FreeBSD Architecture Handbook
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# 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.

> Note: 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.

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