dmm/bm86x: check more bytes in DMM packet (undocumented by vendor)

User johnch reported via IRC that bm86x meters can lose sync to the
packet stream which goes unnoticed and tries to process invalid data.

Tighten the packet validity check, compare four bytes of the third chunk
in the DMM packet. This improves reliability, but isn't backed by vendor
documentation. It's yet to get determined whether other existing devices
don't provide the four-byte magic pattern.

diff --git a/src/dmm/bm86x.c b/src/dmm/bm86x.c
index b38c3123d3d55a616c21616ddc2e72443523f1fa..ea260d8f97180869cfa6441696be549a476323da 100644 (file)
--- a/src/dmm/bm86x.c
+++ b/src/dmm/bm86x.c
@@ -59,6 +59,22 @@ SR_PRIV gboolean sr_brymen_bm86x_packet_valid(const uint8_t *buf)
        if (buf[19] != 0x86)
                return FALSE;
 
+       /*
+        * 2021-05 update: The devices that we have seen in the field do
+        * provide four bytes which we can synchronize to. Which happens
+        * to match the bm52x and bm82x devices' protocol. This condition
+        * is not documented by the vendor, but improves reliability of
+        * the re-synchronization for slight offsets, which were seen
+        * in the field, and which would have kept failing in an earlier
+        * implementation.
+        */
+       if (buf[16] != 0x86)
+               return FALSE;
+       if (buf[17] != 0x86)
+               return FALSE;
+       if (buf[18] != 0x86)
+               return FALSE;
+
        return TRUE;
 }