[PATCH 13/27] libfdt: Add FDT_CREATE_FLAG_NO_NAME_DEDUP flag that trades size for speed

[Sebastian Huber]

From: Nicholas Piggin <npiggin at gmail.com>

Searching for duplicate names scales O(n^2) with the number of names
added to a fdt, which can cause a noticable slowdown with larger device
trees and very slow CPU cores.

Add FDT_CREATE_FLAG_NO_NAME_DEDUP that allow the caller to trade fdt size
for speed in the creation process.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
Message-Id: <20190509094122.834-4-npiggin at gmail.com>
Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
---
 cpukit/dtc/libfdt/fdt_sw.c | 39 ++++++++++++++++++++++++++-------------
 cpukit/include/libfdt.h    |  8 +++++++-
 2 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/cpukit/dtc/libfdt/fdt_sw.c b/cpukit/dtc/libfdt/fdt_sw.c
index ba05868845..e773157d0f 100644
--- a/cpukit/dtc/libfdt/fdt_sw.c
+++ b/cpukit/dtc/libfdt/fdt_sw.c
@@ -284,6 +284,23 @@ int fdt_end_node(void *fdt)
 	return 0;
 }
 
+static int fdt_add_string_(void *fdt, const char *s)
+{
+	char *strtab = (char *)fdt + fdt_totalsize(fdt);
+	int strtabsize = fdt_size_dt_strings(fdt);
+	int len = strlen(s) + 1;
+	int struct_top, offset;
+
+	offset = -strtabsize - len;
+	struct_top = fdt_off_dt_struct(fdt) + fdt_size_dt_struct(fdt);
+	if (fdt_totalsize(fdt) + offset < struct_top)
+		return 0; /* no more room :( */
+
+	memcpy(strtab + offset, s, len);
+	fdt_set_size_dt_strings(fdt, strtabsize + len);
+	return offset;
+}
+
 /* Must only be used to roll back in case of error */
 static void fdt_del_last_string_(void *fdt, const char *s)
 {
@@ -296,10 +313,8 @@ static void fdt_del_last_string_(void *fdt, const char *s)
 static int fdt_find_add_string_(void *fdt, const char *s, int *allocated)
 {
 	char *strtab = (char *)fdt + fdt_totalsize(fdt);
-	const char *p;
 	int strtabsize = fdt_size_dt_strings(fdt);
-	int len = strlen(s) + 1;
-	int struct_top, offset;
+	const char *p;
 
 	*allocated = 0;
 
@@ -307,17 +322,9 @@ static int fdt_find_add_string_(void *fdt, const char *s, int *allocated)
 	if (p)
 		return p - strtab;
 
-	/* Add it */
 	*allocated = 1;
 
-	offset = -strtabsize - len;
-	struct_top = fdt_off_dt_struct(fdt) + fdt_size_dt_struct(fdt);
-	if (fdt_totalsize(fdt) + offset < struct_top)
-		return 0; /* no more room :( */
-
-	memcpy(strtab + offset, s, len);
-	fdt_set_size_dt_strings(fdt, strtabsize + len);
-	return offset;
+	return fdt_add_string_(fdt, s);
 }
 
 int fdt_property_placeholder(void *fdt, const char *name, int len, void **valp)
@@ -328,7 +335,13 @@ int fdt_property_placeholder(void *fdt, const char *name, int len, void **valp)
 
 	FDT_SW_PROBE_STRUCT(fdt);
 
-	nameoff = fdt_find_add_string_(fdt, name, &allocated);
+	/* String de-duplication can be slow, _NO_NAME_DEDUP skips it */
+	if (sw_flags(fdt) & FDT_CREATE_FLAG_NO_NAME_DEDUP) {
+		allocated = 1;
+		nameoff = fdt_add_string_(fdt, name);
+	} else {
+		nameoff = fdt_find_add_string_(fdt, name, &allocated);
+	}
 	if (nameoff == 0)
 		return -FDT_ERR_NOSPACE;
 
diff --git a/cpukit/include/libfdt.h b/cpukit/include/libfdt.h
index c2dee3eaa6..0684ec22ed 100644
--- a/cpukit/include/libfdt.h
+++ b/cpukit/include/libfdt.h
@@ -1433,7 +1433,13 @@ int fdt_nop_node(void *fdt, int nodeoffset);
 /* Sequential write functions                                         */
 /**********************************************************************/
 
-#define FDT_CREATE_FLAGS_ALL	0
+/* fdt_create_with_flags flags */
+#define FDT_CREATE_FLAG_NO_NAME_DEDUP 0x1
+	/* FDT_CREATE_FLAG_NO_NAME_DEDUP: Do not try to de-duplicate property
+	 * names in the fdt. This can result in faster creation times, but
+	 * a larger fdt. */
+
+#define FDT_CREATE_FLAGS_ALL	(FDT_CREATE_FLAG_NO_NAME_DEDUP)
 
 /**
  * fdt_create_with_flags - begin creation of a new fdt
-- 
2.16.4