[LTP] [PATCH] pac_common: redesign function for finding Start Of Frame

["Németh Márton" <nm127@freemail.hu>]

From: Márton Németh <nm127@freemail.hu>

The original implementation of pac_find_sof() does not always find
the Start Of Frame (SOF) marker. Replace it with a state machine
based design.

The change was tested with Labtec Webcam 2200.

Signed-off-by: Márton Németh <nm127@freemail.hu>
---
--- linux-2.6.32-rc1.orig/drivers/media/video/gspca/pac_common.h	2009-09-10 00:13:59.000000000 +0200
+++ linux-2.6.32-rc1/drivers/media/video/gspca/pac_common.h	2009-10-04 21:49:19.000000000 +0200
@@ -33,6 +33,45 @@
 static const unsigned char pac_sof_marker[5] =
 		{ 0xff, 0xff, 0x00, 0xff, 0x96 };

+/*
+   The following state machine finds the SOF marker sequence
+   0xff, 0xff, 0x00, 0xff, 0x96 in a byte stream.
+
+	   +----------+
+	   | 0: START |<---------------\
+	   +----------+<-\             |
+	     |       \---/otherwise    |
+	     v 0xff                    |
+	   +----------+ otherwise      |
+	   |     1    |--------------->*
+	   |          |                ^
+	   +----------+                |
+	     |                         |
+	     v 0xff                    |
+	   +----------+<-\0xff         |
+	/->|          |--/             |
+	|  |     2    |--------------->*
+	|  |          | otherwise      ^
+	|  +----------+                |
+	|    |                         |
+	|    v 0x00                    |
+	|  +----------+                |
+	|  |     3    |                |
+	|  |          |--------------->*
+	|  +----------+ otherwise      ^
+	|    |                         |
+   0xff |    v 0xff                    |
+	|  +----------+                |
+	\--|     4    |                |
+	   |          |----------------/
+	   +----------+ otherwise
+	     |
+	     v 0x96
+	   +----------+
+	   |  FOUND   |
+	   +----------+
+*/
+
 static unsigned char *pac_find_sof(struct gspca_dev *gspca_dev,
 					unsigned char *m, int len)
 {
@@ -41,17 +80,54 @@ static unsigned char *pac_find_sof(struc

 	/* Search for the SOF marker (fixed part) in the header */
 	for (i = 0; i < len; i++) {
-		if (m[i] == pac_sof_marker[sd->sof_read]) {
-			sd->sof_read++;
-			if (sd->sof_read == sizeof(pac_sof_marker)) {
+		switch (sd->sof_read) {
+		case 0:
+			if (m[i] == 0xff)
+				sd->sof_read = 1;
+			break;
+		case 1:
+			if (m[i] == 0xff)
+				sd->sof_read = 2;
+			else
+				sd->sof_read = 0;
+			break;
+		case 2:
+			switch (m[i]) {
+			case 0x00:
+				sd->sof_read = 3;
+				break;
+			case 0xff:
+				/* stay in this state */
+				break;
+			default:
+				sd->sof_read = 0;
+			}
+			break;
+		case 3:
+			if (m[i] == 0xff)
+				sd->sof_read = 4;
+			else
+				sd->sof_read = 0;
+			break;
+		case 4:
+			switch (m[i]) {
+			case 0x96:
+				/* Pattern found */
 				PDEBUG(D_FRAM,
 					"SOF found, bytes to analyze: %u."
 					" Frame starts at byte #%u",
 					len, i + 1);
 				sd->sof_read = 0;
 				return m + i + 1;
+				break;
+			case 0xff:
+				sd->sof_read = 2;
+				break;
+			default:
+				sd->sof_read = 0;
 			}
-		} else {
+			break;
+		default:
 			sd->sof_read = 0;
 		}
 	}

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