stlink: fix computation of trace prescaler

Use integer rounding for the computation of prescaler.
Improve the test of prescaler range, knowing its value would be
decremented before being written in TPIU ACPR.

Change-Id: I041dde1dca41323904e36a6b6975028a6de902b3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5909
Tested-by: jenkins

diff --git a/src/jtag/drivers/stlink_usb.c b/src/jtag/drivers/stlink_usb.c
index 8ab73cd12752d677dfda640ec2416dcc60cdb32f..c54e2cc192e0900eef4392fc0581b2b82eb5b57e 100644 (file)
--- a/src/jtag/drivers/stlink_usb.c
+++ b/src/jtag/drivers/stlink_usb.c
@@ -2989,7 +2989,6 @@ static int stlink_config_trace(void *handle, bool enabled,
                uint16_t *prescaler)
 {
        struct stlink_usb_handle_s *h = handle;
-       uint16_t presc;
 
        if (enabled && (!(h->version.flags & STLINK_F_HAS_TRACE) ||
                        pin_protocol != TPIU_PIN_PROTOCOL_ASYNC_UART)) {
@@ -3012,12 +3011,17 @@ static int stlink_config_trace(void *handle, bool enabled,
        if (!*trace_freq)
                *trace_freq = max_trace_freq;
 
-       presc = traceclkin_freq / *trace_freq;
-
-       if (traceclkin_freq % *trace_freq > 0)
-               presc++;
+       unsigned int presc = (traceclkin_freq + *trace_freq / 2) / *trace_freq;
+       if (presc == 0 || presc > TPIU_ACPR_MAX_SWOSCALER + 1) {
+               LOG_ERROR("SWO frequency is not suitable. Please choose a different "
+                       "frequency.");
+               return ERROR_FAIL;
+       }
 
-       if (presc > TPIU_ACPR_MAX_SWOSCALER) {
+       /* Probe's UART speed must be within 3% of the TPIU's SWO baud rate. */
+       unsigned int max_deviation = (traceclkin_freq * 3) / 100;
+       if (presc * *trace_freq < traceclkin_freq - max_deviation ||
+                       presc * *trace_freq > traceclkin_freq + max_deviation) {
                LOG_ERROR("SWO frequency is not suitable. Please choose a different "
                        "frequency.");
                return ERROR_FAIL;