I386_GET_MTRR(2) BSD/i386 System Calls Manual I386_GET_MTRR(2)NAME
i386_get_mtrr, i386_set_mtrr — access Memory Type Range Registers
LIBRARY
i386 Architecture Library (libi386, -li386)
SYNOPSIS
#include <sys/types.h>
#include <machine/sysarch.h>
#include <machine/mtrr.h>
int
i386_get_mtrr(struct mtrr *mtrrp, int *n);
int
i386_set_mtrr(struct mtrr *mtrrp, int *n);
DESCRIPTION
These functions provide an interface to the MTRR registers found on
686-class processors for controlling processor access to memory ranges.
This is most useful for accessing devices such as video accelerators on
pci(4) and agp(4) buses. For example, enabling write-combining allows
bus-write transfers to be combined into a larger transfer before bursting
over the bus. This can increase performance of write operations 2.5
times or more.
mtrrp is a pointer to one or more mtrr structures, as described below.
The n argument is a pointer to an integer containing the number of struc‐
tures pointed to by mtrrp. For i386_set_mtrr() the integer pointed to by
n will be updated to reflect the actual number of MTRRs successfully set.
For i386_get_mtrr() no more than n structures will be copied out, and the
integer value pointed to by n will be updated to reflect the actual num‐
ber of valid structures retrieved. A NULL argument to mtrrp will result
in just the number of MTRRs available being returned in the integer
pointed to by n.
The argument mtrrp has the following structure:
struct mtrr {
uint64_t base;
uint64_t len;
uint8_t type;
int flags;
pid_t owner;
};
The location of the mapping is described by its physical base address
base and length len. Valid values for type are:
MTRR_TYPE_UC uncached memory
MTRR_TYPE_WC use write-combining
MTRR_TYPE_WT use write-through caching
MTRR_TYPE_WP write-protected memory
MTRR_TYPE_WB use write-back caching
Valid values for flags are:
MTRR_PRIVATE own range, reset the MTRR when the current process
exits
MTRR_FIXED use fixed range MTRR
MTRR_VALID entry is valid
The owner member is the PID of the user process which claims the mapping.
It is only valid if MTRR_PRIVATE is set in flags. To clear/reset MTRRs,
use a flags field without MTRR_VALID set.
RETURN VALUES
Upon successful completion zero is returned, otherwise -1 is returned on
failure, and the global variable errno is set to indicate the error. The
integer value pointed to by n will contain the number of successfully
processed mtrr structures in both cases.
ERRORS
[ENOSYS] The currently running kernel or CPU has no MTRR support.
[EINVAL] The currently running kernel has no MTRR support, or one of the
mtrr structures pointed to by mtrrp is invalid.
[EBUSY] No unused MTRRs are available.
HISTORY
The i386_get_mtrr() and i386_set_mtrr() functions appeared in NetBSD 1.6.
BSD November 10, 2001 BSD
