ida: document IDA_BITMAP_LONGS calculation

IDA_BITMAP_LONGS value is calculated take into account struct ida_bitmap
not to waste memory space. Comment it.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 5 insertions, 2 deletions

diff --git a/include/linux/idr.h b/include/linux/idr.h
index e968db71e33a..88607523e2df 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -117,10 +117,13 @@ void idr_init(struct idr *idp);
 /*
  * IDA - IDR based id allocator, use when translation from id to
  * pointer isn't necessary.
+ *
+ * IDA_BITMAP_LONGS is calculated to be one less to accommodate
+ * ida_bitmap->nr_busy so that the whole struct fits in 128 bytes.
  */
 #define IDA_CHUNK_SIZE          128     /* 128 bytes per chunk */
-#define IDA_BITMAP_LONGS        (128 / sizeof(long) - 1)
-#define IDA_BITMAP_BITS         (IDA_BITMAP_LONGS * sizeof(long) * 8)
+#define IDA_BITMAP_LONGS        (IDA_CHUNK_SIZE / sizeof(long) - 1)
+#define IDA_BITMAP_BITS         (IDA_BITMAP_LONGS * sizeof(long) * 8)
 
 struct ida_bitmap {
         long                    nr_busy;