speed up ftdi by reorder to out-in

When the ftdi driver calls finally the mpsse_flush function, it first
initiate the USB in and finally the corresponding USB out transaction.
Because data in is requested too early the USB device will always answer
the first USB in by a NAK. That can prevented by a simple reordering of
the out and then the in transfer and can improve the Jtag performance for
high JTAG clock rates.

Change-Id: I17abf1487c914c92e2e447ee6d30562ef629f327
Signed-off-by: Peter Henn <Peter.Henn@web.de>
Reviewed-on: http://openocd.zylin.com/942
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Xiaofan <xiaofanc@gmail.com>

diff --git a/src/jtag/drivers/mpsse.c b/src/jtag/drivers/mpsse.c
index d6cbc8404c6e93330f3f38d6ed87317776b0e617..41429693c15fe8a347de26cbf3e2d3a9b526e4b8 100644 (file)
--- a/src/jtag/drivers/mpsse.c
+++ b/src/jtag/drivers/mpsse.c
@@ -779,11 +779,8 @@ int mpsse_flush(struct mpsse_ctx *ctx)
        if (ctx->read_count) {
                buffer_write_byte(ctx, 0x87); /* SEND_IMMEDIATE */
                read_result.done = false;
-               read_transfer = libusb_alloc_transfer(0);
-               libusb_fill_bulk_transfer(read_transfer, ctx->usb_dev, ctx->in_ep, ctx->read_chunk,
-                       ctx->read_chunk_size, read_cb, &read_result,
-                       ctx->usb_read_timeout);
-               retval = libusb_submit_transfer(read_transfer);
+               /* delay read transaction to ensure the FTDI chip can support us with data
+                  immediately after processing the MPSSE commands in the write transaction */
        }
 
        struct transfer_result write_result = { .ctx = ctx, .done = false };
@@ -792,6 +789,14 @@ int mpsse_flush(struct mpsse_ctx *ctx)
                ctx->write_count, write_cb, &write_result, ctx->usb_write_timeout);
        retval = libusb_submit_transfer(write_transfer);
 
+       if (ctx->read_count) {
+               read_transfer = libusb_alloc_transfer(0);
+               libusb_fill_bulk_transfer(read_transfer, ctx->usb_dev, ctx->in_ep, ctx->read_chunk,
+                       ctx->read_chunk_size, read_cb, &read_result,
+                       ctx->usb_read_timeout);
+               retval = libusb_submit_transfer(read_transfer);
+       }
+
        /* Polling loop, more or less taken from libftdi */
        while (!write_result.done || !read_result.done) {
                retval = libusb_handle_events(ctx->usb_ctx);