NAME    [Toc]    [Back]

     amd64_get_mtrr, amd64_set_mtrr - access  Memory  Type  Range
Registers

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <machine/sysarch.h>
     #include <machine/mtrr.h>

     int
     amd64_get_mtrr(struct mtrr *mtrrp, int *n);

     int
     amd64_set_mtrr(struct mtrr *mtrrp, int *n);

DESCRIPTION    [Toc]    [Back]

     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) busses.  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 amd64_set_mtrr() the integer
pointed to
     by n will be updated to reflect the actual number  of  MTRRs
successfully
     set.  For amd64_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    [Toc]    [Back]

     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    [Toc]    [Back]

     [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.

OpenBSD     3.6                        November     10,      2001