\"     $NetBSD: i386_get_mtrr.2,v 1.13 2008/04/30 13:10:50 martin Exp $
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\" by Gregory McGarry.
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Dd November 10, 2001
Dt I386_GET_MTRR 2 i386
Os
Sh NAME
Nm i386_get_mtrr ,
Nm i386_set_mtrr
Nd access Memory Type Range Registers
Sh LIBRARY
Lb libi386
Sh SYNOPSIS
In sys/types.h
In machine/sysarch.h
In machine/mtrr.h
Ft int
Fn i386_get_mtrr "struct mtrr *mtrrp" "int *n"
Ft int
Fn i386_set_mtrr "struct mtrr *mtrrp" "int *n"
Sh 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
Xr pci 4
and
Xr 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.
Pp
Fa mtrrp
is a pointer to one or more mtrr structures, as described below.
The
Fa n
argument is a pointer to an integer containing the number of structures
pointed to by
Fa mtrrp .
For
Fn i386_set_mtrr
the integer pointed to by
Fa n
will be updated to reflect the actual number of MTRRs successfully set.
For
Fn i386_get_mtrr
no more than
Fa n
structures will be copied out, and the integer value pointed to by
Fa n
will be updated to reflect the actual number of valid structures retrieved.
A
Dv NULL
argument to
Fa mtrrp
will result in just the number of MTRRs available being returned
in the integer pointed to by
Fa n .
Pp
The argument
Fa mtrrp
has the following structure:
Bd -literal
struct mtrr {
       uint64_t base;
       uint64_t len;
       uint8_t type;
       int flags;
       pid_t owner;
};
Ed
Pp
The location of the mapping is described by its physical base address
Em base
and length
Em len .
Valid values for
Em type
are:
Pp
Bl -tag -offset indent -width MTRR_TYPE_UNDEF1 -compact
It MTRR_TYPE_UC
uncached memory
It MTRR_TYPE_WC
use write-combining
It MTRR_TYPE_WT
use write-through caching
It MTRR_TYPE_WP
write-protected memory
It MTRR_TYPE_WB
use write-back caching
El
Pp
Valid values for
Em flags
are:
Pp
Bl -tag -offset indent -width MTRR_PRIVATE -compact
It MTRR_PRIVATE
own range, reset the MTRR when the current process exits
It MTRR_FIXED
use fixed range MTRR
It MTRR_VALID
entry is valid
El
Pp
The
Em owner
member is the PID of the user process which claims the mapping.
It is only valid if MTRR_PRIVATE is set in
Em flags .
To clear/reset MTRRs, use a
Em flags
field without MTRR_VALID set.
Sh RETURN VALUES
Upon successful completion zero is returned, otherwise \-1 is returned
on failure, and the global variable
Va errno
is set to indicate the error.
The integer value pointed to by
Fa n
will contain the number of successfully processed mtrr structures
in both cases.
Sh ERRORS
Bl -tag -width [EINVAL]
It Bq Er ENOSYS
The currently running kernel or CPU has no MTRR support.
It Bq Er EINVAL
The currently running kernel has no MTRR support, or one of the mtrr
structures pointed to by
Fa mtrrp
is invalid.
It Bq Er EBUSY
No unused MTRRs are available.
El
Sh HISTORY
The
Fn i386_get_mtrr
and
Fn i386_set_mtrr
functions appeared in
Nx 1.6 .

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