Bit-banged MII bus support

The miiphybb ( Bit-banged MII bus driver ) supports an arbitrary number of MII buses. This feature is useful when a driver uses different MII buses for different PHYs and all (or a part) of these buses are implemented via bit-banging mode.

The driver requires that the following macro is defined in the board configuration file:

• CONFIG_BITBANGMII - Enable the miiphybb driver

The driver code needs to allocate a regular MDIO device using mdio_alloc() and assign .read and .write accessors which wrap bb_miiphy_read() and bb_miiphy_write() functions respectively. The bb_miiphy_read() and bb_miiphy_write() functions take a pointer to a callback structure, struct bb_miiphy_bus_ops. The struct bb_miiphy_bus_ops has the following fields/callbacks (see miiphy.h for details):

int (*mdio_active)()   // Activate the MDIO pin as output
int (*mdio_tristate)() // Activate the MDIO pin as input/tristate pin
int (*set_mdio)()      // Write the MDIO pin
int (*get_mdio)()      // Read the MDIO pin
int (*set_mdc)()       // Write the MDC pin
int (*delay)()         // Delay function

The driver code will look like:

static const struct bb_miiphy_bus_ops ravb_bb_miiphy_bus_ops = {
    .mdio_active      = ravb_bb_mdio_active,
    .mdio_tristate    = ravb_bb_mdio_tristate,
    .set_mdio         = ravb_bb_set_mdio,
    .get_mdio         = ravb_bb_get_mdio,
    .set_mdc          = ravb_bb_set_mdc,
    .delay            = ravb_bb_delay,
};

static int ravb_bb_miiphy_read(struct mii_dev *miidev, int addr,
                               int devad, int reg)
{
    return bb_miiphy_read(miidev, &ravb_bb_miiphy_bus_ops,
                          addr, devad, reg);
}

static int ravb_bb_miiphy_write(struct mii_dev *miidev, int addr,
                                int devad, int reg, u16 value)
{
    return bb_miiphy_write(miidev, &ravb_bb_miiphy_bus_ops,
                           addr, devad, reg, value);
}

static int ravb_probe(struct udevice *dev)
{
    struct mii_dev *mdiodev;
...
    mdiodev = mdio_alloc();
    if (!mdiodev)
        return -ENOMEM;

    mdiodev->read = ravb_bb_miiphy_read;
    mdiodev->write = ravb_bb_miiphy_write;
    mdiodev->priv = eth;
    snprintf(mdiodev->name, sizeof(mdiodev->name), dev->name);

    ret = mdio_register(mdiodev);
...
}