V4L/DVB (13192): gspca - pac_common: redesign function for finding Start Of Frame

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.

Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

diff --git a/drivers/media/video/gspca/pac_common.h b/drivers/media/video/gspca/pac_common.h
index 34d4b14..d300653 100644 (file)
--- a/drivers/media/video/gspca/pac_common.h
+++ b/drivers/media/video/gspca/pac_common.h
@@ -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(struct gspca_dev *gspca_dev,
 
        /* 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;
                }
        }