adiv5_swdp: allow to connect to a sleeping target by pulling SRST

This patch introduces a new command, "connect_srst [enable|disable]"
which allows to enable special mode in which SRST would be pulled low
before the SWD scan till attaching to a target.

Since on Cortex-Mx the SRST signal doesn't gate JTAG and SWD, it's
possible to connect to a target while holding reset, ask it to stop at
reset vector and only then deassert reset, thus allowing to attach to
the kind of firmware that goes immediately to sleep or disables
debugging by other means early on start.

Tested on an STM32VLDiscovery board with STM32F100 configured to go to
STOP mode and executing WFI in the very beginning of main().

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

src/adiv5_swdp.c

@@ -27,6 +27,7 @@
 #include "adiv5.h"
 
 #include "swdptap.h"
+#include "jtagtap.h"
 
 #include "command.h"
 
@@ -53,6 +54,8 @@ int adiv5_swdp_scan(void)
 	ADIv5_DP_t *dp = (void*)calloc(1, sizeof(*dp));
 
 	swdptap_init();
+	if(connect_assert_srst)
+		jtagtap_srst(true); /* will be deasserted after attach */
 	/* Read the SW-DP IDCODE register to syncronise */
 	/* This could be done with adiv_swdp_low_access(), but this doesn't
 	 * allow the ack to be checked here. */

src/command.c

@@ -46,6 +46,7 @@ static bool cmd_jtag_scan(target *t, int argc, char **argv);
 static bool cmd_swdp_scan(void);
 static bool cmd_targets(target *t);
 static bool cmd_morse(void);
+static bool cmd_connect_srst(target *t, int argc, const char **argv);
 #ifdef PLATFORM_HAS_TRACESWO
 static bool cmd_traceswo(void);
 #endif
@@ -57,6 +58,7 @@ const struct command_s cmd_list[] = {
 	{"swdp_scan", (cmd_handler)cmd_swdp_scan, "Scan SW-DP for devices" },
 	{"targets", (cmd_handler)cmd_targets, "Display list of available targets" },
 	{"morse", (cmd_handler)cmd_morse, "Display morse error message" },
+	{"connect_srst", (cmd_handler)cmd_connect_srst, "Configure connect under SRST: (enable|disable)" },
 #ifdef PLATFORM_HAS_TRACESWO
 	{"traceswo", (cmd_handler)cmd_traceswo, "Start trace capture" },
 #endif
@@ -209,6 +211,17 @@ bool cmd_morse(void)
 	return true;
 }
 
+static bool cmd_connect_srst(target *t, int argc, const char **argv)
+{
+	(void)t;
+	if (argc == 1)
+		gdb_outf("Assert SRST during connect: %s\n",
+			 connect_assert_srst ? "enabled" : "disabled");
+	else
+		connect_assert_srst = !strcmp(argv[1], "enable");
+	return true;
+}
+
 #ifdef PLATFORM_HAS_TRACESWO
 static bool cmd_traceswo(void)
 {

src/cortexm.c

@@ -385,7 +385,7 @@ cortexm_attach(struct target_s *target)
 
 	target_halt_request(target);
 	tries = 10;
-	while(!target_halt_wait(target) && --tries)
+	while(!connect_assert_srst && !target_halt_wait(target) && --tries)
 		platform_delay(2);
 	if(!tries)
 		return false;
@@ -429,6 +429,9 @@ cortexm_attach(struct target_s *target)
 	target->clear_hw_wp = cortexm_clear_hw_wp;
 	target->check_hw_wp = cortexm_check_hw_wp;
 
+	if(connect_assert_srst)
+		jtagtap_srst(false);
+
 	return true;
 }
 
@@ -546,7 +549,8 @@ cortexm_reset(struct target_s *target)
 {
 	ADIv5_AP_t *ap = adiv5_target_ap(target);
 
-	jtagtap_srst();
+	jtagtap_srst(true);
+	jtagtap_srst(false);
 
 	/* Read DHCSR here to clear S_RESET_ST bit before reset */
 	adiv5_ap_mem_read(ap, CORTEXM_DHCSR);

src/include/jtagtap.h

@@ -29,7 +29,7 @@ int jtagtap_init(void);
 
 void jtagtap_reset(void);
 
-void jtagtap_srst(void);
+void jtagtap_srst(bool assert);
 
 uint8_t jtagtap_next(const uint8_t TMS, const uint8_t TDI);
 /* tap_next executes one state transision in the JTAG TAP state machine:

src/include/target.h

@@ -188,6 +188,7 @@ struct target_command_s {
 };
 
 extern target *target_list;
+extern bool connect_assert_srst;
 
 target *target_new(unsigned size);
 void target_list_free(void);

src/platforms/libftdi/jtagtap.c

@@ -67,7 +67,7 @@ void jtagtap_reset(void)
 	jtagtap_soft_reset();
 }
 
-void jtagtap_srst(void)
+void jtagtap_srst(bool assert)
 {
 	platform_buffer_flush();
 	//ftdi_write_data(ftdic, "\x80\x88\xAB", 3);

src/platforms/stm32/jtagtap.c

@@ -50,13 +50,16 @@ void jtagtap_reset(void)
 	jtagtap_soft_reset();
 }
 
-void jtagtap_srst(void)
+void jtagtap_srst(bool assert)
 {
-#ifdef SRST_PORT
+	(void)assert;
-	volatile int i;
+#ifdef SRST_SET_VAL
-	gpio_set(SRST_PORT, SRST_PIN);
+	SRST_SET_VAL(assert);
-	for(i = 0; i < 10000; i++) asm("nop");
+	if(assert) {
-	gpio_clear(SRST_PORT, SRST_PIN);
+		int i;
+		for(i = 0; i < 10000; i++)
+			asm volatile("nop");
+	}
 #endif
 }
 

src/target.c

@@ -24,6 +24,7 @@
 #include <stdlib.h>
 
 target *target_list = NULL;
+bool connect_assert_srst;
 
 target *target_new(unsigned size)
 {