ihex: Simplify next record offset calculation

This change “ihex: Simplify next record offset calculation” in Linux kernel is authored by Andrey Smirnov <andrew.smirnov [at] gmail.com> on Thu Dec 20 23:28:39 2018 -0800.

ihex: Simplify next record offset calculation

Next record calucaltion can be reduced to a much more tivial ALIGN
operation as follows:

1. Splitting 5 into 2 + 3 we get

   next = ((be16_to_cpu(rec->len) + 2 + 3) & ~3) - 2            (1)

2. Using ALIGN macro we reduce (1) to:

   ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2                      (2)

3. Subsituting 'next' in original next record calucation we get:

   (void *)&rec->data[ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2]  (3)

4. Converting array index to pointer arithmetic we convert (3) into:

   (void *)rec + sizeof(*rec) +
   	 ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2		(4)

5. Subsituting sizeof(*rec) with its value, 6, and substracting 2,
   in (4) we get:

   (void *)rec + ALIGN(be16_to_cpu(rec->len) + 2, 4) + 4        (5)

6. Since ALIGN(X, 4) + 4 == ALIGN(X + 4, 4), (5) can be converted to:

   (void *)rec + ALIGN(be16_to_cpu(rec->len) + 6, 4)            (6)

5. Subsituting 6 in (6) to sizeof(*rec) we get:

   (void *)rec + ALIGN(be16_to_cpu(rec->len) + sizeof(*rec), 4) (7)

Using expression (7) should make it more clear that next record is
located by adding full size of the current record (payload + auxiliary
data) aligned to 4 bytes, to the location of the current one. No
functional change intended.

Cc: Chris Healy <cphealy@gmail.com>
Cc: Kyle McMartin <kyle@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 9fb4ab4.

There are 10 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 include/linux/ihex.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/ihex.h b/include/linux/ihex.h
index 9130f30..98cb5ce 100644
--- a/include/linux/ihex.h
+++ b/include/linux/ihex.h
@@ -21,14 +21,18 @@ struct ihex_binrec {
 	uint8_t data[0];
 } __attribute__((packed));
 
+static inline uint16_t ihex_binrec_size(const struct ihex_binrec *p)
+{
+	return be16_to_cpu(p->len) + sizeof(*p);
+}
+
 /* Find the next record, taking into account the 4-byte alignment */
 static inline const struct ihex_binrec *
 __ihex_next_binrec(const struct ihex_binrec *rec)
 {
-	int next = ((be16_to_cpu(rec->len) + 5) & ~3) - 2;
-	rec = (void *)&rec->data[next];
+	const void *p = rec;
 
-	return rec;
+	return p + ALIGN(ihex_binrec_size(rec), 4);
 }
 
 static inline const struct ihex_binrec *

The commit for this change in Linux stable tree is 9fb4ab4 (patch).

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