Writing Device Drivers

devmap_map() Entry Point

The syntax for devmap(9E) is as follows:

int xxdevmap_map(devmap_cookie_t handle, dev_t dev, uint_t flags,
    offset_t offset, size_t len, void **new-devprivate);

The devmap_map() entry point is called after the driver returns from its devmap() entry point and the system has established the user mapping to the device memory. The devmap() entry point enables a driver to perform additional processing or to allocate mapping specific private data. For example, in order to support context switching, the driver has to allocate a context structure. The driver must then associate the context structure with the mapping.

The system expects the driver to return a pointer to the allocated private data in *new-devprivate. The driver must store offset and len, which define the range of the mapping, in its private data. Later, when the system calls devmap_unmap(9E), the driver uses this information to determine how much of the mapping is being unmapped.

flags indicates whether the driver should allocate a private context for the mapping. For example, a driver can allocate a memory region to store the device context if flags is set to MAP_PRIVATE. If MAP_SHARED is set, the driver returns a pointer to a shared region.

The following example shows a devmap() entry point. The driver allocates a new context structure. The driver then saves relevant parameters passed in by the entry point. Next, the mapping is assigned a new context either through allocation or by attaching the mapping to an already existing shared context. The minimum time interval that the mapping should have access to the device is set to one millisecond.

Example 11–1 Using the devmap() Routine

static int
int xxdevmap_map(devmap_cookie_t handle, dev_t dev, uint_t flags,
    offset_t offset, size_t len, void **new_devprivate)
    struct xxstate *xsp = ddi_get_soft_state(statep,
    struct xxctx *newctx;

    /* create a new context structure */
    newctx = kmem_alloc(sizeof (struct xxctx), KM_SLEEP);
    newctx->xsp = xsp;
    newctx->handle = handle;
    newctx->offset = offset;
    newctx->flags = flags;
    newctx->len = len;
    if (flags & MAP_PRIVATE) {
        /* allocate a private context and initialize it */
        newctx->context = kmem_alloc(XXCTX_SIZE, KM_SLEEP);
    } else {
        /* set a pointer to the shared context */
        newctx->context = xsp->ctx_shared;
    /* give at least 1 ms access before context switching */
    devmap_set_ctx_timeout(handle, drv_usectohz(1000));
    /* return the context structure */
    *new_devprivate = newctx;