Writing Device Drivers

Exporting Kernel Memory to Applications

devmap_umem_setup(9F) is provided to export kernel memory to user applications. devmap_umem_setup(9F) must be called from the driver's devmap(9E) entry point:

	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, u_int maxprot, u_int flags,
 			ddi_device_acc_attr_t *accattrp);

handle is an opaque structure that the system uses to describe the mapping.

dip is a pointer to the device's dev_info structure.

callbackops is a pointer to a devmap_callback_ctl(9S) structure.

cookie is a kernel memory cookie returned by ddi_umem_alloc(9F).

koff is the offset into the kernel memory specified by cookie.

len is the length in bytes that is exported.

maxprot specifies the maximum protection possible for the exported mapping.

flags must be set to DEVMAP_DEFAULTS.

accattrp is a pointer to a ddi_device_acc_attr(9S) structure.

handle is a device-mapping handle that the system uses to identify the mapping. It is passed in by the devmap(9E) entry point. dip is a pointer to the device's dev_info structure. dip is stored by the driver in its private data structure during attach(9E). callbackops allows the driver to be notified of user events on the mapping. Most drivers will 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 will be made accessible to the user's application mapping at the offset passed in by the devmap(9E) entry point. Usually the driver will pass the devmap(9E) offset directly to devmap_umem_setup(9F). The return address of mmap(2) will then map 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.

Example 11-3 shows how to export kernel memory to an application. The driver first checks if 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 ensures that only applications compiled to the same data model will share the same page.

Example 11-3 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);
#endif  /* _MULTI_DATAMODEL */
	/* export the memory to the application */
	error = devmap_umem_setup(handle, xsp->dip, NULL, xsp-
	*maplen = len;
	return (error);