prevent abort via polling during jtag_reset

Observed:

  openocd: core.c:318: jtag_checks: Assertion `jtag_trst == 0' failed.

The issue was that nothing disabled background polling during calls
from the TCL shell to "jtag_reset 1 1".  Fix by moving the existing
poll-disable mechanism to the JTAG layer where it belongs, and then
augmenting it to always pay attention to TRST and SRST.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/jtag/core.c

@@ -137,6 +137,32 @@ int jtag_error_clear(void)
 	return temp;
 }
 
+/************/
+
+static bool jtag_poll = 1;
+
+bool is_jtag_poll_safe(void)
+{
+	/* Polling can be disabled explicitly with set_enabled(false).
+	 * It is also implicitly disabled while TRST is active and
+	 * while SRST is gating the JTAG clock.
+	 */
+	if (!jtag_poll || jtag_trst != 0)
+		return false;
+	return jtag_srst == 0 || (jtag_reset_config & RESET_SRST_NO_GATING);
+}
+
+bool jtag_poll_get_enabled(void)
+{
+	return jtag_poll;
+}
+
+void jtag_poll_set_enabled(bool value)
+{
+	jtag_poll = value;
+}
+
+/************/
 
 jtag_tap_t *jtag_all_taps(void)
 {

src/jtag/jtag.h

@@ -737,4 +737,21 @@ int jtag_get_error(void);
  */
 int jtag_error_clear(void);
 
+/**
+ * Return true if it's safe for a background polling task to access the
+ * JTAG scan chain.  Polling may be explicitly disallowed, and is also
+ * unsafe while nTRST is active or the JTAG clock is gated off.,
+ */
+bool is_jtag_poll_safe(void);
+
+/**
+ * Return flag reporting whether JTAG polling is disallowed.
+ */
+bool jtag_poll_get_enabled(void);
+
+/**
+ * Assign flag reporting whether JTAG polling is disallowed.
+ */
+void jtag_poll_set_enabled(bool value);
+
 #endif /* JTAG_H */