Chapter 156. Writing Ethernet Device Drivers

Generic Ethernet API

This section provides a simple description of how to write a low-level, hardware dependent ethernet driver. In eCos this is known as a “standard” driver.

There is a high-level driver (which is only code — with no state of its own) that is part of the stack. There will be one or more low-level drivers tied to the actual network hardware. Each of these drivers contains one or more driver instances. The intent is that the low-level drivers know nothing of the details of the stack that will be using them. Thus, the same driver can be used by the eCos supported TCP/IP stack, RedBoot, or any other, with no changes.

A driver instance is contained within a struct eth_drv_sc:

struct eth_hwr_funs {
    // Initialize hardware (including startup)
    void (*start)(struct eth_drv_sc *sc,
                  unsigned char *enaddr,
                  int flags);
    // Shut down hardware
    void (*stop)(struct eth_drv_sc *sc);
    // Device control (ioctl pass-thru)
    int  (*control)(struct eth_drv_sc *sc,
                    unsigned long key,
                    void *data,
                    int   data_length);
    // Query - can a packet be sent?
    int  (*can_send)(struct eth_drv_sc *sc);
    // Send a packet of data
    void (*send)(struct eth_drv_sc *sc,
                 struct eth_drv_sg *sg_list,
                 int sg_len,
                 int total_len,
                 unsigned long key);
    // Receive [unload] a packet of data
    void (*recv)(struct eth_drv_sc *sc,
                 struct eth_drv_sg *sg_list,
                 int sg_len);
    // Deliver data to/from device from/to stack memory space
    // (moves lots of memcpy()s out of DSRs into thread)
    void (*deliver)(struct eth_drv_sc *sc);
    // Poll for interrupts/device service
    void (*poll)(struct eth_drv_sc *sc);
    // Get interrupt information from hardware driver
    int (*int_vector)(struct eth_drv_sc *sc);
    // Logical driver interface
    struct eth_drv_funs *eth_drv, *eth_drv_old;
};

struct eth_drv_sc {
    struct eth_hwr_funs *funs;
    void                *driver_private;
    const char          *dev_name;
    int                  state;
    struct arpcom        sc_arpcom; /* ethernet common */
};

Note: If you have two instances of the same hardware, you only need one struct eth_hwr_funs shared between them.

There is another structure which is used to communicate with the rest of the stack:

struct eth_drv_funs {
    // Logical driver - initialization
    void (*init)(struct eth_drv_sc *sc, 
                 unsigned char *enaddr);
    // Logical driver - incoming packet notifier
    void (*recv)(struct eth_drv_sc *sc, 
                 int total_len);
    // Logical driver - outgoing packet notifier
    void (*tx_done)(struct eth_drv_sc *sc, 
                    CYG_ADDRESS key, 
                    int status);
};
Your driver does not create an instance of this structure. It is provided for driver code to use in the eth_drv member of the function record. Its usage is described below in the Section called Upper Layer Functions

One more function completes the API with which your driver communicates with the rest of the stack:

extern void eth_drv_dsr(cyg_vector_t vector,
                        cyg_ucount32 count,
                        cyg_addrword_t data);

This function is designed so that it can be registered as the DSR for your interrupt handler. It will awaken the “Network Delivery Thread” to call your deliver routine. See the Section called Deliver function.

You create an instance of struct eth_drv_sc using the ETH_DRV_SC() macro which sets up the structure, including the prototypes for the functions, etc. By doing things this way, if the internal design of the ethernet drivers changes (e.g. we need to add a new low-level implementation function), existing drivers will no longer compile until updated. This is much better than to have all of the definitions in the low-level drivers themselves and have them be (quietly) broken if the interfaces change.

The “magic” which gets the drivers started (and indeed, linked) is similar to what is used for the I/O subsystem. This is done using the NETDEVTAB_ENTRY() macro, which defines an initialization function and the basic data structures for the low-level driver.

  typedef struct cyg_netdevtab_entry {
      const char        *name;
      bool             (*init)(struct cyg_netdevtab_entry *tab);
      void              *device_instance;
      unsigned long     status;
  } cyg_netdevtab_entry_t;
The device_instance entry here would point to the struct eth_drv_sc entry previously defined. This allows the network driver setup to work with any class of driver, not just ethernet drivers. In the future, there will surely be serial PPP drivers, etc. These will use the NETDEVTAB_ENTRY() setup to create the basic driver, but they will most likely be built on top of other high-level device driver layers.

To instantiate itself, and connect it to the system, a hardware driver will have a template (boilerplate) which looks something like this:

#include <cyg/infra/cyg_type.h>
#include <cyg/hal/hal_arch.h>
#include <cyg/infra/diag.h>
#include <cyg/hal/drv_api.h>
#include <cyg/io/eth/netdev.h>
#include <cyg/io/eth/eth_drv.h>

ETH_DRV_SC(DRV_sc,
           0,             // No driver specific data needed
           "eth0",        // Name for this interface
           HRDWR_start,
           HRDWR_stop,
           HRDWR_control,
           HRDWR_can_send
           HRDWR_send,
           HRDWR_recv,
           HRDWR_deliver,
           HRDWR_poll,
           HRDWR_int_vector
);

NETDEVTAB_ENTRY(DRV_netdev, 
                "DRV", 
                DRV_HRDWR_init, 
                &DRV_sc);

This, along with the referenced functions, completely define the driver.

Note: If one needed the same low-level driver to handle multiple similar hardware interfaces, you would need multiple invocations of the ETH_DRV_SC()/NETDEVTAB_ENTRY() macros. You would add a pointer to some instance specific data, e.g. containing base addresses, interrupt numbers, etc, where the

        0, // No driver specific data
is currently.

2017-02-09
Documentation license for this page: Open Publication License