Entry Point map()

int pfxmap(dev_t dev, vhandl_t *vt,
          off_t off, int len, int prot);

dev	A dev_t value from which you can extract both the major and minor device numbers.
vt	The address of an opaque structure that describes the assigned address in the user process address space. The structure contents are subject to change.
off, len	The offset and length arguments passed to mmap() by the user process.
prot	Flags showing the access intentions of the user process.

The first task of the driver is to verify that the access specified in prot is allowed. The next task is to validate the off and len values: do they fall in the valid address space of the device?

When the device driver approves of a mapping, it uses a kernel function, v_mapphys(), to establish the mapping. This function (documented in the v_mapphys(D3) reference page) takes the vhandle_t, an address in kernel cached or uncached memory, and a length. It makes the specified region of kernel space a part of the address space of the user process.

For example, a pseudo-device driver that intends to share kernel virtual memory with user processes would first allocate the memory:

caddr_t *kaddr = kmem_alloc (len , KM_CACHEALIGN);

v_mapphys (vt, kaddr, len)

Note: There are no special precautions to take when mapping cached memory into user space, or when mapping device registers or bus addresses. However, you should almost never map uncached memory into user space. The effects of uncached memory access are hardware dependent and differ between multiprocessors and uniprocessors. Among uniprocessors, the IP26 CPU module has highly restrictive rules for the use of uncached memory (see "Uncached Memory Access in the IP26 CPU"). In general, mapping uncached memory makes a driver nonportable and is likely to lead to subtle failures that are hard to resolve. Example 8-3 contains an edited fragment of code from a Silicon Graphics device driver. This pseudo-device driver, whose prefix is flash_, provides access to "flash" PROM in certain computer models. It allows a user process to map the PROM into user space.

Example 8-3 : Edited Fragment of flash_map()

int flash_map(dev_t dev, vhandl_t *vt, off_t off, long len)
{
   long offset = (long) off; /*Actual offset in flash prom*/
/* Don't allow requests which exceed the flash prom size */
   if ((offset + len) > FLASHPROM_SIZE)
      return ENOSPC;
/* Don't allow non page-aligned offsets */
   if ((offset % NBPC) != 0)
      return EIO;
/* Only allow mapping of entire pages */
   if ((len % NBPC) != 0)
      return EIO;
   return v_mapphys(vt, FLASHMAP_ADDR + offset, len);
}