juntek-jds6600: speedup serial communication, reduce timeouts

Wire traffic captures for serial-over-USB communication between the host
and the signal generator suggests that: Device firmware's reponses are
immediate. The 115200 UART is the limiting factor, USB FS adds little.

Comment on these findings for later reference. Reduce timeouts and
remove delays where appropriate. This dramatically speeds up the scan
for the device, and subsequent communication. Identification and getting
the current configuration takes a total of 70ms after this change.

diff --git a/src/hardware/juntek-jds6600/protocol.c b/src/hardware/juntek-jds6600/protocol.c
index 92b888f6ad6e5b443305dfd290103433f925ebcf..5759c6a07722456b4d7eb78112b7d56aa8868348 100644 (file)
--- a/src/hardware/juntek-jds6600/protocol.c
+++ b/src/hardware/juntek-jds6600/protocol.c
@@ -53,7 +53,10 @@
  *   need to read back, this library driver doesn't.
  *
  * Implementation details:
- * - Communicates via USB CDC at 115200/8n1 (virtual COM port).
+ * - Communicates via USB CDC at 115200/8n1 (virtual COM port). The user
+ *   perceives a USB attached device (full speed, CDC/ACM class). The
+ *   implementation needs to remember that a WCH CH340G forwards data
+ *   to a microcontroller. Maximum throughput is in the 10KiB/s range.
  * - Requests are in text format. Start with a ':' colon, followed by a
  *   single letter instruction opcode, followed by a number which either
  *   addresses a parameter (think hardware register) or storage slot for
@@ -144,11 +147,36 @@
  */
 #define MAX_RSP_LENGTH (8 + 2048 * 5)
 
-/* Times are in milliseconds. */
-#define DELAY_AFTER_WRITE      10
+/*
+ * Times are in milliseconds.
+ * - Delay after transmission was an option during initial development.
+ *   Has become obsolete. Support remains because it doesn't harm.
+ * - Delay after flash is essential when writing multiple waveforms to
+ *   the device. Not letting more idle time pass after successful write
+ *   and reception of the "ok" response, and before the next write, will
+ *   result in corrupted waveform storage in the device. The next wave
+ *   that is written waveform will start with several hundred samples
+ *   of all-one bits.
+ * - Timeout per receive attempt at the physical layer can be short.
+ *   Experience suggests that 2ms are a good value. Reception ends when
+ *   the response termination was seen. Or when no receive data became
+ *   available within that per-attemt timeout, and no higher level total
+ *   timeout was specified. Allow some slack for USB FS frame intervals.
+ * - Timeout for identify attempts at the logical level can be short.
+ *   Captures of the microcontroller communication suggest that firmware
+ *   responds immediately (within 2ms). So 10ms per identify attempt
+ *   are plenty for successful communication, yet quick enough to not
+ *   stall on missing peripherals.
+ * - Timeout for waveform upload/download needs to be huge. Textual
+ *   presentation of 2k samples with 12 significant bits (0..4095 range)
+ *   combined with 115200bps UART communication result in a 1s maximum
+ *   transfer time per waveform. So 1.2s is a good value.
+ */
+#define DELAY_AFTER_SEND       0
 #define DELAY_AFTER_FLASH      100
-#define TIMEOUT_READ_CHUNK     20
-#define TIMEOUT_IDENTIFY       200
+#define TIMEOUT_READ_CHUNK     2
+#define TIMEOUT_IDENTIFY       10
+#define TIMEOUT_WAVEFORM       1200
 
 /* Instruction codes. Read/write parameters/waveforms. */
 #define INSN_WRITE_PARA        'w'
@@ -498,7 +526,6 @@ static int serial_recv_textline(const struct sr_dev_inst *sdi,
        log_raw_bytes("serial RX bytes: <-- ", s);
        sr_dbg("serial read, unterminated response, discarded");
 
-       sr_dbg("serial read, no EOL seen");
        return SR_ERR_DATA;
 }
 
@@ -815,7 +842,7 @@ static int quick_send_read_then_recv(const struct sr_dev_inst *sdi,
 
        g_string_truncate(s, 0);
        append_insn_read_para(s, insn, idx);
-       ret = serial_send_textline(sdi, s, DELAY_AFTER_WRITE);
+       ret = serial_send_textline(sdi, s, DELAY_AFTER_SEND);
        if (ret != SR_OK)
                return ret;
 
@@ -860,7 +887,7 @@ static int quick_send_write_then_recv_ok(const struct sr_dev_inst *sdi,
        va_start(args, fmt);
        append_insn_write_para_va(s, insn, idx, fmt, args);
        va_end(args);
-       ret = serial_send_textline(sdi, s, DELAY_AFTER_WRITE);
+       ret = serial_send_textline(sdi, s, DELAY_AFTER_SEND);
        if (ret != SR_OK)
                return ret;