initial "transport" framework

This adds the guts of a transport framework with initialization,
which should work with current JTAG-only configurations (tested
with FT2232).

Each debug adapter can declare the transports it supports, and
exactly one transport is initialized.  (with its commands) in
any given OpenOCD session.

  * Define a new "struct transport with init hooks and a few
 "transport"  subcommands to support it:

     "list" ... list the transports configured (just "jtag" for now)
     "select" ... makes the debug session use that transport
     "init" ... initializes the selected transport (internal)

  * "interface_transports" ... declares transports the current interface
    can support.  (Some will do this from C code instead, when there are
    no hardware versioning (or other) issues to prevent it.

Plus some FT2232 tweaks, including a few to streamline upcoming
support for an SWD transport (initially for Luminary adapters).

Eventually src/jtag should probably become src/transport, moving
jtag-specific stuff  to transport/jtag.

Signed-off-by: David Brownell <db@helium.(none)>

10 files changed, 633 insertions, 30 deletions

diff --git a/NEWS b/NEWS
index db4485da..2dd14cc6 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,9 @@ JTAG Layer:
 
 Boundary Scan:
 
+Transport framework core ... supporting future work for SWD, SPI, and other
+non-JTAG ways to debug targets or program flash.
+
 Target Layer:
 	ARM:
 		- basic semihosting support (ARMv7M).
diff --git a/doc/openocd.texi b/doc/openocd.texi
index a3ca1247..a765727c 100644
--- a/doc/openocd.texi
+++ b/doc/openocd.texi
@@ -2096,6 +2096,14 @@ target.
 List the debug adapter drivers that have been built into
 the running copy of OpenOCD.
 @end deffn
+@deffn Command {interface transports} transport_name+
+Specifies the transports supported by this debug adapter.
+The adapter driver builds-in similar knowledge; use this only
+when external configuration (such as jumpering) changes what
+the hardware can support.
+@end deffn
+
+
 
 @deffn Command {adapter_name}
 Returns the name of the debug adapter driver being used.
@@ -2428,7 +2436,41 @@ Turn power switch to target on/off.
 No arguments: print status.
 @end deffn
 
-@end deffn
+@section Transport Configuration
+As noted earlier, depending on the version of OpenOCD you use,
+and the debug adapter you are using,
+several transports may be available to
+communicate with debug targets (or perhaps to program flash memory).
+@deffn Command {transport list}
+displays the names of the transports supported by this
+version of OpenOCD.
+@end deffn
+
+@deffn Command {transport select} transport_name
+Select which of the supported transports to use in this OpenOCD session.
+The transport must be supported by the debug adapter hardware  and by the
+version of OPenOCD you are using (including the adapter's driver).
+No arguments: print selected transport..
+@end deffn
+
+@subsection JTAG Transport
+JTAG is the original transport supported by OpenOCD, and most
+of the OpenOCD commands support it.
+JTAG transports expose a chain of one or more Test Access Points (TAPs),
+each of which must be explicitly declared.
+JTAG supports both debugging and boundary scan testing.
+Flash programming support is built on top of debug support.
+@subsection SWD ransport
+SWD (Serial Wire Debug) is an ARM-specific transport which exposes one
+Debug Access Point (DAP, which must be explicitly declared.
+(SWD uses fewer signal wires than JTAG.)
+SWD is debug-oriented, and does not support  boundary scan testing.
+Flash programming support is built on top of debug support.
+(Some processors support both JTAG and SWD.)
+@subsection SPI ransport
+The Serial Peripheral Interface (SPI) is a general purpose transport
+which uses four wire signaling.  Some processors use it as part of a
+solution for flash programming.
 
 @anchor{JTAG Speed}
 @section JTAG Speed
diff --git a/src/jtag/Makefile.am b/src/jtag/Makefile.am
index 910affc1..59cd8fff 100644
--- a/src/jtag/Makefile.am
+++ b/src/jtag/Makefile.am
@@ -58,6 +58,7 @@ libjtag_la_SOURCES = \
 	interface.c \
 	interfaces.c \
 	tcl.c \
+	transport.c \
 	$(DRIVERFILES)
 
 noinst_HEADERS = \
@@ -67,6 +68,7 @@ noinst_HEADERS = \
 	interfaces.h \
 	minidriver.h \
 	jtag.h \
+	transport.h \
 	minidriver/minidriver_imp.h \
 	minidummy/jtag_minidriver.h
 
diff --git a/src/jtag/adapter.c b/src/jtag/adapter.c
index 50e2a022..15b3ddc2 100644
--- a/src/jtag/adapter.c
+++ b/src/jtag/adapter.c
@@ -35,6 +35,7 @@
 #include "minidriver.h"
 #include "interface.h"
 #include "interfaces.h"
+#include "transport.h"
 
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
@@ -92,6 +93,25 @@ static int default_srst_asserted(int *srst_asserted)
 	return ERROR_OK;
 }
 
+COMMAND_HANDLER(interface_transport_command)
+{
+	char **transports;
+	int retval;
+
+	retval = CALL_COMMAND_HANDLER(transport_list_parse, &transports);
+	if (retval != ERROR_OK) {
+		return retval;
+
+	retval = allow_transports(CMD_CTX, (const char **)transports);
+	if (retval != ERROR_OK) {
+		for (unsigned i = 0; transports[i]; i++)
+			free(transports[i]);
+		free(transports);
+	}
+	}
+	return retval;
+}
+
 COMMAND_HANDLER(handle_interface_list_command)
 {
 	if (strcmp(CMD_NAME, "interface_list") == 0 && CMD_ARGC > 0)
@@ -452,6 +472,13 @@ static const struct command_registration interface_command_handlers[] = {
 		.usage = "driver_name",
 	},
 	{
+		.name = "interface_transports",
+		.handler = interface_transport_command,
+		.mode = COMMAND_CONFIG,
+		.help = "Declare transports the interface supports.",
+		.usage = "transport ... ",
+	},
+	{
 		.name = "interface_list",
 		.handler = handle_interface_list_command,
 		.mode = COMMAND_ANY,
diff --git a/src/jtag/core.c b/src/jtag/core.c
index 782f10f7..1643d9b6 100644
--- a/src/jtag/core.c
+++ b/src/jtag/core.c
@@ -33,11 +33,15 @@
 
 #include "jtag.h"
 #include "interface.h"
+#include "transport.h"
 
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
 #endif
 
+/* SVF and XSVF are higher level JTAG command sets (for boundary scan) */
+#include "svf/svf.h"
+#include "xsvf/xsvf.h"
 
 /// The number of JTAG queue flushes (for profiling and debugging purposes).
 static int jtag_flush_queue_count;
@@ -1348,6 +1352,21 @@ int adapter_init(struct command_context *cmd_ctx)
 		return ERROR_JTAG_INIT_FAILED;
 	}
 
+	/* LEGACY SUPPORT ... adapter drivers  must declare what
+	 * transports they allow.  Until they all do so, assume
+	 * the legacy drivers are JTAG-only
+	 */
+	if (!transports_are_declared()) {
+		static const char *jtag_only[] = { "jtag", NULL, };
+		LOG_ERROR("Adapter driver '%s' did not declare "
+			"which transports it allows; assuming"
+			"JTAG-only", jtag->name);
+		int retval = allow_transports(cmd_ctx, jtag_only);
+		if (retval != ERROR_OK)
+			return retval;
+	}
+
+
 	int requested_khz = jtag_get_speed_khz();
 	int actual_khz = requested_khz;
 	int retval = jtag_get_speed_readable(&actual_khz);
@@ -1706,3 +1725,45 @@ unsigned jtag_get_ntrst_assert_width(void)
 {
 	return jtag_ntrst_assert_width;
 }
+
+static int jtag_select(struct command_context *ctx)
+{
+	int retval;
+
+	/* NOTE:  interface init must already have been done.
+	 * That works with only C code ... no Tcl glue required.
+	 */
+
+
+	retval = jtag_register_commands(ctx);
+
+	if (retval != ERROR_OK)
+		return retval;
+
+	retval = svf_register_commands(ctx);
+
+	if (retval != ERROR_OK)
+		return retval;
+
+	return xsvf_register_commands(ctx);
+}
+
+static struct transport jtag_transport = {
+	.name = "jtag",
+	.select = jtag_select,
+	.init = jtag_init,
+};
+
+static void jtag_constructor(void) __attribute__((constructor));
+static void jtag_constructor(void)
+{
+	transport_register(&jtag_transport);
+}
+
+/** Returns true if the current debug session
+ * is using JTAG as its transport.
+ */
+bool transport_is_jtag(void)
+{
+	return get_current_transport() == &jtag_transport;
+}
diff --git a/src/jtag/drivers/ft2232.c b/src/jtag/drivers/ft2232.c
index f315d699..f02ae626 100644
--- a/src/jtag/drivers/ft2232.c
+++ b/src/jtag/drivers/ft2232.c
@@ -81,6 +81,7 @@
 
 /* project specific includes */
 #include <jtag/interface.h>
+#include <jtag/transport.h>
 #include <helper/time_support.h>
 
 #if IS_CYGWIN == 1
@@ -167,6 +168,7 @@ struct ft2232_layout {
 	void (*reset)(int trst, int srst);
 	void (*blink)(void);
 	int channel;
+	const char **transports;
 };
 
 /* init procedures for supported layouts */
@@ -210,6 +212,13 @@ static void turtle_jtag_blink(void);
 static void signalyzer_h_blink(void);
 static void ktlink_blink(void);
 
+/* common transport support options */
+static const char *jtag_only[] = { "jtag", NULL };
+
+
+//static const char *jtag_and_swd[] = { "jtag", "swd", NULL };
+#define jtag_and_swd NULL
+
 static const struct ft2232_layout  ft2232_layouts[] =
 {
 	{ .name = "usbjtag",
@@ -235,10 +244,12 @@ static const struct ft2232_layout  ft2232_layouts[] =
 	{ .name = "evb_lm3s811",
 		.init = lm3s811_jtag_init,
 		.reset = ftx23_reset,
+		.transports = jtag_and_swd,
 	},
 	{ .name = "luminary_icdi",
 		.init = icdi_jtag_init,
 		.reset = ftx23_reset,
+		.transports = jtag_and_swd,
 	},
 	{ .name = "olimex-jtag",
 		.init = olimex_jtag_init,
@@ -2393,7 +2404,7 @@ static int ft2232_init(void)
 /** Updates defaults for DBUS signals:  the four JTAG signals
  * (TCK, TDI, TDO, TMS) and * the four GPIOL signals.
  */
-static inline void ftx232_init_head(void)
+static inline void ftx232_dbus_init(void)
 {
 	low_output    = 0x08;
 	low_direction = 0x0b;
@@ -2403,7 +2414,7 @@ static inline void ftx232_init_head(void)
  * the four GPIOL signals.  Initialization covers value and direction,
  * as customized for each layout.
  */
-static int ftx232_init_tail(void)
+static int ftx232_dbus_write(void)
 {
 	uint8_t  buf[3];
 	uint32_t bytes_written;
@@ -2452,19 +2463,19 @@ static int usbjtag_init(void)
 	 * NOTE:  This is now _specific_ to the "usbjtag" layout.
 	 * Don't try cram any more layouts into this.
 	 */
-	ftx232_init_head();
+	ftx232_dbus_init();
 
 	nTRST    = 0x10;
 	nTRSTnOE = 0x10;
 	nSRST    = 0x40;
 	nSRSTnOE = 0x40;
 
-	return ftx232_init_tail();
+	return ftx232_dbus_write();
 }
 
 static int lm3s811_jtag_init(void)
 {
-	ftx232_init_head();
+	ftx232_dbus_init();
 
 	/* There are multiple revisions of LM3S811 eval boards:
 	 * - Rev B (and older?) boards have no SWO trace support.
@@ -2478,12 +2489,12 @@ static int lm3s811_jtag_init(void)
 	low_output    = 0x88;
 	low_direction = 0x8b;
 
-	return ftx232_init_tail();
+	return ftx232_dbus_write();
 }
 
 static int icdi_jtag_init(void)
 {
-	ftx232_init_head();
+	ftx232_dbus_init();
 
 	/* Most Luminary eval boards support SWO trace output,
 	 * and should use this "luminary_icdi" layout.
@@ -2495,18 +2506,18 @@ static int icdi_jtag_init(void)
 	low_output    = 0x88;
 	low_direction = 0xcb;
 
-	return ftx232_init_tail();
+	return ftx232_dbus_write();
 }
 
 static int signalyzer_init(void)
 {
-	ftx232_init_head();
+	ftx232_dbus_init();
 
 	nTRST    = 0x10;
 	nTRSTnOE = 0x10;
 	nSRST    = 0x20;
 	nSRSTnOE = 0x20;
-	return ftx232_init_tail();
+	return ftx232_dbus_write();
 }
 
 static int axm0432_jtag_init(void)
@@ -3195,7 +3206,11 @@ COMMAND_HANDLER(ft2232_handle_layout_command)
 	for (const struct ft2232_layout *l = ft2232_layouts; l->name; l++) {
 		if (strcmp(l->name, CMD_ARGV[0]) == 0) {
 			layout = l;
-			return ERROR_OK;
+			/* This may also select the transport
+			 * if we only suppport one of them.
+			 */
+			return allow_transports(CMD_CTX,
+				l->transports ? : jtag_only);
 		}
 	}
 
diff --git a/src/jtag/jtag.h b/src/jtag/jtag.h
index 97ceb013..3b0a145b 100644
--- a/src/jtag/jtag.h
+++ b/src/jtag/jtag.h
@@ -689,4 +689,6 @@ void jtag_poll_set_enabled(bool value);
  * level APIs that are used in inner loops. */
 #include <jtag/minidriver.h>
 
+bool transport_is_jtag(void);
+
 #endif /* JTAG_H */
diff --git a/src/jtag/transport.c b/src/jtag/transport.c
new file mode 100644
index 00000000..cee8ffb3
--- /dev/null
+++ b/src/jtag/transport.c
@@ -0,0 +1,373 @@
+/*
+ * Copyright (c) 2010 by David Brownell
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+/** @file
+ * Infrastructure for specifying and managing the transport protocol
+ * used in a given debug or programming session.
+ *
+ * Examples of "debug-capable" transports are JTAG or SWD.
+ * Additionally, JTAG supports boundary scan testing.
+ *
+ * Examples of "programming-capable" transports include SPI or UART;
+ * those are used (often mediated by a ROM bootloader) for ISP style
+ * programming, to perform an initial load of code into flash, or
+ * sometimes into SRAM.  Target code could use "variant" options to
+ * decide how to use such protocols.  For example, Cortex-M3 cores
+ * from TI/Luminary and from NXP use different protocols for for
+ * UART or SPI based firmware loading.
+ *
+ * As a rule, there are protocols layered on top of the transport.
+ * For example, different chip families use JTAG in different ways
+ * for debugging.  Also, each family that supports programming over
+ * a UART link for initial firmware loading tends to define its own
+ * messaging and error handling.
+ */
+
+#include <helper/log.h>
+
+#include "transport.h"
+
+/*-----------------------------------------------------------------------*/
+
+/*
+ * Infrastructure internals
+ */
+
+/** List of transports known to OpenOCD. */
+static struct transport *transport_list;
+
+/**
+ * NULL-terminated Vector of names of transports which the
+ * currently selected debug adapter supports.  This is declared
+ * by the time that adapter is fully set up.
+ */
+static const char **allowed_transports;
+
+/** * The transport being used for the current OpenOCD session.  */
+static struct transport *session;
+
+static  int transport_select(struct command_context *ctx, const char *name)
+{
+	/* name may only identify a known transport;
+	 * caller guarantees session's transport isn't yet set.*/
+	for (struct transport *t = transport_list; t; t = t->next) {
+			if (strcmp(t->name, name) == 0) {
+				int retval = t->select(ctx);
+			/* select() registers commands specific to this
+			 * transport, and may also reset the link, e.g.
+			 * forcing it to JTAG or SWD mode.
+			 */
+			if (retval == ERROR_OK)
+				session = t;
+			else
+				LOG_ERROR("Error %d selecting '%s' as "
+					"transport", retval, t->name);
+			return retval;
+		}
+	}
+
+	LOG_ERROR("No transport named '%s' is available.", name);
+	return ERROR_FAIL;
+}
+
+/**
+ * Called by debug adapter drivers, or affiliated Tcl config scripts,
+ * to declare the set of transports supported by an adapter.  When
+ * there is only one member of that set, it is automatically selected.
+ */
+int allow_transports(struct command_context *ctx, const char **vector)
+{
+	/* NOTE:  caller is required to provide only a list
+	 * of *valid* transport names
+	 *
+	 * REVISIT should we validate that?  and insist there's
+	 * at least one non-NULL element in that list?
+	 */
+	if (allowed_transports != NULL || session) {
+		LOG_ERROR("Can't modify the set of allowed transports.");
+		return ERROR_FAIL;
+	}
+
+
+	allowed_transports = vector;
+
+	/* autoselect if there's no choice ... */
+	if (!vector[1]) {
+		LOG_INFO("only one transport option; autoselect '%s'",
+				vector[0]);
+		return transport_select(ctx, vector [0]);
+	} else {
+		while (*vector)
+			LOG_DEBUG("allow transport '%s'", *vector++);
+		return ERROR_OK;
+	}
+}
+
+
+/**
+ * Used to verify corrrect adapter driver initialization.
+ *
+ * @returns true iff the adapter declared one or more transports.
+ */
+bool transports_are_declared(void)
+{
+	return allowed_transports != NULL;
+}
+
+/**
+ * Registers a transport.  There are general purpose transports
+ * (such as JTAG), as well as relatively proprietary ones which are
+ * specific to a given chip (or chip family).
+ *
+ * Code implementing a transport needs to register it before it can
+ * be selected and then activated.  This is a dynamic process, so
+ * that chips (and families) can define transports as needed (without
+ * nneeding error-prone static tables).
+ *
+ * @param new_transport the transport being registered.  On a
+ * successful return, this memory is owned by the transport framework.
+ *
+ * @returns ERROR_OK on success, else a fault code.
+ */
+int transport_register(struct transport *new_transport)
+{
+	struct transport *t;
+
+	for (t = transport_list; t; t = t->next) {
+		if (strcmp(t->name, new_transport->name) == 0) {
+			LOG_ERROR("transport name already used");
+			return ERROR_FAIL;
+		}
+	}
+
+	if (!new_transport->select || !new_transport->init) {
+		LOG_ERROR("invalid transport %s", new_transport->name);
+	}
+
+	/* splice this into the list */
+	new_transport->next = transport_list;
+	transport_list = new_transport;
+	LOG_DEBUG("register '%s'", t->name);
+
+	return ERROR_OK;
+}
+
+/**
+ * Returns the transport currently being used by this debug or
+ * programming session.
+ *
+ * @returns handle to the read-only transport entity.
+ */
+struct transport *get_current_transport(void)
+{
+
+	/* REVISIT -- constify */
+	return session;
+}
+
+
+/*-----------------------------------------------------------------------*/
+
+/*
+ * Infrastructure for Tcl interface to transports.
+ */
+
+/**
+ * Makes and stores a copy of a set of transports passed as
+ * parameters to a command.
+ *
+ * @param vector where the resulting copy is stored, as an argv-style
+ *	NULL-terminated vector.
+ */
+COMMAND_HELPER(transport_list_parse, char ***vector)
+{
+	char **argv;
+	unsigned n = CMD_ARGC;
+	unsigned j = 0;
+
+	*vector = NULL;
+
+	if (n < 1)
+		return ERROR_COMMAND_SYNTAX_ERROR;
+
+	/* our return vector must be NULL terminated */
+	argv = (char **) calloc(n + 1, sizeof(char *));
+	if (argv == NULL)
+		return ERROR_FAIL;
+
+	for (unsigned i = 0; i < n; i++) {
+		struct transport *t;
+
+		for (t = transport_list; t; t = t->next) {
+			if (strcmp(t->name, CMD_ARGV[i]) != 0)
+				continue;
+			argv[j++] = strdup(CMD_ARGV[i]);
+			break;
+		}
+		if (!t) {
+			LOG_ERROR("no such transport '%s'", CMD_ARGV[i]);
+			goto fail;
+		}
+	}
+
+	*vector = argv;
+	return ERROR_OK;
+
+fail:
+	for (unsigned i = 0; i < n; i++)
+		free(argv[i]);
+	free(argv);
+	return ERROR_FAIL;
+}
+
+COMMAND_HANDLER(handle_transport_init)
+{
+	LOG_DEBUG("%s", __func__);
+	if (!session) {
+		LOG_ERROR("session's transport is not selected.");
+		return ERROR_FAIL;
+	}
+
+	return session->init(CMD_CTX);
+}
+
+COMMAND_HANDLER(handle_transport_list)
+{
+	if (CMD_ARGC != 0)
+		return ERROR_COMMAND_SYNTAX_ERROR;
+
+	command_print(CMD_CTX, "The following transports are available:");
+
+	for (struct transport *t = transport_list; t; t = t->next)
+		command_print(CMD_CTX, "\t%s", t->name);
+
+	return ERROR_OK;
+}
+
+/**
+ * Implements the Tcl "transport select" command, choosing the
+ * transport to be used in this debug session from among the
+ * set supported by the debug adapter being used.
+ */
+COMMAND_HANDLER(handle_transport_select)
+{
+	int retval = ERROR_OK;;
+
+	switch (CMD_ARGC) {
+	case 0:			/* "select" */
+		if (session) {
+			goto show;
+		}
+		LOG_ERROR("session's transport is not selected.");
+		return ERROR_FAIL;
+
+	case 1:			/* "select FOO" */
+		if(strcmp(session->name, CMD_ARGV[0]) == 0) {
+			/* NOP */
+			LOG_DEBUG("transport '%s' is already selected",
+					CMD_ARGV[0]);
+			return ERROR_OK;
+		} else {
+
+		/* we can't change this session's transport after-the-fact */
+		if (session) {
+			LOG_ERROR("session's transport is already selected.");
+			return ERROR_FAIL;
+		}
+		}
+		break;
+
+	default:		/* select FOO BAR */
+		/* we only select *one* transport per session */
+		LOG_ERROR("may only select one transport!");
+		return ERROR_COMMAND_SYNTAX_ERROR;
+	}
+
+	/* Is this transport supported by our debug adapter?
+	 * Example, "JTAG-only" means SWD is not supported.
+	 *
+	 * NOTE:  requires adapter to have been set up, including
+	 * declaring transport via C code or Tcl script.
+	 */
+	if (!allowed_transports) {
+		LOG_ERROR("Debug adapter doesn't support any transports?");
+		return ERROR_FAIL;
+	}
+	for (unsigned i = 0; allowed_transports[i]; i++) {
+
+		if (strcmp(allowed_transports[i], CMD_ARGV[0]) == 0)
+			return transport_select(CMD_CTX, CMD_ARGV[0]);
+	}
+
+	LOG_ERROR("Debug adapter doesn't support '%s' "
+			"transport?", CMD_ARGV[0]);
+	return ERROR_FAIL;
+
+
+show:
+	/* report the current transport selection */
+	command_print(CMD_CTX, "%s", session->name);
+	return retval;
+}
+
+static const struct command_registration transport_commands[] = {
+	{
+		.name = "init",
+		.handler = handle_transport_init,
+		/* this would be COMMAND_CONFIG ... except that
+		 * it needs to trigger event handlers that may
+		 * require COMMAND_EXEC ...
+		 */
+		.mode = COMMAND_ANY,
+		.help = "Initialize this session's transport",
+	},
+	{
+		.name = "list",
+		.handler = handle_transport_list,
+		.mode = COMMAND_ANY,
+		.help = "list all built-in transports",
+	},
+	{
+		.name = "select",
+		.handler = handle_transport_select,
+		.mode = COMMAND_ANY,
+		.help = "Select this session's transport",
+		.usage = "[transport_name]",
+	},
+	COMMAND_REGISTRATION_DONE
+};
+
+static const struct command_registration transport_group[] = {
+	{
+		.name = "transport",
+		.mode = COMMAND_ANY,
+		.help = "Transport command group",
+		.chain = transport_commands,
+	},
+	COMMAND_REGISTRATION_DONE
+};
+
+
+int transport_register_commands(struct command_context *ctx)
+{
+	return register_commands(ctx, NULL, transport_group);
+}
diff --git a/src/jtag/transport.h b/src/jtag/transport.h
new file mode 100644
index 00000000..d2d38ec2
--- /dev/null
+++ b/src/jtag/transport.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2010 by David Brownell
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#include "helper/command.h"
+
+/**
+ * Wrapper for transport lifecycle operations.
+ *
+ * OpenOCD talks to targets through some kind of debugging
+ * or programming adapter, using some protocol that probably
+ * has target-specific aspects.
+ *
+ * A "transport" reflects electrical protocol to the target,
+ * e..g jtag, swd, spi, uart, ... NOT the messaging protocols
+ * layered over it (e.g. JTAG has eICE, CoreSight, Nexus, OnCE,
+ * and more).
+ *
+ * In addition to the lifecycle operations packaged by this
+ * structure, a transport also involves  an interface supported
+ * by debug adapters and used by components such as debug targets.
+ * For non-debug transports,  there may be interfaces used to
+ * write to flash chips.
+ */
+struct transport {
+	/**
+	 * Each transport has a unique name, used to select it
+	 * from among the alternatives.  Examples might include
+	 * "jtag", * "swd", "AVR_ISP" and more.
+	 */
+	const char *name;
+
+	/**
+	 * When a transport is selected, this method registers
+	 * its commands and activates the transport (e.g. resets
+	 * the link).
+	 *
+	 * After those commands are registered, they will often
+	 * be used for further configuration of the debug link.
+	 */
+	int (*select)(struct command_context *ctx);
+
+	/**
+	 * server startup uses this method to validate transport
+	 * configuration.  (For example, with JTAG this interrogates
+	 * the scan chain against the list of expected TAPs.)
+	 */
+	int (*init)(struct command_context *ctx);
+
+	/**
+	 * Transports are stored in a singly linked list.
+	 */
+	struct transport *next;
+};
+
+int transport_register(struct transport *new_transport);
+
+struct transport *get_current_transport(void);
+
+int transport_register_commands(struct command_context *ctx);
+
+COMMAND_HELPER(transport_list_parse, char ***vector);
+
+int allow_transports(struct command_context *ctx, const char **vector);
+
+bool transports_are_declared(void);
diff --git a/src/openocd.c b/src/openocd.c
index 96de50c8..bba1e0bf 100644
--- a/src/openocd.c
+++ b/src/openocd.c
@@ -31,10 +31,9 @@
 #include "openocd.h"
 #include <jtag/driver.h>
 #include <jtag/jtag.h>
+#include <jtag/transport.h>
 #include <helper/ioutil.h>
 #include <helper/configuration.h>
-#include <xsvf/xsvf.h>
-#include <svf/svf.h>
 #include <flash/nor/core.h>
 #include <flash/nand/core.h>
 #include <pld/pld.h>
@@ -120,22 +119,24 @@ COMMAND_HANDLER(handle_init_command)
 		/* we must be able to set up the debug adapter */
 		return retval;
 	}
+
 	LOG_DEBUG("Debug Adapter init complete");
 
-	/* Try to initialize & examine the JTAG chain at this point,
-	 * but continue startup regardless.  Note that platforms
-	 * need to be able to provide JTAG event handlers that use
-	 * a variety of JTAG operations in order to do that...
+	/* "transport init" verifies the expected devices are present;
+	 * for JTAG, it checks the list of configured TAPs against
+	 * what's discoverable, possibly with help from the platform's
+	 * JTAG event handlers.  (which require COMMAND_EXEC)
 	 */
 	command_context_mode(CMD_CTX, COMMAND_EXEC);
-	if (command_run_line(CMD_CTX, "jtag init") == ERROR_OK)
-	{
-		LOG_DEBUG("Examining targets...");
-		if (target_examine() != ERROR_OK)
-			LOG_DEBUG("target examination failed");
-	}
-	else
-		LOG_WARNING("jtag initialization failed; try 'jtag init' again.");
+
+	retval = command_run_line(CMD_CTX, "transport init");
+	if (ERROR_OK != retval)
+		return ERROR_FAIL;
+
+	LOG_DEBUG("Examining targets...");
+	if (target_examine() != ERROR_OK)
+		LOG_DEBUG("target examination failed");
+
 	command_context_mode(CMD_CTX, COMMAND_CONFIG);
 
 	if (command_run_line(CMD_CTX, "flash init") != ERROR_OK)
@@ -227,16 +228,13 @@ struct command_context *setup_command_handler(Jim_Interp *interp)
 		&server_register_commands,
 		&gdb_register_commands,
 		&log_register_commands,
+		&transport_register_commands,
 		&interface_register_commands,
-		&jtag_register_commands,
-		&xsvf_register_commands,
-		&svf_register_commands,
 		&target_register_commands,
 		&flash_register_commands,
 		&nand_register_commands,
 		&pld_register_commands,
 		&mflash_register_commands,
-		NULL
 	};
 	for (unsigned i = 0; NULL != command_registrants[i]; i++)
 	{