[edid-decode] [PATCH 2/2] Calculate DisplayID checksums. Refactor do_checksum.

[Mark Ferry]

---
 edid-decode.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/edid-decode.c b/edid-decode.c
index c18697f..6df2b6e 100644
--- a/edid-decode.c
+++ b/edid-decode.c
@@ -47,6 +47,7 @@ static int has_range_descriptor = 0;
 static int has_preferred_timing = 0;
 static int has_valid_checksum = 1;
 static int has_valid_cvt = 1;
+static int has_valid_displayid_checksum = 1;
 static int has_valid_dummy_block = 1;
 static int has_valid_week = 0;
 static int has_valid_year = 0;
@@ -560,23 +561,27 @@ detailed_block(unsigned char *x, int in_extension)
     return 1;
 }
 
-static void
-do_checksum(unsigned char *x)
+static unsigned char
+do_checksum(unsigned char *x, size_t len)
 {
-    printf("Checksum: 0x%hx", x[0x7f]);
-    {
-       unsigned char sum = 0;
-       int i;
-       for (i = 0; i < 128; i++)
-           sum += x[i];
-       if (sum) {
-           printf(" (should be 0x%hx)", (unsigned char)(x[0x7f] - sum));
-           has_valid_checksum = 0;
-       } else printf(" (valid)");
-    }
+    unsigned char sum = 0;
+    int i;
+
+    printf("Checksum: 0x%hx", x[len -1]);
+
+    for (i = 0; i < len; i++)
+        sum += x[i];
+
+    if (sum) {
+        printf(" (should be 0x%hx)", (unsigned char)(x[len-1] - sum));
+    } else printf(" (valid)");
+
     printf("\n");
+
+    return sum;
 }
 
+
 /* CEA extension */
 
 static const char *
@@ -1281,7 +1286,7 @@ parse_cea(unsigned char *x)
                detailed_block(detailed, 1);
     } while (0);
 
-    do_checksum(x);
+    (void) do_checksum(x, 128);
 
     return ret;
 }
@@ -1371,6 +1376,9 @@ parse_displayid(unsigned char *x)
     int ext_count = x[4];
     int i;
     printf("Length %d, version %d, extension count %d\n", length, version, 
ext_count);
+
+    has_valid_displayid_checksum = (do_checksum(x+1, length + 5) == 0x0);
+
     int offset = 5;
     while (length > 0) {
        int tag = x[offset];
@@ -2037,7 +2045,7 @@ int main(int argc, char **argv)
        has_valid_extension_count = 1;
     }
 
-    do_checksum(edid);
+    (void) do_checksum(edid, 128);
 
     x = edid;
     for (edid_lines /= 8; edid_lines > 1; edid_lines--) {
@@ -2127,6 +2135,8 @@ int main(int argc, char **argv)
            printf("\tInvalid detailed timing descriptor ordering\n");
        if (!has_valid_range_descriptor)
            printf("\tRange descriptor contains garbage\n");
+       if (!has_valid_displayid_checksum)
+           printf("\tBlock has broken DisplayID checksum\n");
        if (!has_valid_max_dotclock)
            printf("\tEDID 1.4 block does not set max dotclock\n");
     }
-- 
2.5.0

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