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EXTENT(9)                BSD Kernel Developer's Manual               EXTENT(9)

NAME
     extent_create, extent_destroy, extent_alloc, extent_alloc_with_descr,
     extent_alloc_subregion, extent_alloc_subregion_with_descr,
     extent_alloc_region, extent_free, extent_print -- general purpose extent
     manager

SYNOPSIS
     #include <&lt;sys/malloc.h>&gt;
     #include <&lt;sys/extent.h>&gt;

     struct extent *
     extent_create(char *name, u_long start, u_long end, int mtype,
         caddr_t storage, size_t storagesize, int flags);

     void
     extent_destroy(struct extent *ex);

     int
     extent_alloc(struct extent *ex, u_long size, u_long alignment,
         u_long skew, u_long boundary, int flags, u_long *result);

     int
     extent_alloc_with_descr(struct extent *ex, u_long size, u_long alignment,
         u_long skew, u_long boundary, int flags, struct extent_region *rp,
         u_long *result);

     int
     extent_alloc_subregion(struct extent *ex, u_long substart, u_long subend,
         u_long size, u_long alignment, u_long skew, u_long boundary,
         int flags, u_long *result);

     int
     extent_alloc_subregion_with_descr(struct extent *ex, u_long substart,
         u_long subend, u_long size, u_long alignment, u_long skew,
         u_long boundary, int flags, struct extent_region *rp,
         u_long *result);

     int
     extent_alloc_region(struct extent *ex, u_long start, u_long size,
         int flags);

     int
     extent_free(struct extent *ex, u_long start, u_long size, int flags);

     void
     extent_print(struct extent *ex);

DESCRIPTION
     The extent manager provides management of areas of memory or other enu-
     merable spaces (such as I/O ports).  An opaque structure called an extent
     map keeps track of allocated regions within the enumerable space.

     extent_create() creates an extent map managing the space from start to
     end inclusive.  All memory allocation will use the memory type mtype (see
     malloc(9)).  The extent map will have the name name, used for identifica-
     tion in case of errors or in ddb(4) show extents.  If the flag
     EX_NOCOALESCE is set, internal coalescing of regions is disabled, and
     only entire regions may be freed within the extent map, so that
     extent_free() will never have to allocate a region descriptor.  If the
     flag EX_FILLED is set, the entire space managed by the extent map will be
     allocated upon creation of the extent map, such that selected regions may
     be made available through calls to extent_free().

     Some applications may want to use an extent map but can't use malloc()
     and free().  These applications may provide pre-allocated storage for all
     descriptor overhead with the arguments storage and storagesize.  An
     extent of this type is called a fixed extent.  If the application can
     safely use malloc() and free(), storage should be NULL.  A fixed extent
     has a fixed number of region descriptors, so care should be taken to pro-
     vide enough storage for them; alternatively, the flag EX_MALLOCOK may be
     passed to extent requests to indicate that a fixed extent map may be
     extended using a call to malloc().  Note that passing the flag EX_FILLED
     to extent_create() will consume a region descriptor upon creation of the
     extent map.

     The caller should pass the flag EX_WAITOK or EX_NOWAIT to extent func-
     tions that have a memory overhead, to specify whether it is okay to wait.
     These functions are extent_create() (non fixed extents), extent_free()
     (unless EX_NOCOALESCE is set), extent_alloc(), extent_alloc_subregion()
     and extent_alloc_region().

     extent_destroy() destroys the extent map ex, freeing all allocated
     regions.  If the extent is not a fixed extent, the region and internal
     extent descriptors themselves are freed.  This function always succeeds.

     extent_alloc() allocates a region in the extent map ex of size size that
     fits the provided parameters.  There are two distinct allocation poli-
     cies, which are selected by the flags argument:

           EX_FAST    Allocate the first region that fits the provided parame-
                      ters, regardless of resulting extent fragmentation.

           default    Allocate the smallest region that is capable of holding
                      the request, thus minimizing fragmentation of the
                      extent.

     The caller may specify that it is okay to wait for space to become free
     in the extent by setting the flag EX_WAITSPACE.  If EX_WAITSPACE is not
     set, the allocation will fail if the request can not be satisfied without
     sleeping.

     The request will be aligned to a multiple of alignment.  That value must
     be a power of 2.  If no alignment is necessary, the value EX_NOALIGN
     should be specified.  If skew is non-zero, it modifies the requested
     alignment result in the following way: the value (result - skew) is
     aligned to alignment boundaries.  skew must be a smaller number than
     alignment.  If boundary is not EX_NOBOUNDARY, the allocated region will
     not cross any boundary lines, spaced boundary apart.  If the caller spec-
     ifies the EX_BOUNDZERO flag, boundary lines begin at zero.  Otherwise,
     boundary lines begin at the beginning of the extent.  The allocated
     region may begin on a boundary line, but the end of the region will not
     touch nor cross a boundary line.  A boundary argument smaller than the
     sum of the requested skew and the size of the request is invalid.  Upon
     successful completion, *result will contain the start of the allocated
     region.

     extent_alloc_with_descr() is similar to extent_alloc() but allows the
     caller to provide a pre-allocated region descriptor instead of having the
     function allocate one.  This function can only be used with extents that
     have the EX_NOCOALESCE property.

     extent_alloc_subregion() and extent_alloc_subregion_with_descr() are gen-
     eralized versions of extent_alloc() and extent_alloc_with_descr() that
     allow the caller to specify that the allocated region must fall within
     the subregion from substart to subend inclusive.

     extent_alloc_region() allocates the specific region in the extent map ex
     beginning at start with the size size.  If the caller specifies the
     EX_CONFLICTOK flag, the allocation will succeed even if part of the
     requested region has already been allocated.  The caller may specify that
     it is okay to wait for the indicated region to be free by setting the
     flag EX_WAITSPACE.  If neither EX_WAITSPACE nor EX_CONFLICTOK is set, the
     allocation will fail if the request can not be satisfied without sleep-
     ing.

     extent_free() frees a region of size bytes starting at start in the
     extent map ex.  If the extent has the EX_NOCOALESCE property, only entire
     regions may be freed.  If the extent has the EX_NOCOALESCE property and
     the caller attempts to free a partial region, behavior is undefined.  If
     called on an extent without the EX_NOCOALESCE property, this function can
     fail with error codes listed below, otherwise this function will always
     succeed.

     extent_print() Prints out information about extent ex.  This function
     always succeeds.

RETURN VALUES
     The behavior of all extent manager functions is undefined if given
     invalid arguments.  extent_create() returns the extent map on success, or
     NULL if it fails to allocate storage for the extent map.  It always suc-
     ceeds when creating a fixed extent or when given the flag EX_WAITOK.
     extent_alloc(), extent_alloc_region(), extent_alloc_subregion(), and
     extent_free() return one of the following values:

           0         Operation was successful.

           ENOMEM    If EX_NOWAIT is specified, the extent manager was not
                     able to allocate a region descriptor for the new region
                     or to split a region when freeing a partial region.

           EAGAIN    Requested region is not available and EX_WAITSPACE was
                     not specified.

           EINTR     Process received a signal while waiting for the requested
                     region to become available in the extent.

EXAMPLES
     Here is an example of a (useless) function that uses several of the
     extent manager routines.

     void
     func()
     {
             struct extent *foo_ex;
             u_long region_start;
             int error;

             /*
              * Extent "foo" manages a 256k region starting at 0x0 and
              * only allows complete regions to be freed so that
              * extent_free() never needs to allocate memory.
              */
             foo_ex = extent_create("foo", 0x0, 0x3ffff, M_DEVBUF,
                 NULL, 0, EX_WAITOK | EX_NOCOALESCE);

             /*
              * Allocate an 8k region, aligned to a 4k boundary, which
              * does not cross any of the 3 64k boundaries (at 64k,
              * 128k, and 192k) within the extent.
              */
             error = extent_alloc(foo_ex, 0x2000, 0x1000, 0x10000,
                 EX_NOWAIT, &region_start);
             if (error)
                     panic("you lose");

             /*
              * Give up the extent.
              */
             extent_destroy(foo_ex);
     }

CODE REFERENCES
     The extent manager itself is implemented within the file
     sys/kern/subr_extent.c.

     The i386 bus management code uses the extent manager for managing I/O
     ports and I/O memory.  See sys/arch/i386/i386/machdep.c.

SEE ALSO
     ddb(4), malloc(9)

HISTORY
     The extent manager appeared in NetBSD 1.3.

AUTHORS
     The extent manager was designed and implemented by Jason R. Thorpe
     <thorpej@NetBSD.ORG>.  Matthias Drochner
     <drochner@zelux6.zel.kfa-juelich.de> contributed to the initial testing
     and optimization of the implementation.  Chris Demetriou <cgd@NetBSD.ORG>
     contributed many architectural suggestions.

BSD                            February 8, 2014                            BSD