NAME
i386_get_mtrr,
i386_set_mtrr —
access Memory Type Range
Registers
LIBRARY
library “libi386”
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 structures
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 number 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.