jtag: getting the JTAG speed can fail

[openocd.git] / src / jtag / drivers / jlink.c

/***************************************************************************
 *   Copyright (C) 2007 by Juergen Stuber <juergen@jstuber.net>            *
 *   based on Dominic Rath's and Benedikt Sauter's usbprog.c               *
 *                                                                         *
 *   Copyright (C) 2008 by Spencer Oliver                                  *
 *   spen@spen-soft.co.uk                                                  *
 *                                                                         *
 *   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

#include <jtag/interface.h>
#include <jtag/commands.h>
#include "usb_common.h"

/* See Segger's public documentation:
 *      Reference manual for J-Link USB Protocol
 *      Document RM08001-R6 Date: June 16, 2009
 *      (Or newer, with some SWD information).
 
http://www.segger.com/cms/admin/uploads/productDocs/RM08001_JLinkUSBProtocol.pdf
 */

#define VID 0x1366
#define PID 0x0101

#define JLINK_WRITE_ENDPOINT    0x02
#define JLINK_READ_ENDPOINT             0x81

static unsigned int jlink_write_ep = JLINK_WRITE_ENDPOINT;
static unsigned int jlink_read_ep = JLINK_READ_ENDPOINT;
static unsigned int jlink_hw_jtag_version = 2;

#define JLINK_USB_TIMEOUT               1000

// See Section 1.3.2 of the Segger JLink USB protocol manual
/* 2048 is the max value we can use here */
//#define JLINK_TAP_BUFFER_SIZE 2048
#define JLINK_TAP_BUFFER_SIZE 256
//#define JLINK_TAP_BUFFER_SIZE 384

#define JLINK_IN_BUFFER_SIZE                    2048
#define JLINK_OUT_BUFFER_SIZE                   2*2048 + 4
#define JLINK_EMU_RESULT_BUFFER_SIZE    64

/* Global USB buffers */
static uint8_t usb_in_buffer[JLINK_IN_BUFFER_SIZE];
static uint8_t usb_out_buffer[JLINK_OUT_BUFFER_SIZE];
static uint8_t usb_emu_result_buffer[JLINK_EMU_RESULT_BUFFER_SIZE];

/* Constants for JLink command */
#define EMU_CMD_VERSION                 0x01
#define EMU_CMD_SET_SPEED               0x05
#define EMU_CMD_GET_STATE               0x07
#define EMU_CMD_HW_CLOCK                0xc8
#define EMU_CMD_HW_TMS0                 0xc9
#define EMU_CMD_HW_TMS1                 0xca
#define EMU_CMD_HW_JTAG2                0xce
#define EMU_CMD_HW_JTAG3                0xcf
#define EMU_CMD_GET_MAX_MEM_BLOCK       0xd4
#define EMU_CMD_HW_RESET0               0xdc
#define EMU_CMD_HW_RESET1               0xdd
#define EMU_CMD_HW_TRST0                0xde
#define EMU_CMD_HW_TRST1                0xdf
#define EMU_CMD_GET_CAPS                0xe8
#define EMU_CMD_GET_HW_VERSION  0xf0

/* bits return from EMU_CMD_GET_CAPS */
#define EMU_CAP_GET_HW_VERSION          1
#define EMU_CAP_GET_MAX_BLOCK_SIZE      11

/* max speed 12MHz v5.0 jlink */
#define JLINK_MAX_SPEED 12000

/* Queue command functions */
static void jlink_end_state(tap_state_t state);
static void jlink_state_move(void);
static void jlink_path_move(int num_states, tap_state_t *path);
static void jlink_runtest(int num_cycles);
static void jlink_scan(bool ir_scan, enum scan_type type, uint8_t *buffer,
                int scan_size, struct scan_command *command);
static void jlink_reset(int trst, int srst);
static void jlink_simple_command(uint8_t command);
static int jlink_get_status(void);

/* J-Link tap buffer functions */
static void jlink_tap_init(void);
static int jlink_tap_execute(void);
static void jlink_tap_ensure_space(int scans, int bits);
static void jlink_tap_append_step(int tms, int tdi);
static void jlink_tap_append_scan(int length, uint8_t *buffer,
                struct scan_command *command);

/* Jlink lowlevel functions */
struct jlink {
        struct usb_dev_handle* usb_handle;
};

static struct jlink *jlink_usb_open(void);
static void jlink_usb_close(struct jlink *jlink);
static int jlink_usb_message(struct jlink *jlink, int out_length, int in_length);
static int jlink_usb_write(struct jlink *jlink, int out_length);
static int jlink_usb_read(struct jlink *jlink, int expected_size);
static int jlink_usb_read_emu_result(struct jlink *jlink);

/* helper functions */
static int jlink_get_version_info(void);

#ifdef _DEBUG_USB_COMMS_
static void jlink_debug_buffer(uint8_t *buffer, int length);
#endif

static enum tap_state jlink_last_state = TAP_RESET;

static struct jlink* jlink_handle;

/***************************************************************************/
/* External interface implementation */

static void jlink_execute_runtest(struct jtag_command *cmd)
{
        DEBUG_JTAG_IO("runtest %i cycles, end in %i",
                        cmd->cmd.runtest->num_cycles,
                        cmd->cmd.runtest->end_state);

        jlink_end_state(cmd->cmd.runtest->end_state);

        jlink_runtest(cmd->cmd.runtest->num_cycles);
}

static void jlink_execute_statemove(struct jtag_command *cmd)
{
        DEBUG_JTAG_IO("statemove end in %i", cmd->cmd.statemove->end_state);

        jlink_end_state(cmd->cmd.statemove->end_state);
        jlink_state_move();
}

static void jlink_execute_pathmove(struct jtag_command *cmd)
{
        DEBUG_JTAG_IO("pathmove: %i states, end in %i",
                cmd->cmd.pathmove->num_states,
                cmd->cmd.pathmove->path[cmd->cmd.pathmove->num_states - 1]);

        jlink_path_move(cmd->cmd.pathmove->num_states,
                        cmd->cmd.pathmove->path);
}

static void jlink_execute_scan(struct jtag_command *cmd)
{
        int scan_size;
        enum scan_type type;
        uint8_t *buffer;

        DEBUG_JTAG_IO("scan end in %s", tap_state_name(cmd->cmd.scan->end_state));

        jlink_end_state(cmd->cmd.scan->end_state);

        scan_size = jtag_build_buffer(cmd->cmd.scan, &buffer);
        DEBUG_JTAG_IO("scan input, length = %d", scan_size);

#ifdef _DEBUG_USB_COMMS_
        jlink_debug_buffer(buffer, (scan_size + 7) / 8);
#endif
        type = jtag_scan_type(cmd->cmd.scan);
        jlink_scan(cmd->cmd.scan->ir_scan,
                        type, buffer, scan_size, cmd->cmd.scan);
}

static void jlink_execute_reset(struct jtag_command *cmd)
{
        DEBUG_JTAG_IO("reset trst: %i srst %i",
                        cmd->cmd.reset->trst, cmd->cmd.reset->srst);

        jlink_tap_execute();
        jlink_reset(cmd->cmd.reset->trst, cmd->cmd.reset->srst);
        jlink_tap_execute();
}

static void jlink_execute_sleep(struct jtag_command *cmd)
{
        DEBUG_JTAG_IO("sleep %" PRIi32 "", cmd->cmd.sleep->us);
        jlink_tap_execute();
        jtag_sleep(cmd->cmd.sleep->us);
}

static void jlink_execute_command(struct jtag_command *cmd)
{
        switch (cmd->type)
        {
        case JTAG_RUNTEST:   jlink_execute_runtest(cmd); break;
        case JTAG_TLR_RESET: jlink_execute_statemove(cmd); break;
        case JTAG_PATHMOVE:  jlink_execute_pathmove(cmd); break;
        case JTAG_SCAN:      jlink_execute_scan(cmd); break;
        case JTAG_RESET:     jlink_execute_reset(cmd); break;
        case JTAG_SLEEP:     jlink_execute_sleep(cmd); break;
        default:
                LOG_ERROR("BUG: unknown JTAG command type encountered");
                exit(-1);
        }
}

static int jlink_execute_queue(void)
{
        struct jtag_command *cmd = jtag_command_queue;

        while (cmd != NULL)
        {
                jlink_execute_command(cmd);
                cmd = cmd->next;
        }

        return jlink_tap_execute();
}

/* Sets speed in kHz. */
static int jlink_speed(int speed)
{
        int result;

        if (speed > JLINK_MAX_SPEED)
        {
                LOG_INFO("Ignoring speed request: %dkHz exceeds %dkHz maximum",
                                speed, JLINK_MAX_SPEED);
                return ERROR_OK;
        }

        /* check for RTCK setting */
        if (speed == 0)
                speed = -1;

        usb_out_buffer[0] = EMU_CMD_SET_SPEED;
        usb_out_buffer[1] = (speed >> 0) & 0xff;
        usb_out_buffer[2] = (speed >> 8) & 0xff;

        result = jlink_usb_write(jlink_handle, 3);
        if (result != 3)
        {
                LOG_ERROR("J-Link setting speed failed (%d)", result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        return ERROR_OK;
}

static int jlink_speed_div(int speed, int* khz)
{
        *khz = speed;

        return ERROR_OK;
}

static int jlink_khz(int khz, int *jtag_speed)
{
        *jtag_speed = khz;

        return ERROR_OK;
}

static int jlink_init(void)
{
        int i;

        jlink_handle = jlink_usb_open();

        if (jlink_handle == 0)
        {
                LOG_ERROR("Cannot find jlink Interface! Please check "
                                "connection and permissions.");
                return ERROR_JTAG_INIT_FAILED;
        }

        /*
         * The next three instructions were added after discovering a problem
         * while using an oscilloscope.
         * For the V8 SAM-ICE dongle (and likely other j-link device variants),
         * the reset line to the target microprocessor was found to cycle only
         * intermittently during emulator startup (even after encountering the
         * downstream reset instruction later in the code).
         * This was found to create two issues:
         * 1) In general it is a bad practice to not reset a CPU to a known
         * state when starting an emulator and
         * 2) something critical happens inside the dongle when it does the
         * first read following a new USB session.
         * Keeping the processor in reset during the first read collecting
         * version information seems to prevent errant
         * "J-Link command EMU_CMD_VERSION failed" issues.
         */

        LOG_INFO("J-Link initialization started / target CPU reset initiated");
        jlink_simple_command(EMU_CMD_HW_TRST0);
        jlink_simple_command(EMU_CMD_HW_RESET0);
        usleep(1000);

        jlink_hw_jtag_version = 2;

        if (jlink_get_version_info() == ERROR_OK)
        {
                /* attempt to get status */
                jlink_get_status();
        }

        LOG_INFO("J-Link JTAG Interface ready");

        jlink_reset(0, 0);
        jtag_sleep(3000);
        jlink_tap_init();
        int jtag_speed_var;
        int retval = jtag_get_speed(&jtag_speed_var);
        if (retval != ERROR_OK)
                return retval;
        jlink_speed(jtag_speed_var);

        /* v5/6 jlink seems to have an issue if the first tap move
         * is not divisible by 8, so we send a TLR on first power up */
        for (i = 0; i < 8; i++) {
                jlink_tap_append_step(1, 0);
        }
        jlink_tap_execute();

        return ERROR_OK;
}

static int jlink_quit(void)
{
        jlink_usb_close(jlink_handle);
        return ERROR_OK;
}

/***************************************************************************/
/* Queue command implementations */

static void jlink_end_state(tap_state_t state)
{
        if (tap_is_state_stable(state))
        {
                tap_set_end_state(state);
        }
        else
        {
                LOG_ERROR("BUG: %i is not a valid end state", state);
                exit(-1);
        }
}

/* Goes to the end state. */
static void jlink_state_move(void)
{
        int i;
        int tms = 0;
        uint8_t tms_scan = tap_get_tms_path(tap_get_state(), tap_get_end_state());
        uint8_t tms_scan_bits = tap_get_tms_path_len(tap_get_state(), tap_get_end_state());

        for (i = 0; i < tms_scan_bits; i++)
        {
                tms = (tms_scan >> i) & 1;
                jlink_tap_append_step(tms, 0);
        }

        tap_set_state(tap_get_end_state());
}

static void jlink_path_move(int num_states, tap_state_t *path)
{
        int i;

        for (i = 0; i < num_states; i++)
        {
                if (path[i] == tap_state_transition(tap_get_state(), false))
                {
                        jlink_tap_append_step(0, 0);
                }
                else if (path[i] == tap_state_transition(tap_get_state(), true))
                {
                        jlink_tap_append_step(1, 0);
                }
                else
                {
                        LOG_ERROR("BUG: %s -> %s isn't a valid TAP transition",
                                        tap_state_name(tap_get_state()), tap_state_name(path[i]));
                        exit(-1);
                }

                tap_set_state(path[i]);
        }

        tap_set_end_state(tap_get_state());
}

static void jlink_runtest(int num_cycles)
{
        int i;

        tap_state_t saved_end_state = tap_get_end_state();

        jlink_tap_ensure_space(1,num_cycles + 16);

        /* only do a state_move when we're not already in IDLE */
        if (tap_get_state() != TAP_IDLE)
        {
                jlink_end_state(TAP_IDLE);
                jlink_state_move();
//              num_cycles--;
        }

        /* execute num_cycles */
        for (i = 0; i < num_cycles; i++)
        {
                jlink_tap_append_step(0, 0);
        }

        /* finish in end_state */
        jlink_end_state(saved_end_state);
        if (tap_get_state() != tap_get_end_state())
        {
                jlink_state_move();
        }
}

static void jlink_scan(bool ir_scan, enum scan_type type, uint8_t *buffer,
                int scan_size, struct scan_command *command)
{
        tap_state_t saved_end_state;

        jlink_tap_ensure_space(1, scan_size + 16);

        saved_end_state = tap_get_end_state();

        /* Move to appropriate scan state */
        jlink_end_state(ir_scan ? TAP_IRSHIFT : TAP_DRSHIFT);

        /* Only move if we're not already there */
        if (tap_get_state() != tap_get_end_state())
                jlink_state_move();

        jlink_end_state(saved_end_state);

        /* Scan */
        jlink_tap_append_scan(scan_size, buffer, command);

        /* We are in Exit1, go to Pause */
        jlink_tap_append_step(0, 0);

        tap_set_state(ir_scan ? TAP_IRPAUSE : TAP_DRPAUSE);

        if (tap_get_state() != tap_get_end_state())
        {
                jlink_state_move();
        }
}

static void jlink_reset(int trst, int srst)
{
        LOG_DEBUG("trst: %i, srst: %i", trst, srst);

        /* Signals are active low */
        if (srst == 0)
        {
                jlink_simple_command(EMU_CMD_HW_RESET1);
        }
        if (srst == 1)
        {
                jlink_simple_command(EMU_CMD_HW_RESET0);
        }

        if (trst == 1)
        {
                jlink_simple_command(EMU_CMD_HW_TRST0);
        }

        if (trst == 0)
        {
                jlink_simple_command(EMU_CMD_HW_TRST1);
        }
}

static void jlink_simple_command(uint8_t command)
{
        int result;

        DEBUG_JTAG_IO("0x%02x", command);

        usb_out_buffer[0] = command;
        result = jlink_usb_write(jlink_handle, 1);

        if (result != 1)
        {
                LOG_ERROR("J-Link command 0x%02x failed (%d)", command, result);
        }
}

static int jlink_get_status(void)
{
        int result;

        jlink_simple_command(EMU_CMD_GET_STATE);

        result = jlink_usb_read(jlink_handle, 8);
        if (result != 8)
        {
                LOG_ERROR("J-Link command EMU_CMD_GET_STATE failed (%d)\n", result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        int vref = usb_in_buffer[0] + (usb_in_buffer[1] << 8);
        LOG_INFO("Vref = %d.%d TCK = %d TDI = %d TDO = %d TMS = %d SRST = %d TRST = %d", \
                vref / 1000, vref % 1000, \
                usb_in_buffer[2], usb_in_buffer[3], usb_in_buffer[4], \
                usb_in_buffer[5], usb_in_buffer[6], usb_in_buffer[7]);

        if (vref < 1500)
                LOG_ERROR("Vref too low. Check Target Power");

        return ERROR_OK;
}

static int jlink_get_version_info(void)
{
        int result;
        int len;
        uint32_t jlink_caps, jlink_max_size;

        /* query hardware version */
        jlink_simple_command(EMU_CMD_VERSION);

        result = jlink_usb_read(jlink_handle, 2);
        if (2 != result)
        {
                LOG_ERROR("J-Link command EMU_CMD_VERSION failed (%d)", result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        len = buf_get_u32(usb_in_buffer, 0, 16);
        if (len > JLINK_IN_BUFFER_SIZE)
        {
                LOG_ERROR("J-Link command EMU_CMD_VERSION impossible return length 0x%0x", len);
                len = JLINK_IN_BUFFER_SIZE;
        }

        result = jlink_usb_read(jlink_handle, len);
        if (result != len)
        {
                LOG_ERROR("J-Link command EMU_CMD_VERSION failed (%d)", result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        usb_in_buffer[result] = 0;
        LOG_INFO("%s", (char *)usb_in_buffer);

        /* query hardware capabilities */
        jlink_simple_command(EMU_CMD_GET_CAPS);

        result = jlink_usb_read(jlink_handle, 4);
        if (4 != result)
        {
                LOG_ERROR("J-Link command EMU_CMD_GET_CAPS failed (%d)", result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        jlink_caps = buf_get_u32(usb_in_buffer, 0, 32);
        LOG_INFO("JLink caps 0x%x", (unsigned)jlink_caps);

        if (jlink_caps & (1 << EMU_CAP_GET_HW_VERSION))
        {
                /* query hardware version */
                jlink_simple_command(EMU_CMD_GET_HW_VERSION);

                result = jlink_usb_read(jlink_handle, 4);
                if (4 != result)
                {
                        LOG_ERROR("J-Link command EMU_CMD_GET_HW_VERSION failed (%d)", result);
                        return ERROR_JTAG_DEVICE_ERROR;
                }

                uint32_t jlink_hw_version = buf_get_u32(usb_in_buffer, 0, 32);
                uint32_t major_revision = (jlink_hw_version / 10000) % 100;
                if (major_revision >= 5)
                        jlink_hw_jtag_version = 3;

                LOG_INFO("JLink hw version %i", (int)jlink_hw_version);
        }

        if (jlink_caps & (1 << EMU_CAP_GET_MAX_BLOCK_SIZE))
        {
                /* query hardware maximum memory block */
                jlink_simple_command(EMU_CMD_GET_MAX_MEM_BLOCK);

                result = jlink_usb_read(jlink_handle, 4);
                if (4 != result)
                {
                        LOG_ERROR("J-Link command EMU_CMD_GET_MAX_MEM_BLOCK failed (%d)", result);
                        return ERROR_JTAG_DEVICE_ERROR;
                }

                jlink_max_size = buf_get_u32(usb_in_buffer, 0, 32);
                LOG_INFO("JLink max mem block %i", (int)jlink_max_size);
        }

        return ERROR_OK;
}

COMMAND_HANDLER(jlink_handle_jlink_info_command)
{
        if (jlink_get_version_info() == ERROR_OK)
        {
                /* attempt to get status */
                jlink_get_status();
        }

        return ERROR_OK;
}

COMMAND_HANDLER(jlink_handle_jlink_hw_jtag_command)
{
        switch (CMD_ARGC) {
        case 0:
                command_print(CMD_CTX, "jlink hw jtag  %i", jlink_hw_jtag_version);
                break;
        case 1: {
                int request_version = atoi(CMD_ARGV[0]);
                switch (request_version) {
                case 2: case 3:
                        jlink_hw_jtag_version = request_version;
                        break;
                default:
                        return ERROR_COMMAND_SYNTAX_ERROR;
                }
                break;
        }
        default:
                return ERROR_COMMAND_SYNTAX_ERROR;
        }

        return ERROR_OK;
}

static const struct command_registration jlink_command_handlers[] = {
        {
                .name = "jlink_info",
                .handler = &jlink_handle_jlink_info_command,
                .mode = COMMAND_EXEC,
                .help = "show jlink info",
        },
        {
                .name = "jlink_hw_jtag",
                .handler = &jlink_handle_jlink_hw_jtag_command,
                .mode = COMMAND_EXEC,
                .help = "access J-Link HW JTAG command version",
                .usage = "[2|3]",
        },
        COMMAND_REGISTRATION_DONE
};

struct jtag_interface jlink_interface = {
        .name = "jlink",
        .commands = jlink_command_handlers,

        .execute_queue = jlink_execute_queue,
        .speed = jlink_speed,
        .speed_div = jlink_speed_div,
        .khz = jlink_khz,
        .init = jlink_init,
        .quit = jlink_quit,
};

/***************************************************************************/
/* J-Link tap functions */


static unsigned tap_length = 0;
static uint8_t tms_buffer[JLINK_TAP_BUFFER_SIZE];
static uint8_t tdi_buffer[JLINK_TAP_BUFFER_SIZE];
static uint8_t tdo_buffer[JLINK_TAP_BUFFER_SIZE];

struct pending_scan_result {
        int first;      /* First bit position in tdo_buffer to read */
        int length; /* Number of bits to read */
        struct scan_command *command; /* Corresponding scan command */
        uint8_t *buffer;
};

#define MAX_PENDING_SCAN_RESULTS 256

static int pending_scan_results_length;
static struct pending_scan_result pending_scan_results_buffer[MAX_PENDING_SCAN_RESULTS];

static void jlink_tap_init(void)
{
        tap_length = 0;
        pending_scan_results_length = 0;
}

static void jlink_tap_ensure_space(int scans, int bits)
{
        int available_scans = MAX_PENDING_SCAN_RESULTS - pending_scan_results_length;
        int available_bits = JLINK_TAP_BUFFER_SIZE * 8 - tap_length - 32;

        if (scans > available_scans || bits > available_bits)
        {
                jlink_tap_execute();
        }
}

static void jlink_tap_append_step(int tms, int tdi)
{
        int index_var = tap_length / 8;

        if (index_var >= JLINK_TAP_BUFFER_SIZE)
        {
                LOG_ERROR("jlink_tap_append_step: overflow");
                *(uint32_t *)0xFFFFFFFF = 0;
                exit(-1);
        }

        int bit_index = tap_length % 8;
        uint8_t bit = 1 << bit_index;

        // we do not pad TMS, so be sure to initialize all bits
        if (0 == bit_index)
        {
                tms_buffer[index_var] = tdi_buffer[index_var] = 0;
        }

        if (tms)
                tms_buffer[index_var] |= bit;
        else
                tms_buffer[index_var] &= ~bit;

        if (tdi)
                tdi_buffer[index_var] |= bit;
        else
                tdi_buffer[index_var] &= ~bit;

        tap_length++;
}

static void jlink_tap_append_scan(int length, uint8_t *buffer,
                struct scan_command *command)
{
        struct pending_scan_result *pending_scan_result =
                &pending_scan_results_buffer[pending_scan_results_length];
        int i;

        pending_scan_result->first = tap_length;
        pending_scan_result->length = length;
        pending_scan_result->command = command;
        pending_scan_result->buffer = buffer;

        for (i = 0; i < length; i++)
        {
                int tms = (i < (length - 1)) ? 0 : 1;
                int tdi = (buffer[i / 8] & (1 << (i % 8))) != 0;
                jlink_tap_append_step(tms, tdi);
        }
        pending_scan_results_length++;
}

/* Pad and send a tap sequence to the device, and receive the answer.
 * For the purpose of padding we assume that we are in idle or pause state. */
static int jlink_tap_execute(void)
{
        int byte_length;
        int i;
        int result;

        if (!tap_length)
                return ERROR_OK;

        /* JLink returns an extra NULL in packet when size of incoming
         * message is a multiple of 64, creates problems with USB comms.
         * WARNING: This will interfere with tap state counting. */
        while ((DIV_ROUND_UP(tap_length, 8) % 64) == 0)
        {
                jlink_tap_append_step((tap_get_state() == TAP_RESET) ? 1 : 0, 0);
        }

        // number of full bytes (plus one if some would be left over)
        byte_length = DIV_ROUND_UP(tap_length, 8);

        bool use_jtag3 = jlink_hw_jtag_version >= 3;
        usb_out_buffer[0] = use_jtag3 ? EMU_CMD_HW_JTAG3 : EMU_CMD_HW_JTAG2;
        usb_out_buffer[1] = 0;
        usb_out_buffer[2] = (tap_length >> 0) & 0xff;
        usb_out_buffer[3] = (tap_length >> 8) & 0xff;
        memcpy(usb_out_buffer + 4, tms_buffer, byte_length);
        memcpy(usb_out_buffer + 4 + byte_length, tdi_buffer, byte_length);

        jlink_last_state = jtag_debug_state_machine(tms_buffer, tdi_buffer,
                        tap_length, jlink_last_state);

        result = jlink_usb_message(jlink_handle, 4 + 2 * byte_length, byte_length);
        if (result != byte_length)
        {
                LOG_ERROR("jlink_tap_execute, wrong result %d (expected %d)",
                                result, byte_length);
                jlink_tap_init();
                return ERROR_JTAG_QUEUE_FAILED;
        }

        memcpy(tdo_buffer, usb_in_buffer, byte_length);

        for (i = 0; i < pending_scan_results_length; i++)
        {
                struct pending_scan_result *pending_scan_result = &pending_scan_results_buffer[i];
                uint8_t *buffer = pending_scan_result->buffer;
                int length = pending_scan_result->length;
                int first = pending_scan_result->first;
                struct scan_command *command = pending_scan_result->command;

                /* Copy to buffer */
                buf_set_buf(tdo_buffer, first, buffer, 0, length);

                DEBUG_JTAG_IO("pending scan result, length = %d", length);

#ifdef _DEBUG_USB_COMMS_
                jlink_debug_buffer(buffer, DIV_ROUND_UP(length, 8));
#endif

                if (jtag_read_buffer(buffer, command) != ERROR_OK)
                {
                        jlink_tap_init();
                        return ERROR_JTAG_QUEUE_FAILED;
                }

                if (pending_scan_result->buffer != NULL)
                {
                        free(pending_scan_result->buffer);
                }
        }

        jlink_tap_init();
        return ERROR_OK;
}

/*****************************************************************************/
/* JLink USB low-level functions */

static struct jlink* jlink_usb_open()
{
        usb_init();

        const uint16_t vids[] = { VID, 0 };
        const uint16_t pids[] = { PID, 0 };
        struct usb_dev_handle *dev;
        if (jtag_usb_open(vids, pids, &dev) != ERROR_OK)
                return NULL;

        /* BE ***VERY CAREFUL*** ABOUT MAKING CHANGES IN THIS
         * AREA!!!!!!!!!!!  The behavior of libusb is not completely
         * consistent across Windows, Linux, and Mac OS X platforms.
         * The actions taken in the following compiler conditionals may
         * not agree with published documentation for libusb, but were
         * found to be necessary through trials and tribulations.  Even
         * little tweaks can break one or more platforms, so if you do
         * make changes test them carefully on all platforms before
         * committing them!
         */

#if IS_WIN32 == 0

        usb_reset(dev);

#if IS_DARWIN == 0

        int timeout = 5;
        /* reopen jlink after usb_reset
         * on win32 this may take a second or two to re-enumerate */
        int retval;
        while ((retval = jtag_usb_open(vids, pids, &dev)) != ERROR_OK)
        {
                usleep(1000);
                timeout--;
                if (!timeout) {
                        break;
                }
        }
        if (ERROR_OK != retval)
                return NULL;
#endif

#endif

        /* usb_set_configuration required under win32 */
        struct usb_device *udev = usb_device(dev);
        usb_set_configuration(dev, udev->config[0].bConfigurationValue);
        usb_claim_interface(dev, 0);

#if 0
        /*
         * This makes problems under Mac OS X. And is not needed
         * under Windows. Hopefully this will not break a linux build
         */
        usb_set_altinterface(result->usb_handle, 0);
#endif
        struct usb_interface *iface = udev->config->interface;
        struct usb_interface_descriptor *desc = iface->altsetting;
        for (int i = 0; i < desc->bNumEndpoints; i++)
        {
                uint8_t epnum = desc->endpoint[i].bEndpointAddress;
                bool is_input = epnum & 0x80;
                LOG_DEBUG("usb ep %s %02x", is_input ? "in" : "out", epnum);
                if (is_input)
                        jlink_read_ep = epnum;
                else
                        jlink_write_ep = epnum;
        }

        struct jlink *result = malloc(sizeof(struct jlink));
        result->usb_handle = dev;
        return result;
}

static void jlink_usb_close(struct jlink *jlink)
{
        usb_close(jlink->usb_handle);
        free(jlink);
}

/* Send a message and receive the reply. */
static int jlink_usb_message(struct jlink *jlink, int out_length, int in_length)
{
        int result;

        result = jlink_usb_write(jlink, out_length);
        if (result != out_length)
        {
                LOG_ERROR("usb_bulk_write failed (requested=%d, result=%d)",
                                out_length, result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        result = jlink_usb_read(jlink, in_length);
        if ((result != in_length) && (result != (in_length + 1)))
        {
                LOG_ERROR("usb_bulk_read failed (requested=%d, result=%d)",
                                in_length, result);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        if (jlink_hw_jtag_version < 3)
                return result;

        int result2 = ERROR_OK;
        if (result == in_length)
        {
                /* Must read the result from the EMU too */
                result2 = jlink_usb_read_emu_result(jlink);
                if (1 != result2)
                {
                        LOG_ERROR("jlink_usb_read_emu_result retried requested = 1, "
                                        "result=%d, in_length=%i", result2, in_length);
                        /* Try again once, should only happen if (in_length%64 == 0) */
                        result2 = jlink_usb_read_emu_result(jlink);
                        if (1 != result2)
                        {
                                LOG_ERROR("jlink_usb_read_emu_result failed "
                                        "(requested = 1, result=%d)", result2);
                                return ERROR_JTAG_DEVICE_ERROR;
                        }
                }

                /* Check the result itself */
                result2 = usb_emu_result_buffer[0];
        }
        else
        {
                /* Save the result, then remove it from return value */
                result2 = usb_in_buffer[result--];
        }

        if (result2)
        {
                LOG_ERROR("jlink_usb_message failed with result=%d)", result2);
                return ERROR_JTAG_DEVICE_ERROR;
        }

        return result;
}

/* calls the given usb_bulk_* function, allowing for the data to
 * trickle in with some timeouts  */
static int usb_bulk_with_retries(
                int (*f)(usb_dev_handle *, int, char *, int, int),
                usb_dev_handle *dev, int ep,
                char *bytes, int size, int timeout)
{
        int tries = 3, count = 0;

        while (tries && (count < size))
        {
                int result = f(dev, ep, bytes + count, size - count, timeout);
                if (result > 0)
                        count += result;
                else if ((-ETIMEDOUT != result) || !--tries)
                        return result;
        }
        return count;
}

static int wrap_usb_bulk_write(usb_dev_handle *dev, int ep,
                               char *buff, int size, int timeout)
{
        /* usb_bulk_write() takes const char *buff */
        return usb_bulk_write(dev, ep, buff, size, timeout);
}

static inline int usb_bulk_write_ex(usb_dev_handle *dev, int ep,
                char *bytes, int size, int timeout)
{
        return usb_bulk_with_retries(&wrap_usb_bulk_write,
                        dev, ep, bytes, size, timeout);
}

static inline int usb_bulk_read_ex(usb_dev_handle *dev, int ep,
                char *bytes, int size, int timeout)
{
        return usb_bulk_with_retries(&usb_bulk_read,
                        dev, ep, bytes, size, timeout);
}

/* Write data from out_buffer to USB. */
static int jlink_usb_write(struct jlink *jlink, int out_length)
{
        int result;

        if (out_length > JLINK_OUT_BUFFER_SIZE)
        {
                LOG_ERROR("jlink_write illegal out_length=%d (max=%d)",
                                out_length, JLINK_OUT_BUFFER_SIZE);
                return -1;
        }

        result = usb_bulk_write_ex(jlink->usb_handle, jlink_write_ep,
                (char *)usb_out_buffer, out_length, JLINK_USB_TIMEOUT);

        DEBUG_JTAG_IO("jlink_usb_write, out_length = %d, result = %d",
                        out_length, result);

#ifdef _DEBUG_USB_COMMS_
        jlink_debug_buffer(usb_out_buffer, out_length);
#endif
        return result;
}

/* Read data from USB into in_buffer. */
static int jlink_usb_read(struct jlink *jlink, int expected_size)
{
        int result = usb_bulk_read_ex(jlink->usb_handle, jlink_read_ep,
                (char *)usb_in_buffer, expected_size, JLINK_USB_TIMEOUT);

        DEBUG_JTAG_IO("jlink_usb_read, result = %d", result);

#ifdef _DEBUG_USB_COMMS_
        jlink_debug_buffer(usb_in_buffer, result);
#endif
        return result;
}

/* Read the result from the previous EMU cmd into result_buffer. */
static int jlink_usb_read_emu_result(struct jlink *jlink)
{
        int result = usb_bulk_read_ex(jlink->usb_handle, jlink_read_ep,
                (char *)usb_emu_result_buffer, 1 /* JLINK_EMU_RESULT_BUFFER_SIZE */,
                JLINK_USB_TIMEOUT);

        DEBUG_JTAG_IO("jlink_usb_read_result, result = %d", result);

#ifdef _DEBUG_USB_COMMS_
        jlink_debug_buffer(usb_emu_result_buffer, result);
#endif
        return result;
}

#ifdef _DEBUG_USB_COMMS_
#define BYTES_PER_LINE  16

static void jlink_debug_buffer(uint8_t *buffer, int length)
{
        char line[81];
        char s[4];
        int i;
        int j;

        for (i = 0; i < length; i += BYTES_PER_LINE)
        {
                snprintf(line, 5, "%04x", i);
                for (j = i; j < i + BYTES_PER_LINE && j < length; j++)
                {
                        snprintf(s, 4, " %02x", buffer[j]);
                        strcat(line, s);
                }
                LOG_DEBUG("%s", line);
        }
}
#endif