Embedded FreeBSD Cookbook phần 4 doc

61 Chapter Four

Device Driver

The sample DIO device driver source contains a shell implementation of both

ISA and PCI device drivers. Because the PCI-DIO24 is a PCI card, the first

modification I made to the DIO source code was to remove all references to

the ISA callback functions. Once this was complete, I performed a complete

build to make sure the remaining code was still correct.

DIO Driver Functions

The dio_pci_probe Function

The first task of the device driver is to detect if its hardware is present. This

is called probing the device. Device probing is performed by the probe device

method. Dio.c contains a function for probing a PCI device implemented

in the function dio_pci_probe.

dio_pci_probe calls the kernel function pci_get_devid to return the

Vendor ID and Device ID for this device. pci_get_dev returns a 32-bit long￾word of which the upper 16 bits contain the PCI Device ID and the lower 16

bits contains the PCI vendor ID for this card. The following code shows the

modifications to the pci_ids structure for the PCI-DIO24.

static struct _pcsid

{

u_int32_t type;

const char *desc;

} pci_ids[] = {

{ 0x00281307, “Measurement Computing PCI-DIO24 Digital

I/O Card” },

{ 0x00000000, NULL }

};

The DIO device driver can now be built and loaded, and it successfully

recognizes the PCI-DIO24 card. The complete dio_pci_probe function is

listed below.

static int

dio_pci_probe (device_t device)

{

u_int32_t type = pci_get_devid(device);

struct _pcsid *ep =pci_ids;

while (ep->type && ep->type != type)

62 Embedded FreeBSD

Cookbook

++ep;

if (ep->desc) {

device_set_desc(device, ep->desc);

return 0; /* If there might be a better driver, return -2 */

} else {

return ENXIO;

}

}

dio_pci_probe scans the pci_ids structure in an attempt to match the

vendor and device IDs. When a match is found, the verbal description of the

device is updated using the kernel function device_set_desc.

TIP

FreeBSD has a utility that dumps information about PCI devices, called pciconf.

Utilities such as pciconf are used for debugging and verifying your hardware is work￾ing correctly before developing a device driver.

The pciconf utility is used to detect that the hardware is successfully loaded and

recognized by FreeBSD. pciconf –l will give you a list of PCI devices present in your

system. Once you have installed a hardware peripheral, it should always be listed in the

output of the pciconf command.

The dio_pci_attach Function

Once a device driver successfully probes the hardware, the kernel calls the

driver attach method. The attach method is responsible for allocating hard￾ware resources and for the PCI-DIO24 making the character device. The

PCI_DIO24 board has three hardware resources, an interrupt and two IO

ports. dio.c contains the attach method dio_pci_attach.

static int

dio_pci_attach(device_t device)

{

interror;

struct dio_softc *scp = DEVICE2SOFTC(device);

error = dio_attach(device, scp);

if (error) {

dio_pci_detach(device);

}