mmap_device_memory()

Map a device's physical memory into a process's address space

Synopsis:

#include <sys/mman.h>

void * mmap_device_memory( void * addr,
                           size_t len,
                           int prot,
                           int flags,
                           uint64_t physical );

Arguments:

addr

NULL, or a pointer to where you want to map the object in the calling process's address space.

len

The number of bytes you want to map into the caller's address space. It can't be 0.

prot

The access capabilities that you want to use for the memory region being mapped. You can use a combination of at least the following protection bits, as defined in <sys/mman.h>:

• PROT_EXEC -- the region can be executed.
• PROT_NOCACHE -- disable the caching of the region (e.g. to access dual-ported memory).

  ---

  Read the architecture guide for your processor; you may need to add special instructions. For example, if you specify PROT_NOCACHE on a PPC device, you may need to issue special eieio (Enforce In-Order Execution of I/O) instructions to ensure that writes occur in a desired order.

  ---

• PROT_NONE -- the region can't be accessed.
• PROT_READ -- the region can be read.
• PROT_WRITE -- the region can be written.

flags

Specifies further information about handling the mapped region. You can use the following flag:

• MAP_FIXED -- map the object to the address specified by addr. If this area is already mapped, the call changes the existing mapping of the area.

  ---

  Use MAP_FIXED with caution. Not all memory models support it. In general, you should assume that you can MAP_FIXED only at an address (and size) that a call to mmap() without MAP_FIXED returned. These restrictions will be removed from the user-to-user protection model with the full VM (available with a later version of the QNX Neutrino OS).

  ---

  A memory area being mapped with MAP_FIXED is first unmapped by the system using the same memory area. See munmap() for details.

This function already uses MAP_SHARED ORed with MAP_PHYS (see mmap() for a description of these flags).

physical

The physical address of the memory to map into the caller's address space.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The mmap_device_memory() function maps len bytes of a device's physical memory address into the caller's address space at the location returned by mmap_device_memory().

You should use this function instead of using mmap() with the MAP_PHYS flag.

Typically, you don't need to use addr; you can just pass NULL instead. If you set addr to a non-NULL value, whether the object is mapped depends on whether or not you set MAP_FIXED in flags:

MAP_FIXED is set

The object is mapped to the address in addr, or the function fails.

MAP_FIXED isn't set

The value of addr is taken as a hint as to where to map the object in the calling process's address space. The mapped area won't overlay any current mapped areas.

Returns:

The address of the mapped-in object, or MAP_FAILED if an error occurs (errno is set).

Errors:

EINVAL

Invalid flags type, or len is 0.

ENOMEM

The address range requested is outside of the allowed process address range, or there wasn't enough memory to satisfy the request.

ENXIO

The address from physical for len bytes is invalid for the requested object, or MAP_FIXED was specified and addr, len, and physical are invalid for the requested object.

Examples:

/* map in the physical memory, 0xb8000 is text mode VGA video memory */
    
ptr = mmap_device_memory( 0, len, PROT_READ|PROT_WRITE|PROT_NOCACHE, 0, 0xb8000 );
if ( ptr == MAP_FAILED ) {
    perror( "mmap_device_memory for physical address 0xb8000 failed" );
    exit( EXIT_FAILURE );
}

Classification:

QNX Neutrino

Caveats:

You need I/O privileges to use the result of the mmap_device_memory() function. The calling thread may call ThreadCtl() with the _NTO_TCTL_IO command to establish these privileges.

See also:

mmap(), mmap_device_io(), munmap_device_memory()