rdtech-tc: rephrase PAC consistency check, discuss sample data layout

Rephrase access to the PAC chunks and the consistency check of the total
192 bytes data image in terms of one 64 bytes unit. Which eliminates
other magic numbers. Add a comment for awareness during development.

Prefer read_u32le() endianess conversion helpers for reliability. Use
big endian readers for four-character-codes to increase readability.
Address style nits in the process: Use booleans for checksum and
consistency checks. Eliminate else after return. Consolidate error
messages. Prefer indexing an array over pointer arithmetics. Group
related instructions in the same block.

diff --git a/src/hardware/rdtech-tc/protocol.c b/src/hardware/rdtech-tc/protocol.c
index 0c126b14b3f660f59b95ee989888089035948783..c6b7ce7707927bdf5fb7983583bafb71610c03b6 100644 (file)
--- a/src/hardware/rdtech-tc/protocol.c
+++ b/src/hardware/rdtech-tc/protocol.c
@@ -31,25 +31,31 @@
 
 #define SERIAL_WRITE_TIMEOUT_MS 1
 
-#define TC_POLL_LEN 192
 #define TC_POLL_PERIOD_MS 100
 #define TC_TIMEOUT_MS 1000
 
 static const char *poll_cmd = "getva";
 
-#define MAGIC_PAC1 0x31636170UL
-#define MAGIC_PAC2 0x32636170UL
-#define MAGIC_PAC3 0x33636170UL
+/*
+ * Response data (raw sample data) consists of three adjacent chunks
+ * of 64 bytes each. These chunks start with their magic string, and
+ * end in a 32bit checksum field. Measurement values are scattered
+ * across these 192 bytes total size. All multi-byte integer values
+ * are represented in little endian format. Typical size is 32 bits.
+ */
+
+#define MAGIC_PAC1     0x70616331      /* 'pac1' */
+#define MAGIC_PAC2     0x70616332      /* 'pac2' */
+#define MAGIC_PAC3     0x70616333      /* 'pac3' */
 
-/* Length of PAC block excluding CRC */
-#define PAC_DATA_LEN 60
-/* Length of PAC block including CRC */
 #define PAC_LEN 64
+#define PAC_CRC_POS (PAC_LEN - sizeof(uint32_t))
 
 /* Offset to PAC block from start of poll data */
 #define OFF_PAC1 (0 * PAC_LEN)
 #define OFF_PAC2 (1 * PAC_LEN)
 #define OFF_PAC3 (2 * PAC_LEN)
+#define TC_POLL_LEN (3 * PAC_LEN)
 
 #define OFF_MODEL 4
 #define LEN_MODEL 4
@@ -76,42 +82,45 @@ static const struct binary_analog_channel rdtech_tc_channels[] = {
        ALL_ZERO,
 };
 
-static int check_pac_crc(uint8_t *data)
+static gboolean check_pac_crc(uint8_t *data)
 {
        uint16_t crc;
        uint32_t crc_field;
 
-       crc = sr_crc16(SR_CRC16_DEFAULT_INIT, data, PAC_DATA_LEN);
-       crc_field = RL32(data + PAC_DATA_LEN);
-
+       crc = sr_crc16(SR_CRC16_DEFAULT_INIT, data, PAC_CRC_POS);
+       crc_field = read_u32le(&data[PAC_CRC_POS]);
        if (crc != crc_field) {
                sr_spew("CRC error. Calculated: %0x" PRIx16 ", expected: %0x" PRIx32,
                        crc, crc_field);
-               return 0;
-       } else {
-               return 1;
+               return FALSE;
        }
+
+       return TRUE;
 }
 
 static int process_poll_pkt(struct dev_context *devc, uint8_t *dst)
 {
        struct aes256_ctx ctx;
+       gboolean ok;
 
        aes256_set_decrypt_key(&ctx, AES_KEY);
        aes256_decrypt(&ctx, TC_POLL_LEN, dst, devc->buf);
 
-       if (RL32(dst + OFF_PAC1) != MAGIC_PAC1 ||
-           RL32(dst + OFF_PAC2) != MAGIC_PAC2 ||
-           RL32(dst + OFF_PAC3) != MAGIC_PAC3) {
-               sr_err("Invalid poll packet magic values!");
-               return SR_ERR;
+       ok = TRUE;
+       ok &= read_u32be(&dst[OFF_PAC1]) == MAGIC_PAC1;
+       ok &= read_u32be(&dst[OFF_PAC2]) == MAGIC_PAC2;
+       ok &= read_u32be(&dst[OFF_PAC3]) == MAGIC_PAC3;
+       if (!ok) {
+               sr_err("Invalid poll response packet (magic values).");
+               return SR_ERR_DATA;
        }
 
-       if (!check_pac_crc(dst + OFF_PAC1) ||
-           !check_pac_crc(dst + OFF_PAC2) ||
-           !check_pac_crc(dst + OFF_PAC3)) {
-               sr_err("Invalid poll checksum!");
-               return SR_ERR;
+       ok &= check_pac_crc(&dst[OFF_PAC1]);
+       ok &= check_pac_crc(&dst[OFF_PAC2]);
+       ok &= check_pac_crc(&dst[OFF_PAC3]);
+       if (!ok) {
+               sr_err("Invalid poll response packet (checksum).");
+               return SR_ERR_DATA;
        }
 
        return SR_OK;
@@ -124,7 +133,7 @@ SR_PRIV int rdtech_tc_probe(struct sr_serial_dev_inst *serial, struct dev_contex
 
        if (serial_write_blocking(serial, poll_cmd, strlen(poll_cmd),
                        SERIAL_WRITE_TIMEOUT_MS) < 0) {
-               sr_err("Unable to send probe request.");
+               sr_err("Failed to send probe request.");
                return SR_ERR;
        }
 
@@ -140,9 +149,9 @@ SR_PRIV int rdtech_tc_probe(struct sr_serial_dev_inst *serial, struct dev_contex
        }
 
        devc->channels = rdtech_tc_channels;
-       devc->dev_info.model_name = g_strndup((const char *)poll_pkt + OFF_MODEL, LEN_MODEL);
-       devc->dev_info.fw_ver = g_strndup((const char *)poll_pkt + OFF_FW_VER, LEN_FW_VER);
-       devc->dev_info.serial_num = RL32(poll_pkt + OFF_SERIAL);
+       devc->dev_info.model_name = g_strndup((const char *)&poll_pkt[OFF_MODEL], LEN_MODEL);
+       devc->dev_info.fw_ver = g_strndup((const char *)&poll_pkt[OFF_FW_VER], LEN_FW_VER);
+       devc->dev_info.serial_num = read_u32le(&poll_pkt[OFF_SERIAL]);
 
        return SR_OK;
 }