Chapter 11. PCI Devices

This chapter will talk about the FreeBSD mechanisms for writing a device driver for a device on a PCI bus.

11.1. Probe and Attach

Information here about how the PCI bus code iterates through the unattached devices and see if a newly loaded kld will attach to any of them.

11.1.1. Sample Driver Source (mypci.c)

/*
 * Simple KLD to play with the PCI functions.
 *
 * Murray Stokely
 */

#include <sys/param.h>		/* defines used in kernel.h */
#include <sys/module.h>
#include <sys/systm.h>
#include <sys/errno.h>
#include <sys/kernel.h>		/* types used in module initialization */
#include <sys/conf.h>		/* cdevsw struct */
#include <sys/uio.h>		/* uio struct */
#include <sys/malloc.h>
#include <sys/bus.h>		/* structs, prototypes for pci bus stuff and DEVMETHOD macros! */

#include <machine/bus.h>
#include <sys/rman.h>
#include <machine/resource.h>

#include <dev/pci/pcivar.h>	/* For pci_get macros! */
#include <dev/pci/pcireg.h>

/* The softc holds our per-instance data. */
struct mypci_softc {
	device_t	my_dev;
	struct cdev	*my_cdev;
};

/* Function prototypes */
static d_open_t		mypci_open;
static d_close_t	mypci_close;
static d_read_t		mypci_read;
static d_write_t	mypci_write;

/* Character device entry points */

static struct cdevsw mypci_cdevsw = {
	.d_version =	D_VERSION,
	.d_open =	mypci_open,
	.d_close =	mypci_close,
	.d_read =	mypci_read,
	.d_write =	mypci_write,
	.d_name =	"mypci",
};

/*
 * In the cdevsw routines, we find our softc by using the si_drv1 member
 * of struct cdev.  We set this variable to point to our softc in our
 * attach routine when we create the /dev entry.
 */

int
mypci_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
{
	struct mypci_softc *sc;

	/* Look up our softc. */
	sc = dev->si_drv1;
	device_printf(sc->my_dev, "Opened successfully.\n");
	return (0);
}

int
mypci_close(struct cdev *dev, int fflag, int devtype, struct thread *td)
{
	struct mypci_softc *sc;

	/* Look up our softc. */
	sc = dev->si_drv1;
	device_printf(sc->my_dev, "Closed.\n");
	return (0);
}

int
mypci_read(struct cdev *dev, struct uio *uio, int ioflag)
{
	struct mypci_softc *sc;

	/* Look up our softc. */
	sc = dev->si_drv1;
	device_printf(sc->my_dev, "Asked to read %zd bytes.\n", uio->uio_resid);
	return (0);
}

int
mypci_write(struct cdev *dev, struct uio *uio, int ioflag)
{
	struct mypci_softc *sc;

	/* Look up our softc. */
	sc = dev->si_drv1;
	device_printf(sc->my_dev, "Asked to write %zd bytes.\n", uio->uio_resid);
	return (0);
}

/* PCI Support Functions */

/*
 * Compare the device ID of this device against the IDs that this driver
 * supports.  If there is a match, set the description and return success.
 */
static int
mypci_probe(device_t dev)
{

	device_printf(dev, "MyPCI Probe\nVendor ID : 0x%x\nDevice ID : 0x%x\n",
	    pci_get_vendor(dev), pci_get_device(dev));

	if (pci_get_vendor(dev) == 0x11c1) {
		printf("We've got the Winmodem, probe successful!\n");
		device_set_desc(dev, "WinModem");
		return (BUS_PROBE_DEFAULT);
	}
	return (ENXIO);
}

/* Attach function is only called if the probe is successful. */

static int
mypci_attach(device_t dev)
{
	struct mypci_softc *sc;

	printf("MyPCI Attach for : deviceID : 0x%x\n", pci_get_devid(dev));

	/* Look up our softc and initialize its fields. */
	sc = device_get_softc(dev);
	sc->my_dev = dev;

	/*
	 * Create a /dev entry for this device.  The kernel will assign us
	 * a major number automatically.  We use the unit number of this
	 * device as the minor number and name the character device
	 * "mypci<unit>".
	 */
	sc->my_cdev = make_dev(&mypci_cdevsw, device_get_unit(dev),
	    UID_ROOT, GID_WHEEL, 0600, "mypci%u", device_get_unit(dev));
	sc->my_cdev->si_drv1 = sc;
	printf("Mypci device loaded.\n");
	return (0);
}

/* Detach device. */

static int
mypci_detach(device_t dev)
{
	struct mypci_softc *sc;

	/* Teardown the state in our softc created in our attach routine. */
	sc = device_get_softc(dev);
	destroy_dev(sc->my_cdev);
	printf("Mypci detach!\n");
	return (0);
}

/* Called during system shutdown after sync. */

static int
mypci_shutdown(device_t dev)
{

	printf("Mypci shutdown!\n");
	return (0);
}

/*
 * Device suspend routine.
 */
static int
mypci_suspend(device_t dev)
{

	printf("Mypci suspend!\n");
	return (0);
}

/*
 * Device resume routine.
 */
static int
mypci_resume(device_t dev)
{

	printf("Mypci resume!\n");
	return (0);
}

static device_method_t mypci_methods[] = {
	/* Device interface */
	DEVMETHOD(device_probe,		mypci_probe),
	DEVMETHOD(device_attach,	mypci_attach),
	DEVMETHOD(device_detach,	mypci_detach),
	DEVMETHOD(device_shutdown,	mypci_shutdown),
	DEVMETHOD(device_suspend,	mypci_suspend),
	DEVMETHOD(device_resume,	mypci_resume),

	DEVMETHOD_END
};

static devclass_t mypci_devclass;

DEFINE_CLASS_0(mypci, mypci_driver, mypci_methods, sizeof(struct mypci_softc));
DRIVER_MODULE(mypci, pci, mypci_driver, mypci_devclass, 0, 0);

11.1.2. Makefile for Sample Driver

# Makefile for mypci driver

KMOD=	mypci
SRCS=	mypci.c
SRCS+=	device_if.h bus_if.h pci_if.h

.include <bsd.kmod.mk>

If you place the above source file and Makefile into a directory, you may run make to compile the sample driver. Additionally, you may run make load to load the driver into the currently running kernel and make unload to unload the driver after it is loaded.

11.1.3. Additional Resources

11.2. Bus Resources

FreeBSD provides an object-oriented mechanism for requesting resources from a parent bus. Almost all devices will be a child member of some sort of bus (PCI, ISA, USB, SCSI, etc) and these devices need to acquire resources from their parent bus (such as memory segments, interrupt lines, or DMA channels).

11.2.1. Base Address Registers

To do anything particularly useful with a PCI device you will need to obtain the Base Address Registers (BARs) from the PCI Configuration space. The PCI-specific details of obtaining the BAR are abstracted in the bus_alloc_resource() function.

For example, a typical driver might have something similar to this in the attach() function:

    sc->bar0id = PCIR_BAR(0);
    sc->bar0res = bus_alloc_resource(dev, SYS_RES_MEMORY, &sc->bar0id,
				  0, ~0, 1, RF_ACTIVE);
    if (sc->bar0res == NULL) {
        printf("Memory allocation of PCI base register 0 failed!\n");
        error = ENXIO;
        goto fail1;
    }

    sc->bar1id = PCIR_BAR(1);
    sc->bar1res = bus_alloc_resource(dev, SYS_RES_MEMORY, &sc->bar1id,
				  0, ~0, 1, RF_ACTIVE);
    if (sc->bar1res == NULL) {
        printf("Memory allocation of PCI base register 1 failed!\n");
        error =  ENXIO;
        goto fail2;
    }
    sc->bar0_bt = rman_get_bustag(sc->bar0res);
    sc->bar0_bh = rman_get_bushandle(sc->bar0res);
    sc->bar1_bt = rman_get_bustag(sc->bar1res);
    sc->bar1_bh = rman_get_bushandle(sc->bar1res);

Handles for each base address register are kept in the softc structure so that they can be used to write to the device later.

These handles can then be used to read or write from the device registers with the bus_space_* functions. For example, a driver might contain a shorthand function to read from a board specific register like this:

uint16_t
board_read(struct ni_softc *sc, uint16_t address)
{
    return bus_space_read_2(sc->bar1_bt, sc->bar1_bh, address);
}

Similarly, one could write to the registers with:

void
board_write(struct ni_softc *sc, uint16_t address, uint16_t value)
{
    bus_space_write_2(sc->bar1_bt, sc->bar1_bh, address, value);
}

These functions exist in 8bit, 16bit, and 32bit versions and you should use bus_space_{read|write}_{1|2|4} accordingly.

In FreeBSD 7.0 and later, you can use the bus_* functions instead of bus_space_*. The bus_* functions take a struct resource * pointer instead of a bus tag and handle. Thus, you could drop the bus tag and bus handle members from the softc and rewrite the board_read() function as:

uint16_t
board_read(struct ni_softc *sc, uint16_t address)
{
	return (bus_read(sc->bar1res, address));
}

11.2.2. Interrupts

Interrupts are allocated from the object-oriented bus code in a way similar to the memory resources. First an IRQ resource must be allocated from the parent bus, and then the interrupt handler must be set up to deal with this IRQ.

Again, a sample from a device attach() function says more than words.

/* Get the IRQ resource */

    sc->irqid = 0x0;
    sc->irqres = bus_alloc_resource(dev, SYS_RES_IRQ, &(sc->irqid),
				  0, ~0, 1, RF_SHAREABLE | RF_ACTIVE);
    if (sc->irqres == NULL) {
	printf("IRQ allocation failed!\n");
	error = ENXIO;
	goto fail3;
    }

    /* Now we should set up the interrupt handler */

    error = bus_setup_intr(dev, sc->irqres, INTR_TYPE_MISC,
			   my_handler, sc, &(sc->handler));
    if (error) {
	printf("Couldn't set up irq\n");
	goto fail4;
    }

Some care must be taken in the detach routine of the driver. You must quiesce the device’s interrupt stream, and remove the interrupt handler. Once bus_teardown_intr() has returned, you know that your interrupt handler will no longer be called and that all threads that might have been executing this interrupt handler have returned. Since this function can sleep, you must not hold any mutexes when calling this function.

11.2.3. DMA

This section is obsolete, and present only for historical reasons. The proper methods for dealing with these issues is to use the bus_space_dma*() functions instead. This paragraph can be removed when this section is updated to reflect that usage. However, at the moment, the API is in a bit of flux, so once that settles down, it would be good to update this section to reflect that.

On the PC, peripherals that want to do bus-mastering DMA must deal with physical addresses. This is a problem since FreeBSD uses virtual memory and deals almost exclusively with virtual addresses. Fortunately, there is a function, vtophys() to help.

#include <vm/vm.h>
#include <vm/pmap.h>

#define vtophys(virtual_address) (...)

The solution is a bit different on the alpha however, and what we really want is a function called vtobus().

#if defined(__alpha__)
#define vtobus(va)      alpha_XXX_dmamap((vm_offset_t)va)
#else
#define vtobus(va)      vtophys(va)
#endif

11.2.4. Deallocating Resources

It is very important to deallocate all of the resources that were allocated during attach(). Care must be taken to deallocate the correct stuff even on a failure condition so that the system will remain usable while your driver dies.


All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.