Specifications

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The mmap Device Operation

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Otherwise, pfn_to_page gets the necessary struct page pointer; we can increment its

reference count (with a call to get_page) and return it.

The nopage method normally returns a pointer to a

struct page. If, for some reason,

a normal page cannot be returned (e.g., the requested address is beyond the device’s

memory region),

NOPAGE_SIGBUS can be returned to signal the error; that is what the

simple code above does. nopage can also return NOPAGE_OOM to indicate failures caused

by resource limitations.

Note that this implementation works for ISA memory regions but not for those on

the PCI bus. PCI memory is mapped above the highest system memory, and there are

no entries in the system memory map for those addresses. Because there is no

struct

page

to return a pointer to, nopage cannot be used in these situations; you must use

remap_pfn_range instead.

If the nopage method is left

NULL, kernel code that handles page faults maps the zero

page to the faulting virtual address. The zero page is a copy-on-write page that reads

0 and that is used, for example, to map the BSS segment. Any process referencing

the zero page sees exactly that: a page filled with zeroes. If the process writes to the

page, it ends up modifying a private copy. Therefore, if a process extends a mapped

region by calling mremap, and the driver hasn’t implemented nopage, the process

ends up with zero-filled memory instead of a segmentation fault.

Remapping Speciﬁc I/O Regions

All the examples we’ve seen so far are reimplementations of /dev/mem; they remap

physical addresses into user space. The typical driver, however, wants to map only

the small address range that applies to its peripheral device, not all memory. In order

to map to user space only a subset of the whole memory range, the driver needs only

to play with the offsets. The following does the trick for a driver mapping a region of

simple_region_size bytes, beginning at physical address simple_region_start (which

should be page-aligned):

unsigned long off = vma->vm_pgoff << PAGE_SHIFT;

unsigned long physical = simple_region_start + off;

unsigned long vsize = vma->vm_end - vma->vm_start;

unsigned long psize = simple_region_size - off;

if (vsize > psize)

return -EINVAL; /* spans too high */

remap_pfn_range(vma, vma_>vm_start, physical, vsize, vma->vm_page_prot);

In addition to calculating the offsets, this code introduces a check that reports an

error when the program tries to map more memory than is available in the I/O region

of the target device. In this code,

psize is the physical I/O size that is left after the off-

set has been specified, and

vsize is the requested size of virtual memory; the func-

tion refuses to map addresses that extend beyond the allowed memory range.

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