diff --git a/board/can_definitions.h b/board/can_definitions.h
index e0b075961e..2ef8c7e84b 100644
--- a/board/can_definitions.h
+++ b/board/can_definitions.h
@@ -14,7 +14,7 @@ typedef struct {
   unsigned char data[CANPACKET_DATA_SIZE_MAX];
 } __attribute__((packed, aligned(4))) CANPacket_t;
 
-#define CANPACKET_HEAD_SIZE (sizeof(CANPacket_t) - (CANPACKET_DATA_SIZE_MAX / 8))
+#define CANPACKET_HEAD_SIZE 5U
 
 #define GET_BUS(msg) ((msg)->bus)
 #define GET_LEN(msg) (dlc_to_len[(msg)->data_len_code])
diff --git a/board/main_comms.h b/board/main_comms.h
index e8b1930a63..f715bf168b 100644
--- a/board/main_comms.h
+++ b/board/main_comms.h
@@ -58,9 +58,10 @@ asm_buffer can_write_overflow = {.ptr = 0U, .tail_size = 0U};
 
 bool add_magic = true;
 
-// read out as many CAN messages as we can fit in a max_len chunk,
-// including partial messages, which will be continued in the
-// beginning of the next chunk.
+// for CAN read and send, we write out a buffer consisting of:
+// CAN_TRANSACTION_MAGIC + (N * CANPacket_t)
+// comms_can_read will output this buffer into chunks of size max_len.
+this buffer is split into chunks output by the comms_can_xxx functions.
 int comms_can_read(uint8_t *data, uint32_t max_len) {
   uint32_t pos = 0U;  // count of bytes written
   bool added_magic = false;