jffs2: Reduce excessive scan of empty blocks

Scanning 1024 bytes to see if an EB is empty is a bit much.
Lower it to 256 bytes and make sure the while loop is
optimized.
Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

fs/jffs2/scan.c

@@ -20,7 +20,7 @@
 #include "summary.h"
 #include "debug.h"
 
-#define DEFAULT_EMPTY_SCAN_SIZE 1024
+#define DEFAULT_EMPTY_SCAN_SIZE 256
 
 #define noisy_printk(noise, args...) do { \
 	if (*(noise)) { \
@@ -435,7 +435,7 @@ static int jffs2_scan_eraseblock (struct jffs2_sb_info *c, struct jffs2_eraseblo
 				  unsigned char *buf, uint32_t buf_size, struct jffs2_summary *s) {
 	struct jffs2_unknown_node *node;
 	struct jffs2_unknown_node crcnode;
-	uint32_t ofs, prevofs;
+	uint32_t ofs, prevofs, max_ofs;
 	uint32_t hdr_crc, buf_ofs, buf_len;
 	int err;
 	int noise = 0;
@@ -550,12 +550,12 @@ static int jffs2_scan_eraseblock (struct jffs2_sb_info *c, struct jffs2_eraseblo
 
 	/* We temporarily use 'ofs' as a pointer into the buffer/jeb */
 	ofs = 0;
-
-	/* Scan only 4KiB of 0xFF before declaring it's empty */
-	while(ofs < EMPTY_SCAN_SIZE(c->sector_size) && *(uint32_t *)(&buf[ofs]) == 0xFFFFFFFF)
+	max_ofs = EMPTY_SCAN_SIZE(c->sector_size);
+	/* Scan only EMPTY_SCAN_SIZE of 0xFF before declaring it's empty */
+	while(ofs < max_ofs && *(uint32_t *)(&buf[ofs]) == 0xFFFFFFFF)
 		ofs += 4;
 
-	if (ofs == EMPTY_SCAN_SIZE(c->sector_size)) {
+	if (ofs == max_ofs) {
 #ifdef CONFIG_JFFS2_FS_WRITEBUFFER
 		if (jffs2_cleanmarker_oob(c)) {
 			/* scan oob, take care of cleanmarker */