mm: Document GRUB internal memory management structures

I spent more than a trivial quantity of time figuring out pre_size and
whether a memory region's size contains the header cell or not.

Document the meanings of all the properties. Hopefully now no-one else
has to figure it out!

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/mm_private.h

@@ -21,15 +21,27 @@
 
 #include <grub/mm.h>
 
+/* For context, see kern/mm.c */
+
 /* Magic words.  */
 #define GRUB_MM_FREE_MAGIC	0x2d3c2808
 #define GRUB_MM_ALLOC_MAGIC	0x6db08fa4
 
+/* A header describing a block of memory - either allocated or free */
 typedef struct grub_mm_header
 {
+  /*
+   * The 'next' free block in this region's circular free list.
+   * Only meaningful if the block is free.
+   */
   struct grub_mm_header *next;
+  /* The block size, not in bytes but the number of cells of
+   * GRUB_MM_ALIGN bytes. Includes the header cell.
+   */
   grub_size_t size;
+  /* either free or alloc magic, depending on the block type. */
   grub_size_t magic;
+  /* pad to cell size: see the top of kern/mm.c. */
 #if GRUB_CPU_SIZEOF_VOID_P == 4
   char padding[4];
 #elif GRUB_CPU_SIZEOF_VOID_P == 8
@@ -48,11 +60,27 @@ typedef struct grub_mm_header
 
 #define GRUB_MM_ALIGN	(1 << GRUB_MM_ALIGN_LOG2)
 
+/* A region from which we can make allocations. */
 typedef struct grub_mm_region
 {
+  /* The first free block in this region. */
   struct grub_mm_header *first;
+
+  /*
+   * The next region in the linked list of regions. Regions are initially
+   * sorted in order of increasing size, but can grow, in which case the
+   * ordering may not be preserved.
+   */
   struct grub_mm_region *next;
+
+  /*
+   * A grub_mm_region will always be aligned to cell size. The pre-size is
+   * the number of bytes we were given but had to skip in order to get that
+   * alignment.
+   */
   grub_size_t pre_size;
+
+  /* How many bytes are in this region? (free and allocated) */
   grub_size_t size;
 }
 *grub_mm_region_t;