Chapter 61. Ethernet PHY Device Support

Ethernet PHY Device API

Modern ethernet subsystems are often separated into two pieces, the media access controller (sometimes known as a MAC) and the physical device or line interface (often referred to as a PHY). In this case, the MAC handles generating and parsing physical frames and the PHY handles how this data is actually moved to/from the wire. The MAC and PHY communicate via a special protocol, known as MII. This MII protocol can handle control over the PHY which allows for selection of such transmission criteria as line speed, duplex mode, etc.

In most cases, ethernet drivers only need to bother with the PHY during system initialization. Since the details of the PHY are separate from the MAC, there are different drivers for each. The drivers for the PHY are described by a set of exported functions which are commonly used by the MAC. The primary use of these functions currently is to initialize the PHY and determine the status of the line connection.

The connection between the MAC and the PHY differs from MAC to MAC, so the actual routines to manipulate this data channel are a property of the MAC instance. Furthermore, there are many PHY devices each with their own internal operations. A complete MAC/PHY driver setup will be comprised of the MAC MII access functions and the PHY internal driver.

A driver instance is contained within a eth_phy_access_t:


typedef struct {
    int ops_type;  // 0 => bit level, 1 => register level
    bool init_done;
    void (*init)(void);
    void (*reset)(void);
    union {
        struct {
            void (*set_data)(int);
            int  (*get_data)(void);
            void (*set_clock)(int);
            void (*set_dir)(int);
        } bit_level_ops;
        struct {
            void (*put_reg)(int reg, int unit, unsigned short data);
            bool (*get_reg)(int reg, int unit, unsigned short *data);
        } reg_level_ops;
    } ops;
    int phy_addr;
    struct _eth_phy_dev_entry *dev;  // Chip access functions
} eth_phy_access_t;

struct _eth_phy_dev_entry {
    char          *name;
    unsigned long  id;
    bool         (*stat)(eth_phy_access_t *f, int *stat);
The dev element points to the PHY specific support functions. Currently, the only function which must be defined is stat().

The MAC-MII-PHY interface is a narrow connection, with commands and status moving between the MAC and PHY using a bit-serial protocol. Some MAC devices contain the intelligence to run this protocol, exposing a mechanism to access PHY registers one at a time. Other MAC devices may only provide access to the MII data lines (or even still, this may be considered completely separate from the MAC). In these cases, the PHY support layer must handle the serial protocol. The choice between the access methods is in the ops_type field. If it has the value PHY_BIT_LEVEL_ACCESS_TYPE, then the PHY device layer will run the protocol, using the access functions set_data(), get_data(), set_clock(), set_dir() are used to control the MII signals and run the protocol. If ops_type has the value PHY_REG_LEVEL_ACCESS_TYPE, then the routines put_reg(), and get_reg() are used to access the PHY registers.

Two additional functions may be defined. These are init(), and reset(). The purpose of these functions is for gross-level management of the MII interface. The init() function will be called once, at system initialization time. It should do whatever operations are necessary to prepare the MII channel. In the case of PHY_BIT_LEVEL_ACCESS_TYPE devices, init() should prepare the signals for use, i.e. set up the appropriate parallel port registers, etc. The reset() function may be called by a driver to cause the PHY device to be reset to a known state. Not all drivers will require this and this function may not even be possible, so it's use and behavior is somewhat target specific.

Currently, the only function required of device specific drivers is stat(). This routine should query appropriate registers in the PHY and return a status bitmap indicating the state of the physical connection. In the case where the PHY can auto-negotiate a line speed and condition, this information may be useful to the MAC to indicate what speed it should provide data, etc. The status bitmask contains these bits:

#define ETH_PHY_STAT_LINK   0x0001   // Link up/down
#define ETH_PHY_STAT_100MB  0x0002   // Connection is 100Mb/10Mb
#define ETH_PHY_STAT_FDX    0x0004   // Connection is full/half duplex
#define ETH_PHY_STAT_1000MB 0x0008   // Connection is 1Gb
Note: the usage here is that if the bit is set, then the condition exists. For example, if the ETH_PHY_STAT_LINK is set, then a physical link has been established.