbaio]

I'd prefer to avoid extending the PHY trait with methods for every single feature one might want from a PHY.

The trait exists so you can customize the behavior of SMI operations the ethernet driver needs to do. For example, the embassy-net-driver trait requires Eth to be able to report whether the link is up or down. This means Eth has to know how to get this info out of the phy, and since it varies based on the phy it's a trait method so that it can be customized by the user.

This is not the case for compliance test mode. The Eth driver never needs to call it, only the end user. So there's no need for including these functions in the trait.

I think exposing access to SMI instead should be enough for the user to do any SMI operation they want, without having to extend the trait for each possible feature:

impl Eth {
    fn station_management(&mut self) -> &mut impl StationManagement {
        &mut self.station_management
    }
}

[Gerharddc]

@Dirbaio thanks for the feedback! I agree that it is indeed much cleaner to simply expose SMI directly and have made the suggested change. I opted to mark the function as unsafe though since I believe it could unexpectedly mess things up and most users probably should not be accessing the SMI directly under normal circumstances.

[Dirbaio]

thanks!