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Writing Device Drivers
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Interrupt Handler Functionality

The driver framework and the device each place demands on the interrupt handler. All interrupt handlers are required to do the following tasks:

  • Determine whether the device is interrupting and possibly reject the interrupt.

    The interrupt handler first examines the device to determine whether this device issued the interrupt. If this device did not issue the interrupt, the handler must return DDI_INTR_UNCLAIMED. This step enables the implementation of device polling. Any device at the given interrupt priority level might have issued the interrupt. Device polling tells the system whether this device issued the interrupt.

  • Inform the device that the device is being serviced.

    Informing a device about servicing is a device-specific operation that is required for the majority of devices. For example, SBus devices are required to interrupt until the driver tells the SBus devices to stop. This approach guarantees that all SBus devices that interrupt at the same priority level are serviced.

  • Perform any I/O request-related processing.

    Devices interrupt for different reasons, such as transfer done or transfer error. This step can involve using data access functions to read the device's data buffer, examine the device's error register, and set the status field in a data structure accordingly. Interrupt dispatching and processing are relatively time consuming.

  • Do any additional processing that could prevent another interrupt.

    For example, read the next item of data from the device.


  • MSI interrupts must always be claimed.

    Claiming an interrupt is optional for MSI-X interrupts. In either case, the ownership of the interrupt need not be checked, because MSI and MSI-X interrupts are not shared with other devices.

  • Drivers that support hotplugging and multiple MSI or MSI-X interrupts should retain a separate interrupt for hotplug events and register a separate ISR (interrupt service routine) for that interrupt.

The following example shows an interrupt routine for a device called mydev.

Example 8-9 Interrupt Example
static uint_t
mydev_intr(caddr_t arg1, caddr_t arg2)
    struct mydevstate *xsp = (struct mydevstate *)arg1;
    uint8_t     status; 
    volatile    uint8_t  temp;

     * Claim or reject the interrupt.This example assumes
     * that the device's CSR includes this information.

    /* use data access routines to read status */
    status = ddi_get8(xsp->data_access_handle, &xsp->regp->csr);
    if (!(status & INTERRUPTING)) {
        return (DDI_INTR_UNCLAIMED); /* dev not interrupting */
     * Inform the device that it is being serviced, and re-enable
     * interrupts. The example assumes that writing to the
     * CSR accomplishes this. The driver must ensure that this data
     * access operation makes it to the device before the interrupt
     * service routine returns. For example, using the data access
     * functions to read the CSR, if it does not result in unwanted
     * effects, can ensure this.
    ddi_put8(xsp->data_access_handle, &xsp->regp->csr,

    /* flush store buffers */
    temp = ddi_get8(xsp->data_access_handle, &xsp->regp->csr);
    return (DDI_INTR_CLAIMED);

Most of the steps performed by the interrupt routine depend on the specifics of the device itself. Consult the hardware manual for the device to determine the cause of the interrupt, detect error conditions, and access the device data registers.

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  Published under the terms fo the Public Documentation License Version 1.01. Design by Interspire