From b953002f8f917b96139f198a2628ba3d36d0a0db Mon Sep 17 00:00:00 2001
From: Olliver Schinagl <oliver@schinagl.nl>
Date: Thu, 20 Oct 2016 17:36:04 +0200
Subject: [PATCH] drivers: pn532_i2c: Clarify preamble and start byte

The pn532 documentation differs slightly from the included ascii art
documentation on how a packet looks like. The preamble was omitted
however the postamble is mentioned. This patch adds the Preamble to the
ascii frame documentation.

The code later refers incorrectly to the start byte as the preamble.
This variable was renamed to more descriptively state that it is the
preambe and the start bytes.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
---
 libnfc/chips/pn53x-internal.h | 48 ++++++++++++++++++++---------------
 libnfc/drivers/pn532_i2c.c    | 11 +++++---
 2 files changed, 34 insertions(+), 25 deletions(-)

diff --git a/libnfc/chips/pn53x-internal.h b/libnfc/chips/pn53x-internal.h
index c43934a..e4869cd 100644
--- a/libnfc/chips/pn53x-internal.h
+++ b/libnfc/chips/pn53x-internal.h
@@ -81,32 +81,38 @@
 #define TgGetTargetStatus 0x8A
 
 /** @note PN53x's normal frame:
+ * See the PN532 (firmware) user manual, section 6.2.1.1: Normal information
+ * frame, figure 13. Normal information frame, page 28 rev. 02 - 2007-11-07.
  *
- *   .-- Start
- *   |   .-- Packet length
- *   |   |  .-- Length checksum
- *   |   |  |  .-- Direction (D4 Host to PN, D5 PN to Host)
- *   |   |  |  |  .-- Code
- *   |   |  |  |  |  .-- Packet checksum
- *   |   |  |  |  |  |  .-- Postamble
- *   V   |  |  |  |  |  |
- * ----- V  V  V  V  V  V
- * 00 FF 02 FE D4 02 2A 00
+ * .-- Preamble
+ * |    .-- Start
+ * |    |   .-- Packet length
+ * |    |   |  .-- Length checksum
+ * |    |   |  |  .-- Direction (D4 Host to PN, D5 PN to Host)
+ * |    |   |  |  |  .-- Code
+ * |    |   |  |  |  |  .-- Packet checksum
+ * |    |   |  |  |  |  |  .-- Postamble
+ * |    V   |  |  |  |  |  |
+ * V  ----- V  V  V  V  V  V
+ * 00 00 FF 02 FE D4 02 2A 00
  */
 
 /** @note PN53x's extended frame:
+ * See the PN532 (firmware) user manual, section 6.2.1.2: Extended information
+ * frame, figure 14. Normal information frame, page 29 rev. 02 - 2007-11-07.
  *
- *   .-- Start
- *   |     .-- Fixed to FF to enable extended frame
- *   |     |     .-- Packet length
- *   |     |     |   .-- Length checksum
- *   |     |     |   |  .-- Direction (D4 Host to PN, D5 PN to Host)
- *   |     |     |   |  |  .-- Code
- *   |     |     |   |  |  |  .-- Packet checksum
- *   |     |     |   |  |  |  |  .-- Postamble
- *   V     V     V   |  |  |  |  |
- * ----- ----- ----- V  V  V  V  V
- * 00 FF FF FF 00 02 FE D4 02 2A 00
+ * .-- Preamble
+ * |    .-- Start
+ * |    |     .-- Fixed to FF to enable extended frame
+ * |    |     |     .-- Packet length
+ * |    |     |     |   .-- Length checksum
+ * |    |     |     |   |  .-- Direction (D4 Host to PN, D5 PN to Host)
+ * |    |     |     |   |  |  .-- Code
+ * |    |     |     |   |  |  |  .-- Packet checksum
+ * |    |     |     |   |  |  |  |  .-- Postamble
+ * |    V     V     V   |  |  |  |  |
+ * V  ----- ----- ----- V  V  V  V  V
+ * 00 00 FF FF FF 00 02 FE D4 02 2A 00
  */
 
 /**
diff --git a/libnfc/drivers/pn532_i2c.c b/libnfc/drivers/pn532_i2c.c
index b62c7de..fe3345c 100644
--- a/libnfc/drivers/pn532_i2c.c
+++ b/libnfc/drivers/pn532_i2c.c
@@ -75,6 +75,10 @@ const struct timespec rdyDelay = {
   .tv_nsec = PN532_RDY_LOOP_DELAY * 1000 * 1000
 };
 
+/* preamble and start bytes, see pn532-internal.h for details */
+const uint8_t pn53x_preamble_and_start[] = { 0x00, 0x00, 0xff };
+#define PN53X_PREAMBLE_AND_START_LEN	(sizeof(pn53x_preamble_and_start) / sizeof(pn53x_preamble_and_start[0]))
+
 /* Private Functions Prototypes */
 
 static nfc_device *pn532_i2c_open(const nfc_context *context, const nfc_connstring connstring);
@@ -334,9 +338,10 @@ pn532_i2c_send(nfc_device *pnd, const uint8_t *pbtData, const size_t szData, int
       break;
   };
 
-  uint8_t abtFrame[PN532_BUFFER_LEN] = { 0x00, 0x00, 0xff };       // Every packet must start with "00 00 ff"
+  uint8_t abtFrame[PN532_BUFFER_LEN];
   size_t szFrame = 0;
 
+  memcpy(abtFrame, pn53x_preamble_and_start, PN53X_PREAMBLE_AND_START_LEN);	// Every packet must start with the preamble and start bytes.
   if ((res = pn53x_build_frame(abtFrame, &szFrame, pbtData, szData)) < 0) {
     pnd->last_error = res;
     return pnd->last_error;
@@ -477,9 +482,7 @@ pn532_i2c_receive(nfc_device *pnd, uint8_t *pbtData, const size_t szDataLen, int
     goto error;
   }
 
-  const uint8_t pn53x_preamble[3] = { 0x00, 0x00, 0xff };
-
-  if (0 != (memcmp(frameBuf, pn53x_preamble, 3))) {
+  if (0 != (memcmp(frameBuf, pn53x_preamble_and_start, PN53X_PREAMBLE_AND_START_LEN))) {
     log_put(LOG_GROUP, LOG_CATEGORY, NFC_LOG_PRIORITY_ERROR, "%s", "Frame preamble+start code mismatch");
     pnd->last_error = NFC_EIO;
     goto error;