[PATCH] swsusp: struct snapshot_handle cleanup

Add comments describing struct snapshot_handle and its members, change the
confusing name of its member 'page' to 'cur'.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 56 insertions, 8 deletions

diff --git a/kernel/power/power.h b/kernel/power/power.h
index 59ce712f082d..1cefcf87a694 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -50,17 +50,65 @@ extern asmlinkage int swsusp_arch_resume(void);
 
 extern unsigned int count_data_pages(void);
 
+/**
+ *      Auxiliary structure used for reading the snapshot image data and
+ *      metadata from and writing them to the list of page backup entries
+ *      (PBEs) which is the main data structure of swsusp.
+ *
+ *      Using struct snapshot_handle we can transfer the image, including its
+ *      metadata, as a continuous sequence of bytes with the help of
+ *      snapshot_read_next() and snapshot_write_next().
+ *
+ *      The code that writes the image to a storage or transfers it to
+ *      the user land is required to use snapshot_read_next() for this
+ *      purpose and it should not make any assumptions regarding the internal
+ *      structure of the image.  Similarly, the code that reads the image from
+ *      a storage or transfers it from the user land is required to use
+ *      snapshot_write_next().
+ *
+ *      This may allow us to change the internal structure of the image
+ *      in the future with considerably less effort.
+ */
+
 struct snapshot_handle {
-        loff_t          offset;
-        unsigned int    page;
-        unsigned int    page_offset;
-        unsigned int    prev;
-        struct pbe      *pbe, *last_pbe;
-        void            *buffer;
-        unsigned int    buf_offset;
-        int             sync_read;
+        loff_t          offset; /* number of the last byte ready for reading
+                                 * or writing in the sequence
+                                 */
+        unsigned int    cur;    /* number of the block of PAGE_SIZE bytes the
+                                 * next operation will refer to (ie. current)
+                                 */
+        unsigned int    cur_offset;     /* offset with respect to the current
+                                         * block (for the next operation)
+                                         */
+        unsigned int    prev;   /* number of the block of PAGE_SIZE bytes that
+                                 * was the current one previously
+                                 */
+        struct pbe      *pbe;   /* PBE that corresponds to 'buffer' */
+        struct pbe      *last_pbe;      /* When the image is restored (eg. read
+                                         * from disk) we can store some image
+                                         * data directly in the page frames
+                                         * in which they were before suspend.
+                                         * In such a case the PBEs that
+                                         * correspond to them will be unused.
+                                         * This is the last PBE, so far, that
+                                         * does not correspond to such data.
+                                         */
+        void            *buffer;        /* address of the block to read from
+                                         * or write to
+                                         */
+        unsigned int    buf_offset;     /* location to read from or write to,
+                                         * given as a displacement from 'buffer'
+                                         */
+        int             sync_read;      /* Set to one to notify the caller of
+                                         * snapshot_write_next() that it may
+                                         * need to call wait_on_bio_chain()
+                                         */
 };
 
+/* This macro returns the address from/to which the caller of
+ * snapshot_read_next()/snapshot_write_next() is allowed to
+ * read/write data after the function returns
+ */
 #define data_of(handle) ((handle).buffer + (handle).buf_offset)
 
 extern int snapshot_read_next(struct snapshot_handle *handle, size_t count);