quoted

All the horrible casts go away, the driver is structured like every
other driver, sparse is probably happy, etc.

This looks more like a matter cosmetic preferences.  I mean, I didn't
notice anything "horrible" in the code so far.

#define bus_to_enetc_regs(bus)  (struct enetc_mdio_regs __iomem *)((bus)->priv)

You should not need a cast here, bus->priv is a void *. But bus->priv
is being abused to hold a __iomem pointer.

enetc_wr_reg(&regs->mdio_cfg, mdio_cfg);

This is also rather odd, passing the address of something to an IO
operator? I also don't know the C standard well enough to know if it
is guaranteed that:

struct enetc_mdio_regs {
        u32     mdio_cfg;       /* MDIO configuration and status */
        u32     mdio_ctl;       /* MDIO control */
        u32     mdio_data;      /* MDIO data */
        u32     mdio_addr;      /* MDIO address */
};

actually works. On a 64bit system is the compiler allowed to put in
padding to keep the u32 64 bit aligned?

I actually find it more
ugly to define a new structure with only one element inside, like:
struct enetc_mdio_priv {
       struct enetc_hw *hw;
}

One advantage of this is that struct enetc_hw correctly has all the
__iomem attributes. All the casts to __iomem go away, and sparse is
happy.

Anyway, if others already did this in the kernel, what can I do?

Clean it up. Make the code more readable and easy to maintain.

      Andrew