Specifications

This is the Title of the Book, eMatter Edition

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Chapter 15: Memory Mapping and DMA

Note that the user process can always use mremap to extend its mapping, possibly

past the end of the physical device area. If your driver fails to define a nopage

method, it is never notified of this extension, and the additional area maps to the

zero page. As a driver writer, you may well want to prevent this sort of behavior;

mapping the zero page onto the end of your region is not an explicitly bad thing to

do, but it is highly unlikely that the programmer wanted that to happen.

The simplest way to prevent extension of the mapping is to implement a simple

nopage method that always causes a bus signal to be sent to the faulting process.

Such a method would look like this:

struct page *simple_nopage(struct vm_area_struct *vma,

unsigned long address, int *type);

{ return NOPAGE_SIGBUS; /* send a SIGBUS */}

As we have seen, the nopage method is called only when the process dereferences an

address that is within a known VMA but for which there is currently no valid page

table entry. If we have used remap_pfn_range to map the entire device region, the

nopage method shown here is called only for references outside of that region. Thus,

it can safely return

NOPAGE_SIGBUS to signal an error. Of course, a more thorough

implementation of nopage could check to see whether the faulting address is within

the device area, and perform the remapping if that is the case. Once again, however,

nopage does not work with PCI memory areas, so extension of PCI mappings is not

possible.

Remapping RAM

An interesting limitation of remap_pfn_range is that it gives access only to reserved

pages and physical addresses above the top of physical memory. In Linux, a page of

physical addresses is marked as “reserved” in the memory map to indicate that it is

not available for memory management. On the PC, for example, the range between

640 KB and 1 MB is marked as reserved, as are the pages that host the kernel code

itself. Reserved pages are locked in memory and are the only ones that can be safely

mapped to user space; this limitation is a basic requirement for system stability.

Therefore, remap_pfn_range won’t allow you to remap conventional addresses,

which include the ones you obtain by calling get_free_page. Instead, it maps in the

zero page. Everything appears to work, with the exception that the process sees pri-

vate, zero-filled pages rather than the remapped RAM that it was hoping for. None-

theless, the function does everything that most hardware drivers need it to do,

because it can remap high PCI buffers and ISA memory.

The limitations of remap_pfn_range can be seen by running mapper, one of the sam-

ple programs in misc-progs in the files provided on O’Reilly’s FTP site. mapper is a

simple tool that can be used to quickly test the mmap system call; it maps read-only

parts of a file specified by command-line options and dumps the mapped region to

standard output. The following session, for instance, shows that /dev/mem doesn’t

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