scpi_usbtmc_libusb: Retry if a Bulk-IN request starts with an empty packet

This seems to make the Rigol DS1054Z work.  It's still a bit janky --
on a live capture, sample 688 (zero-based) out of the 1200-sample
frame seems to consistently contain garbage.  I'm not sure what's
going on.

diff --git a/src/scpi/scpi_usbtmc_libusb.c b/src/scpi/scpi_usbtmc_libusb.c
index c8bcd0dc4e3e42cd389fc9a243096f1866c61a19..2da67030ed2495c41e9a0c3dcd56f9255d72f201 100644 (file)
--- a/src/scpi/scpi_usbtmc_libusb.c
+++ b/src/scpi/scpi_usbtmc_libusb.c
@@ -524,19 +524,36 @@ static int scpi_usbtmc_bulkin_start(struct scpi_usbtmc_libusb *uscpi,
                                     uint8_t *transfer_attributes)
 {
        struct sr_usb_dev_inst *usb = uscpi->usb;
-       int ret, transferred, message_size;
+       int ret, transferred, message_size, tries;
+
+       for (tries = 0; ; tries++) {
+               ret = libusb_bulk_transfer(usb->devhdl, uscpi->bulk_in_ep, data,
+                                          size, &transferred,
+                                          TRANSFER_TIMEOUT);
+               if (ret < 0) {
+                       sr_err("USBTMC bulk in transfer error: %s.",
+                              libusb_error_name(ret));
+                       return SR_ERR;
+               }
 
-       ret = libusb_bulk_transfer(usb->devhdl, uscpi->bulk_in_ep, data, size,
-                                  &transferred, TRANSFER_TIMEOUT);
-       if (ret < 0) {
-               sr_err("USBTMC bulk in transfer error: %s.",
-                      libusb_error_name(ret));
-               return SR_ERR;
-       }
+               if (transferred == 0 && tries < 1) {
+                       /*
+                        * The DEV_DEP_MSG_IN message is empty, and the TMC
+                        * spec says it should at least contain a header.
+                        * The Rigol DS1054Z seems to do this sometimes, and
+                        * it follows up with a valid message.  Give the device
+                        * one more chance to send a header.
+                        */
+                       sr_warn("USBTMC bulk in start was empty; retrying\n");
+                       continue;
+               }
 
-       if (transferred < USBTMC_BULK_HEADER_SIZE) {
-               sr_err("USBTMC bulk in returned too little data: %d/%d bytes\n", transferred, size);
-               return SR_ERR;
+               if (transferred < USBTMC_BULK_HEADER_SIZE) {
+                       sr_err("USBTMC bulk in returned too little data: %d/%d bytes\n", transferred, size);
+                       return SR_ERR;
+               }
+
+               break;
        }
 
        if (usbtmc_bulk_in_header_read(data, msg_id, uscpi->bTag, &message_size,