[git:edid-decode/master] edid-decode: specify checksum position for do_checksum()

[Hans Verkuil]

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Subject: edid-decode: specify checksum position for do_checksum()
Author:  Hans Verkuil <hverkuil-ci...@xs4all.nl>
Date:    Wed Jul 31 15:59:01 2024 +0200

This prepares for the upcoming InfoFrame parser where the
checksum isn't at the end. So explicitly pass the checksum
position to do_checksum().

Signed-off-by: Hans Verkuil <hverkuil-ci...@xs4all.nl>

 edid-decode.cpp           | 13 ++++++++-----
 edid-decode.h             |  3 ++-
 parse-base-block.cpp      |  2 +-
 parse-displayid-block.cpp |  2 +-
 4 files changed, 12 insertions(+), 8 deletions(-)

---

diff --git a/edid-decode.cpp b/edid-decode.cpp
index ce4f1fd5a4af..4f39ee544598 100644
--- a/edid-decode.cpp
+++ b/edid-decode.cpp
@@ -249,14 +249,17 @@ void replace_checksum(unsigned char *x, size_t len)
        x[len - 1] = -sum & 0xff;
 }
 
-void do_checksum(const char *prefix, const unsigned char *x, size_t len, 
unsigned unused_bytes)
+void do_checksum(const char *prefix, const unsigned char *x, size_t len, 
size_t checksum_pos,
+                unsigned unused_bytes)
 {
-       unsigned char check = x[len - 1];
+       unsigned char check = x[checksum_pos];
        unsigned char sum = 0;
        unsigned i;
 
-       for (i = 0; i < len - 1; i++)
-               sum += x[i];
+       for (i = 0; i < len; i++) {
+               if (i != checksum_pos)
+                       sum += x[i];
+       }
 
        printf("%sChecksum: 0x%02hhx", prefix, check);
        if ((unsigned char)(check + sum) != 0) {
@@ -1232,7 +1235,7 @@ void edid_state::parse_extension(const unsigned char *x)
        }
 
        data_block.clear();
-       do_checksum("", x, EDID_PAGE_SIZE, unused_bytes);
+       do_checksum("", x, EDID_PAGE_SIZE, EDID_PAGE_SIZE - 1, unused_bytes);
 }
 
 void edid_state::print_preferred_timings()
diff --git a/edid-decode.h b/edid-decode.h
index 0a06b17a2122..2a06563f0c0b 100644
--- a/edid-decode.h
+++ b/edid-decode.h
@@ -554,7 +554,8 @@ bool memchk(const unsigned char *x, unsigned len, unsigned 
char v = 0);
 void hex_block(const char *prefix, const unsigned char *x, unsigned length,
               bool show_ascii = true, unsigned step = 16);
 std::string block_name(unsigned char block);
-void do_checksum(const char *prefix, const unsigned char *x, size_t len, 
unsigned unused_bytes = 0);
+void do_checksum(const char *prefix, const unsigned char *x, size_t len, 
size_t checksum_pos,
+                unsigned unused_bytes = 0);
 void replace_checksum(unsigned char *x, size_t len);
 void calc_ratio(struct timings *t);
 const char *oui_name(unsigned oui, unsigned *ouinum = NULL);
diff --git a/parse-base-block.cpp b/parse-base-block.cpp
index 6565932f24a7..e97dad42539b 100644
--- a/parse-base-block.cpp
+++ b/parse-base-block.cpp
@@ -1753,7 +1753,7 @@ void edid_state::parse_base_block(const unsigned char *x)
 
        block = block_name(0x00);
        data_block.clear();
-       do_checksum("", x, EDID_PAGE_SIZE);
+       do_checksum("", x, EDID_PAGE_SIZE, EDID_PAGE_SIZE - 1);
        if (base.edid_minor >= 3) {
                if (!base.has_name_descriptor)
                        msg(base.edid_minor >= 4, "Missing Display Product 
Name.\n");
diff --git a/parse-displayid-block.cpp b/parse-displayid-block.cpp
index 54a691be0cb0..f4c8fc047737 100644
--- a/parse-displayid-block.cpp
+++ b/parse-displayid-block.cpp
@@ -2153,7 +2153,7 @@ void edid_state::parse_displayid_block(const unsigned 
char *x)
         * (excluding DisplayID-in-EDID magic byte)
         */
        data_block.clear();
-       do_checksum("  ", x + 1, x[2] + 5);
+       do_checksum("  ", x + 1, x[2] + 5, x[2] + 4);
 
        unused_bytes = 0x7f - (1 + x[2] + 5);
        if (!memchk(x + 1 + x[2] + 5, unused_bytes)) {