Add a PHY Driver in U-Boot

If the Ethernet PHY on your board is not already supported by the U-Boot PHY driver framework, you need to implement and register a new PHY driver. This section describes the U-Boot PHY driver data structure and the steps to add, enable, and initialize a new PHY driver.

U-Boot PHY Driver Structure

U-Boot models each supported PHY as a phy_driver structure. All supported PHY drivers are registered at startup through a vendor-specific initialization function.

The following code block shows the U-Boot PHY driver structure of Motorcomm YT8531.

static struct phy_driver YT8531_driver = {
    .name     = "Motorcomm YT8531",
    .uid      = 0x4f51e91a,
    .mask     = 0x00000fff,
    .features = PHY_GBIT_FEATURES,
    .config   = &yt8531_config,
    .startup  = &genphy_startup,
    .shutdown = &genphy_shutdown,
};

Table 1. PHY Driver Parameters
Parameter	Description
`.name`	PHY identifier string. Use the device-specific name, for example Motorcomm YT8531
`.uid`	Concatenation of the PHY manufacturer ID and device ID, as specified in the PHY datasheet.
`.mask`	UID match mask. Bits set to `1` must match; bits set to `0` are ignored. For example, `0x00000fff` matches the lower 12 bits of the UID.
`.features`	Capability flags indicating whether the PHY supports Gigabit Ethernet (e.g., `PHY_GBIT_FEATURES`).
`.config`	Pointer to the PHY configuration function. For simple PHYs this can be `genphy_config`; for PHYs with QSGMII or RMII modes, a custom function is required to set the operating mode.
`.startup`	Pointer to the startup function (link establishment). Typically `genphy_startup` or a vendor-specific implementation.
`.shutdown`	Pointer to the shutdown function. Typically `genphy_shutdown`.

Add a PHY Driver

To add support for a new PHY (for example, the Motorcomm YT8531), follow these steps in the corresponding vendor file under drivers/net/phy/.

Create a new phy_driver structure following the existing definitions in the vendor file. The structure must conform to the U-Boot phy_driver data format.
Reuse the standard genphy_startup and genphy_shutdown functions unless the PHY has specific requirements.

static int phy_motorcomm_init(void)
{
    phy_register(&YT8512_driver);
    phy_register(&YT8521_driver);
    phy_register(&YT8531_driver);  /* New entry */

    return 0;
}

Enable PHY Driver

After implementing the PHY driver, enable it in the board-specific U-Boot configuration.

In the board header file (for example, include/configs/starfive-visionfive.h), define the PHY address.
The following example defines the PHY address as 0.
```
#define DWC_NET_PHYADDR   0
```
Note: Make sure the PHY address matches the hardware strap configuration on your board. An incorrect address forces U-Boot to enumerate all possible PHY addresses, increasing boot time.
Add the PHY driver definition to the board configuration file.

The following image shows an example of adding the YT8531 PHY in the configuration file.
Figure 1. Add PHY in Configuration File

Initialize PHY Device

The following image shows an example of the YT8521 PHY device initialization code.

The following images show an example of the YT8531 PHY device initialization code.

The above function calls specify how to initialize the Ethernet PHY. You have to use the MDIO bus to access the PHY control registers. Thus, you need to make sure your MDIO interface is configured properly before the configuration.