Review of the functions

Now a brief review of the functions. This discussion will use generic names for the functions — your driver should use hardware-specific names to maintain uniqueness against any other drivers.

Init function

static bool DRV_HDWR_init(struct cyg_netdevtab_entry *tab)
This function is called as part of system initialization. Its primary function is to decide if the hardware (as indicated via tab->device_instance) is working and if the interface needs to be made available in the system. If this is the case, this function needs to finish with a call to the ethernet driver function:
    struct eth_drv_sc *sc = (struct eth_drv_sc *)tab->device_instance;
    ....initialization code....
    // Initialize upper level driver
    (sc->funs->eth_drv->init)( sc, unsigned char *enaddr );
where enaddr is a pointer to the ethernet station address for this unit, to inform the stack of this device's readiness and availability.

Note: The ethernet station address (ESA) is supposed to be a world-unique, 48 bit address for this particular ethernet interface. Typically it is provided by the board/hardware manufacturer in ROM.

In many packages it is possible for the ESA to be set from RedBoot, (perhaps from 'fconfig' data), hard-coded from CDL, or from an EPROM. A driver should choose a run-time specified ESA (e.g. from RedBoot) preferentially, otherwise (in order) it should use a CDL specified ESA if one has been set, otherwise an EPROM set ESA, or otherwise fail. See the cl/cs8900a ethernet driver for an example.

Start function

static void
HRDWR_start(struct eth_drv_sc *sc, unsigned char *enaddr, int flags)
This function is called, perhaps much later than system initialization time, when the system (an application) is ready for the interface to become active. The purpose of this function is to set up the hardware interface to start accepting packets from the network and be able to send packets out. The receiver hardware should not be enabled prior to this call.

Note: This function will be called whenever the up/down state of the logical interface changes, e.g. when the IP address changes, or when promiscuous mode is selected by means of an ioctl() call in the application. This may occur more than once, so this function needs to be prepared for that case.

Note: In future, the flags field (currently unused) may be used to tell the function how to start up, e.g. whether interrupts will be used, alternate means of selecting promiscuous mode etc.

Stop function

static void HRDWR_stop(struct eth_drv_sc *sc)
This function is the inverse of “start.” It should shut down the hardware, disable the receiver, and keep it from interacting with the physical network.

Control function

static int
        struct eth_drv_sc *sc, unsigned long key,
        void *data, int len)
This function is used to perform low-level “control” operations on the interface. These operations would typically be initiated via ioctl() calls in the BSD stack, and would be anything that might require the hardware setup to change (i.e. cannot be performed totally by the platform-independent layers).

The key parameter selects the operation, and the data and len params point describe, as required, some data for the operation in question.

Available Operations:


This operation sets the ethernet station address (ESA or MAC) for the device. Normally this address is kept in non-volatile memory and is unique in the world. This function must at least set the interface to use the new address. It may also update the NVM as appropriate.


These acquire a set of statistical counters from the interface, and write the information into the memory pointed to by data. The “UD” variant explicitly instructs the driver to acquire up-to-date values. This is a separate option because doing so may take some time, depending on the hardware.

The definition of the data structure is in cyg/io/eth/eth_drv_stats.h.

This call is typically made by SNMP.


This entry instructs the device to set up multicast packet filtering to receive only packets addressed to the multicast ESAs in the list pointed to by data.

The format of the data is a 32-bit count of the ESAs in the list, followed by packed bytes which are the ESAs themselves, thus:

#define ETH_DRV_MAX_MC 8
struct eth_drv_mc_list {
    int len;
    unsigned char addrs[ETH_DRV_MAX_MC][ETHER_ADDR_LEN];


This entry instructs the device to receive all multicast packets, and delete any explicit filtering which had been set up.

This function should return zero if the specified operation was completed successfully. It should return non-zero if the operation could not be performed, for any reason.

Can-send function

static int HRDWR_can_send(struct eth_drv_sc *sc)
This function is called to determine if it is possible to start the transmission of a packet on the interface. Some interfaces will allow multiple packets to be "queued" and this function allows for the highest possible utilization of that mode.

Return the number of packets which could be accepted at this time, zero implies that the interface is saturated/busy.

Send function

struct eth_drv_sg {
    CYG_ADDRESS  buf;

static void
        struct eth_drv_sc *sc,
        struct eth_drv_sg *sg_list, int sg_len,
        int total_len, unsigned long key)
This function is used to send a packet of data to the network. It is the responsibility of this function to somehow hand the data over to the hardware interface. This will most likely require copying, but just the address/length values could be used by smart hardware.

Note: All data in/out of the driver is specified via a “scatter-gather” list. This is just an array of address/length pairs which describe sections of data to move (in the order given by the array), as in the struct eth_drv_sg defined above and pointed to by sg_list.

Once the data has been successfully sent by the interface (or if an error occurs), the driver should call (sc->funs->eth_drv->tx_done)() (see the Section called Callback Tx-Done function) using the specified key. Only then will the upper layers release the resources for that packet and start another transmission.

Note: In future, this function may be extended so that the data need not be copied by having the function return a “disposition” code (done, send pending, etc). At this point, you should move the data to some “safe” location before returning.

Deliver function

static void
HRDWR_deliver(struct eth_drv_sc *sc)
This function is called from the “Network Delivery Thread” in order to let the device driver do the time-consuming work associated with receiving a packet — usually copying the entire packet from the hardware or a special memory location into the network stack's memory.

After handling any outstanding incoming packets or pending transmission status, it can unmask the device's interrupts, and free any relevant resources so it can process further packets.

It will be called when the interrupt handler for the network device has called

    eth_drv_dsr( vector, count, (cyg_addrword_t)sc );
to alert the system that “something requires attention.” This eth_drv_dsr() call must occur from within the interrupt handler's DSR (not the ISR) or actually be the DSR, whenever it is determined that the device needs attention from the foreground. The third parameter (data in the prototype of eth_drv_dsr() must be a valid struct eth_drv_sc pointer sc.

The reason for this slightly convoluted train of events is to keep the DSR (and ISR) execution time as short as possible, so that other activities of higher priority than network servicing are not denied the CPU by network traffic.

To deliver a newly-received packet into the network stack, the deliver routine must call

(sc->funs->eth_drv->recv)(sc, len);
which will in turn call the receive function, which we talk about next. See also the Section called Callback Receive function below.

Receive function

static void
        struct eth_drv_sc *sc,
        struct eth_drv_sg *sg_list, int sg_len)
This function is a call back, only invoked after the upper-level function
(sc->funs->eth_drv->recv)(struct eth_drv_sc *sc, int total_len)
has been called itself from your deliver function when it knows that a packet of data is available on the interface. The (sc->funs->eth_drv->recv)() function then arranges network buffers and structures for the data and then calls HRDWR_recv() to actually move the data from the interface.

A scatter-gather list (struct eth_drv_sg) is used once more, just like in the send case.

Poll function

static void
HRDWR_poll(struct eth_drv_sc *sc)
This function is used when in a non-interrupt driven system, e.g. when interrupts are completely disabled. This allows the driver time to check whether anything needs doing either for transmission, or to check if anything has been received, or if any other processing needs doing.

It is perfectly correct and acceptable for the poll function to look like this:

static void
HRDWR_poll(struct eth_drv_sc *sc)
   HRDWR_deliver(struct eth_drv_sc *sc);
provided that both the ISR and the deliver functions are idempotent and harmless if called when there is no attention needed by the hardware. Some devices might not need a call to the ISR here if the deliver function contains all the “intelligence.”

Interrupt-vector function

static int
HRDWR_int_vector(struct eth_drv_sc *sc)
This function returns the interrupt vector number used for receive interrupts. This is so that the common GDB stubs can detect when to check for incoming “CTRL-C” packets (used to asynchronously halt the application) when debugging over ethernet. The GDB stubs need to know which interrupt the ethernet device uses so that they can mask or unmask that interrupt as required.

Documentation license for this page: Open Publication License