Writing Device Drivers

Exporting Kernel Memory to Applications

Use devmap_umem_setup(9F) to export kernel memory to user applications. devmap_umem_setup() must be called from the driver's devmap(9E) entry point. The syntax for devmap_umem_setup() is as follows:

int devmap_umem_setup(devmap_cookie_t handle, dev_info_t *dip,
    struct devmap_callback_ctl *callbackops, ddi_umem_cookie_t cookie,
    offset_t koff, size_t len, uint_t maxprot, uint_t flags,
    ddi_device_acc_attr_t *accattrp);



Opaque structure used to describe the mapping.


Pointer to the device's dev_info structure.


Pointer to a devmap_callback_ctl(9S) structure.


Kernel memory cookie returned by ddi_umem_alloc(9F).


Offset into the kernel memory specified by cookie.


Length in bytes that is exported.


Specifies the maximum protection possible for the exported mapping.


Must be set to DEVMAP_DEFAULTS.


Pointer to a ddi_device_acc_attr(9S) structure.

handle is a device-mapping handle that the system uses to identify the mapping. handle is passed in by the devmap(9E) entry point. dip is a pointer to the device's dev_info structure. callbackops enables the driver to be notified of user events on the mapping. Most drivers set callbackops to NULL when kernel memory is exported.

koff and len specify a range within the kernel memory allocated by ddi_umem_alloc(9F). This range is made accessible to the user's application mapping at the offset that is passed in by the devmap(9E) entry point. Usually, the driver passes the devmap(9E) offset directly to devmap_umem_setup(9F). The return address of mmap(2) then maps to the kernel address returned by ddi_umem_alloc(9F). koff and len must be page-aligned.

maxprot enables the driver to specify different protections for different regions within the exported kernel memory. For example, one region might not allow write access by only setting PROT_READ and PROT_USER.

The following example shows how to export kernel memory to an application. The driver first checks whether the requested mapping falls within the allocated kernel memory region. If a 64-bit driver receives a mapping request from a 32-bit application, the request is redirected to the second page of the kernel memory area. This redirection ensures that only applications compiled to the same data model share the same page.

Example 10–5 devmap_umem_setup(9F) Routine

static int
xxdevmap(dev_t dev, devmap_cookie_t handle, offset_t off, size_t len,
    size_t *maplen, uint_t model)
    struct xxstate *xsp;
    int    error;

    /* round up len to a multiple of a page size */
    len = ptob(btopr(len));
    /* check if the requested range is ok */
    if (off + len > ptob(1))
        return (ENXIO);
    xsp = ddi_get_soft_state(statep, getminor(dev));
    if (xsp == NULL)
        return (ENXIO);

    if (ddi_model_convert_from(model) == DDI_MODEL_ILP32)
        /* request from 32-bit application. Skip first page */
        off += ptob(1);

    /* export the memory to the application */
    error = devmap_umem_setup(handle, xsp->dip, NULL, xsp->ucookie,
        off, len, PROT_ALL, DEVMAP_DEFAULTS, NULL);
    *maplen = len;
    return (error);