[PATCH v2] libfdt: Document sequential write mechanism

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When generating a flattened devicetree on the fly, we employ some clever
techniques to avoid excessive copying and adjustments of offset values.

To avoid people scratching their heads over all those negative string
offsets and weird magic values, let's document the nifty tricks we
pull here.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>

another old patch I found in my local repo. I addressed the comments that
Simon and David had back in October:


 libfdt/fdt_sw.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/libfdt/fdt_sw.c b/libfdt/fdt_sw.c
index 4c569ee..d637c18 100644
--- a/libfdt/fdt_sw.c
+++ b/libfdt/fdt_sw.c
@@ -3,6 +3,38 @@
  * libfdt - Flat Device Tree manipulation
  * Copyright (C) 2006 David Gibson, IBM Corporation.
+ * DOC: libfdt sequential write support
+ *
+ * When creating a flattened device tree on the fly, the fixed structure
+ * of a DTB would require constant adjustment of memory offsets, also copying
+ * of whole regions like the string table.
+ * To avoid this, we use an intermediate representation of a flattened tree,
+ * which needs to be finalised explicitly to create a spec-conformant DTB.
+ *
+ * This is achieved by the following means:
+ * - The magic number in the header differs (FDT_SW_MAGIC), to identify such an
+ *   unfinished tree and to avoid it to be mistaken for a proper DTB, should
+ *   the fdt_finish() call have been missed.
+ * - The string table is located at the end of the allocated buffer, and
+ *   is growing *downwards*,  as new strings are *prepended*.
+ * - The string offsets in the dt_struct are negative, since the string
+ *   table pointer points to the end of the string table. This allows
+ *   offsets to stay fixed, even when new strings are added (before the
+ *   old ones).
+ * - The dt_struct is located at its usual place, but leaves room after it,
+ *   to grow *upwards*, towards the string table.
+ * - Upon finalising the DTB, all negative string offsets in dt_struct are
+ *   adjusted, by adding the offset of the new end of the string table.
+ * - To finish the tree, the final offsets of all structures are written into
+ *   the header, and the magic will be changed to be the spec defined one.
+ *
+ * While the tree is in "sequential write" mode, only the functions defined
+ * in this file and the read-only and write-in-place functions can be used.
+ * Any functions marked as "read-write" functions in libfdt.h are not
+ * supported until the tree is finalised with fdt_finish().
+ */
 #include "libfdt_env.h"
 #include <fdt.h>

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