[openocd.git] / src / target / adi_v5_jtag.c

/***************************************************************************
 *   Copyright (C) 2006 by Magnus Lundin
 *   lundin@mlu.mine.nu
 *
 *   Copyright (C) 2008 by Spencer Oliver
 *   spen@spen-soft.co.uk
 *
 *   Copyright (C) 2009 by Oyvind Harboe
 *   oyvind.harboe@zylin.com
 *
 *   Copyright (C) 2009-2010 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.,
 *   51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 ***************************************************************************/

/**
 * @file
 * This file implements JTAG transport support for cores implementing
 the ARM Debug Interface version 5 (ADIv5).
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "arm.h"
#include "arm_adi_v5.h"
#include <helper/time_support.h>
#include <helper/list.h>

/*#define DEBUG_WAIT*/

/* JTAG instructions/registers for JTAG-DP and SWJ-DP */
#define JTAG_DP_ABORT           0x8
#define JTAG_DP_DPACC           0xA
#define JTAG_DP_APACC           0xB
#define JTAG_DP_IDCODE          0xE

/* three-bit ACK values for DPACC and APACC reads */
#define JTAG_ACK_OK_FAULT       0x2
#define JTAG_ACK_WAIT           0x1

static int jtag_ap_q_abort(struct adiv5_dap *dap, uint8_t *ack);

#ifdef DEBUG_WAIT
static const char *dap_reg_name(int instr, int reg_addr)
{
        char *reg_name = "UNK";

        if (instr == JTAG_DP_DPACC) {
                switch (reg_addr) {
                case DP_ABORT:
                        reg_name =  "ABORT";
                        break;
                case DP_CTRL_STAT:
                        reg_name =  "CTRL/STAT";
                        break;
                case DP_SELECT:
                        reg_name = "SELECT";
                        break;
                case DP_RDBUFF:
                        reg_name =  "RDBUFF";
                        break;
                case DP_WCR:
                        reg_name =  "WCR";
                        break;
                default:
                        reg_name = "UNK";
                        break;
                }
        }

        if (instr == JTAG_DP_APACC) {
                switch (reg_addr) {
                case MEM_AP_REG_CSW:
                        reg_name = "CSW";
                        break;
                case MEM_AP_REG_TAR:
                        reg_name = "TAR";
                        break;
                case MEM_AP_REG_DRW:
                        reg_name = "DRW";
                        break;
                case MEM_AP_REG_BD0:
                        reg_name = "BD0";
                        break;
                case MEM_AP_REG_BD1:
                        reg_name = "BD1";
                        break;
                case MEM_AP_REG_BD2:
                        reg_name = "BD2";
                        break;
                case MEM_AP_REG_BD3:
                        reg_name = "BD3";
                        break;
                case MEM_AP_REG_CFG:
                        reg_name = "CFG";
                        break;
                case MEM_AP_REG_BASE:
                        reg_name = "BASE";
                        break;
                case AP_REG_IDR:
                        reg_name = "IDR";
                        break;
                default:
                        reg_name = "UNK";
                        break;
                }
        }

        return reg_name;
}
#endif

struct dap_cmd {
        struct list_head lh;
        uint8_t instr;
        uint8_t reg_addr;
        uint8_t RnW;
        uint8_t *invalue;
        uint8_t ack;
        uint32_t memaccess_tck;
        uint32_t dp_select;

        struct scan_field fields[2];
        uint8_t out_addr_buf;
        uint8_t invalue_buf[4];
        uint8_t outvalue_buf[4];
};

static void log_dap_cmd(const char *header, struct dap_cmd *el)
{
#ifdef DEBUG_WAIT
        LOG_DEBUG("%s: %2s %6s %5s 0x%08x 0x%08x %2s", header,
                el->instr == JTAG_DP_APACC ? "AP" : "DP",
                dap_reg_name(el->instr, el->reg_addr),
                el->RnW == DPAP_READ ? "READ" : "WRITE",
                buf_get_u32(el->outvalue_buf, 0, 32),
                buf_get_u32(el->invalue, 0, 32),
                el->ack == JTAG_ACK_OK_FAULT ? "OK" :
                        (el->ack == JTAG_ACK_WAIT ? "WAIT" : "INVAL"));
#endif
}

static struct dap_cmd *dap_cmd_new(uint8_t instr,
                uint8_t reg_addr, uint8_t RnW,
                uint8_t *outvalue, uint8_t *invalue,
                uint32_t memaccess_tck)
{
        struct dap_cmd *cmd;

        cmd = (struct dap_cmd *)calloc(1, sizeof(struct dap_cmd));
        if (cmd != NULL) {
                INIT_LIST_HEAD(&cmd->lh);
                cmd->instr = instr;
                cmd->reg_addr = reg_addr;
                cmd->RnW = RnW;
                if (outvalue != NULL)
                        memcpy(cmd->outvalue_buf, outvalue, 4);
                cmd->invalue = (invalue != NULL) ? invalue : cmd->invalue_buf;
                cmd->memaccess_tck = memaccess_tck;
        }

        return cmd;
}

static void flush_journal(struct list_head *lh)
{
        struct dap_cmd *el, *tmp;

        list_for_each_entry_safe(el, tmp, lh, lh) {
                list_del(&el->lh);
                free(el);
        }
}

/***************************************************************************
 *
 * DPACC and APACC scanchain access through JTAG-DP (or SWJ-DP)
 *
***************************************************************************/

static int adi_jtag_dp_scan_cmd(struct adiv5_dap *dap, struct dap_cmd *cmd, uint8_t *ack)
{
        struct jtag_tap *tap = dap->tap;
        int retval;

        retval = arm_jtag_set_instr(tap, cmd->instr, NULL, TAP_IDLE);
        if (retval != ERROR_OK)
                return retval;

        /* Scan out a read or write operation using some DP or AP register.
         * For APACC access with any sticky error flag set, this is discarded.
         */
        cmd->fields[0].num_bits = 3;
        buf_set_u32(&cmd->out_addr_buf, 0, 3, ((cmd->reg_addr >> 1) & 0x6) | (cmd->RnW & 0x1));
        cmd->fields[0].out_value = &cmd->out_addr_buf;
        cmd->fields[0].in_value = (ack != NULL) ? ack : &cmd->ack;

        /* NOTE: if we receive JTAG_ACK_WAIT, the previous operation did not
         * complete; data we write is discarded, data we read is unpredictable.
         * When overrun detect is active, STICKYORUN is set.
         */

        cmd->fields[1].num_bits = 32;
        cmd->fields[1].out_value = cmd->outvalue_buf;
        cmd->fields[1].in_value = cmd->invalue;

        jtag_add_dr_scan(tap, 2, cmd->fields, TAP_IDLE);

        /* Add specified number of tck clocks after starting memory bus
         * access, giving the hardware time to complete the access.
         * They provide more time for the (MEM) AP to complete the read ...
         * See "Minimum Response Time" for JTAG-DP, in the ADIv5 spec.
         */
        if (cmd->instr == JTAG_DP_APACC) {
                if (((cmd->reg_addr == MEM_AP_REG_DRW)
                        || ((cmd->reg_addr & 0xF0) == MEM_AP_REG_BD0))
                        && (cmd->memaccess_tck != 0))
                        jtag_add_runtest(cmd->memaccess_tck, TAP_IDLE);
        }

        return ERROR_OK;
}

static int adi_jtag_dp_scan_cmd_sync(struct adiv5_dap *dap, struct dap_cmd *cmd, uint8_t *ack)
{
        int retval;

        retval = adi_jtag_dp_scan_cmd(dap, cmd, ack);
        if (retval != ERROR_OK)
                return retval;

        return jtag_execute_queue();
}

/**
 * Scan DPACC or APACC using target ordered uint8_t buffers.  No endianness
 * conversions are performed.  See section 4.4.3 of the ADIv5 spec, which
 * discusses operations which access these registers.
 *
 * Note that only one scan is performed.  If RnW is set, a separate scan
 * will be needed to collect the data which was read; the "invalue" collects
 * the posted result of a preceding operation, not the current one.
 *
 * @param dap the DAP
 * @param instr JTAG_DP_APACC (AP access) or JTAG_DP_DPACC (DP access)
 * @param reg_addr two significant bits; A[3:2]; for APACC access, the
 *      SELECT register has more addressing bits.
 * @param RnW false iff outvalue will be written to the DP or AP
 * @param outvalue points to a 32-bit (little-endian) integer
 * @param invalue NULL, or points to a 32-bit (little-endian) integer
 * @param ack points to where the three bit JTAG_ACK_* code will be stored
 * @param memaccess_tck number of idle cycles to add after AP access
 */

static int adi_jtag_dp_scan(struct adiv5_dap *dap,
                uint8_t instr, uint8_t reg_addr, uint8_t RnW,
                uint8_t *outvalue, uint8_t *invalue,
                uint32_t memaccess_tck, uint8_t *ack)
{
        struct dap_cmd *cmd;
        int retval;

        cmd = dap_cmd_new(instr, reg_addr, RnW, outvalue, invalue, memaccess_tck);
        if (cmd != NULL)
                cmd->dp_select = dap->select;
        else
                return ERROR_JTAG_DEVICE_ERROR;

        retval = adi_jtag_dp_scan_cmd(dap, cmd, ack);
        if (retval == ERROR_OK)
                list_add_tail(&cmd->lh, &dap->cmd_journal);

        return retval;
}

/**
 * Scan DPACC or APACC out and in from host ordered uint32_t buffers.
 * This is exactly like adi_jtag_dp_scan(), except that endianness
 * conversions are performed (so the types of invalue and outvalue
 * must be different).
 */
static int adi_jtag_dp_scan_u32(struct adiv5_dap *dap,
                uint8_t instr, uint8_t reg_addr, uint8_t RnW,
                uint32_t outvalue, uint32_t *invalue,
                uint32_t memaccess_tck, uint8_t *ack)
{
        uint8_t out_value_buf[4];
        int retval;

        buf_set_u32(out_value_buf, 0, 32, outvalue);

        retval = adi_jtag_dp_scan(dap, instr, reg_addr, RnW,
                        out_value_buf, (uint8_t *)invalue, memaccess_tck, ack);
        if (retval != ERROR_OK)
                return retval;

        if (invalue)
                jtag_add_callback(arm_le_to_h_u32,
                                (jtag_callback_data_t) invalue);

        return retval;
}

static int adi_jtag_finish_read(struct adiv5_dap *dap)
{
        int retval = ERROR_OK;

        if (dap->last_read != NULL) {
                retval = adi_jtag_dp_scan_u32(dap, JTAG_DP_DPACC,
                                DP_RDBUFF, DPAP_READ, 0, dap->last_read, 0, NULL);
                dap->last_read = NULL;
        }

        return retval;
}

static int adi_jtag_scan_inout_check_u32(struct adiv5_dap *dap,
                uint8_t instr, uint8_t reg_addr, uint8_t RnW,
                uint32_t outvalue, uint32_t *invalue, uint32_t memaccess_tck)
{
        int retval;

        /* Issue the read or write */
        retval = adi_jtag_dp_scan_u32(dap, instr, reg_addr,
                        RnW, outvalue, NULL, memaccess_tck, NULL);
        if (retval != ERROR_OK)
                return retval;

        /* For reads,  collect posted value; RDBUFF has no other effect.
         * Assumes read gets acked with OK/FAULT, and CTRL_STAT says "OK".
         */
        if ((RnW == DPAP_READ) && (invalue != NULL)) {
                retval = adi_jtag_dp_scan_u32(dap, JTAG_DP_DPACC,
                                DP_RDBUFF, DPAP_READ, 0, invalue, 0, NULL);
                if (retval != ERROR_OK)
                        return retval;
        }

        return jtag_execute_queue();
}

static int jtagdp_overrun_check(struct adiv5_dap *dap)
{
        int retval;
        struct dap_cmd *el, *tmp, *prev = NULL;
        int found_wait = 0;
        uint64_t time_now;
        LIST_HEAD(replay_list);

        /* make sure all queued transactions are complete */
        retval = jtag_execute_queue();
        if (retval != ERROR_OK)
                goto done;

        /* skip all completed transactions up to the first WAIT */
        list_for_each_entry(el, &dap->cmd_journal, lh) {
                if (el->ack == JTAG_ACK_OK_FAULT) {
                        log_dap_cmd("LOG", el);
                } else if (el->ack == JTAG_ACK_WAIT) {
                        found_wait = 1;
                        break;
                } else {
                        LOG_ERROR("Invalid ACK (%1x) in DAP response", el->ack);
                        log_dap_cmd("ERR", el);
                        retval = ERROR_JTAG_DEVICE_ERROR;
                        goto done;
                }
        }

        /*
         * If we found a stalled transaction and a previous transaction
         * exists, check if it's a READ access.
         */
        if (found_wait && el != list_first_entry(&dap->cmd_journal, struct dap_cmd, lh)) {
                prev = list_entry(el->lh.prev, struct dap_cmd, lh);
                if (prev->RnW == DPAP_READ) {
                        log_dap_cmd("PND", prev);
                        /* search for the next OK transaction, it contains
                         * the result of the previous READ */
                        tmp = el;
                        list_for_each_entry_from(tmp, &dap->cmd_journal, lh) {
                                if (tmp->ack == JTAG_ACK_OK_FAULT) {
                                        /* recover the read value */
                                        log_dap_cmd("FND", tmp);
                                        if (el->invalue != el->invalue_buf) {
                                                uint32_t invalue = le_to_h_u32(tmp->invalue);
                                                memcpy(el->invalue, &invalue, sizeof(uint32_t));
                                        }
                                        prev = NULL;
                                        break;
                                }
                        }

                        if (prev != NULL) {
                                log_dap_cmd("LST", el);

                                /*
                                * At this point we're sure that no previous
                                * transaction completed and the DAP/AP is still
                                * in busy state. We know that the next "OK" scan
                                * will return the READ result we need to recover.
                                * To complete the READ, we just keep polling RDBUFF
                                * until the WAIT condition clears
                                */
                                tmp = dap_cmd_new(JTAG_DP_DPACC,
                                                DP_RDBUFF, DPAP_READ, NULL, NULL, 0);
                                if (tmp == NULL) {
                                        retval = ERROR_JTAG_DEVICE_ERROR;
                                        goto done;
                                }
                                /* synchronously retry the command until it succeeds */
                                time_now = timeval_ms();
                                do {
                                        retval = adi_jtag_dp_scan_cmd_sync(dap, tmp, NULL);
                                        if (retval != ERROR_OK)
                                                break;
                                        if (tmp->ack == JTAG_ACK_OK_FAULT) {
                                                log_dap_cmd("FND", tmp);
                                                if (el->invalue != el->invalue_buf) {
                                                        uint32_t invalue = le_to_h_u32(tmp->invalue);
                                                        memcpy(el->invalue, &invalue, sizeof(uint32_t));
                                                }
                                                break;
                                        }
                                        if (tmp->ack != JTAG_ACK_WAIT) {
                                                LOG_ERROR("Invalid ACK (%1x) in DAP response", tmp->ack);
                                                log_dap_cmd("ERR", tmp);
                                                retval = ERROR_JTAG_DEVICE_ERROR;
                                                break;
                                        }

                                } while (timeval_ms() - time_now < 1000);

                                if (retval == ERROR_OK) {
                                        /* timeout happened */
                                        if (tmp->ack != JTAG_ACK_OK_FAULT) {
                                                LOG_ERROR("Timeout during WAIT recovery");
                                                jtag_ap_q_abort(dap, NULL);
                                                retval = ERROR_JTAG_DEVICE_ERROR;
                                        }
                                }

                                /* we're done with this command, release it */
                                free(tmp);

                                if (retval != ERROR_OK)
                                        goto done;

                        }
                        /* make el->invalue point to the default invalue
                        * so that we can safely retry it without clobbering
                        * the result we just recovered */
                        el->invalue = el->invalue_buf;
                }
        }

        /* move all remaining transactions over to the replay list */
        list_for_each_entry_safe_from(el, tmp, &dap->cmd_journal, lh) {
                log_dap_cmd("REP", el);
                list_move_tail(&el->lh, &replay_list);
        }

        /* we're done with the journal, flush it */
        flush_journal(&dap->cmd_journal);

        /* check for overrun condition in the last batch of transactions */
        if (found_wait) {
                LOG_INFO("DAP transaction stalled (WAIT) - slowing down");
                /* clear the sticky overrun condition */
                retval = adi_jtag_scan_inout_check_u32(dap, JTAG_DP_DPACC,
                                DP_CTRL_STAT, DPAP_WRITE,
                                dap->dp_ctrl_stat | SSTICKYORUN, NULL, 0);
                if (retval != ERROR_OK)
                        goto done;

                /* restore SELECT register first */
                if (!list_empty(&replay_list)) {
                        el = list_first_entry(&replay_list, struct dap_cmd, lh);
                        tmp = dap_cmd_new(JTAG_DP_DPACC,
                                          DP_SELECT, DPAP_WRITE, (uint8_t *)&el->dp_select, NULL, 0);
                        if (tmp == NULL) {
                                retval = ERROR_JTAG_DEVICE_ERROR;
                                goto done;
                        }
                        list_add(&tmp->lh, &replay_list);

                        dap->select = DP_SELECT_INVALID;
                }

                list_for_each_entry_safe(el, tmp, &replay_list, lh) {
                        time_now = timeval_ms();
                        do {
                                retval = adi_jtag_dp_scan_cmd_sync(dap, el, NULL);
                                if (retval != ERROR_OK)
                                        break;
                                log_dap_cmd("REC", el);
                                if (el->ack == JTAG_ACK_OK_FAULT) {
                                        if (el->invalue != el->invalue_buf) {
                                                uint32_t invalue = le_to_h_u32(el->invalue);
                                                memcpy(el->invalue, &invalue, sizeof(uint32_t));
                                        }
                                        break;
                                }
                                if (el->ack != JTAG_ACK_WAIT) {
                                        LOG_ERROR("Invalid ACK (%1x) in DAP response", el->ack);
                                        log_dap_cmd("ERR", el);
                                        retval = ERROR_JTAG_DEVICE_ERROR;
                                        break;
                                }
                        } while (timeval_ms() - time_now < 1000);

                        if (retval == ERROR_OK) {
                                if (el->ack != JTAG_ACK_OK_FAULT) {
                                        LOG_ERROR("Timeout during WAIT recovery");
                                        jtag_ap_q_abort(dap, NULL);
                                        retval = ERROR_JTAG_DEVICE_ERROR;
                                }
                        } else
                                break;
                }
        }

 done:
        flush_journal(&replay_list);
        flush_journal(&dap->cmd_journal);
        return retval;
}

static int jtagdp_transaction_endcheck(struct adiv5_dap *dap)
{
        int retval;
        uint32_t ctrlstat;

        /* too expensive to call keep_alive() here */

        /* Post CTRL/STAT read; discard any previous posted read value
         * but collect its ACK status.
         */
        retval = adi_jtag_scan_inout_check_u32(dap, JTAG_DP_DPACC,
                        DP_CTRL_STAT, DPAP_READ, 0, &ctrlstat, 0);
        if (retval != ERROR_OK)
                goto done;

        /* REVISIT also STICKYCMP, for pushed comparisons (nyet used) */

        /* Check for STICKYERR */
        if (ctrlstat & SSTICKYERR) {
                LOG_DEBUG("jtag-dp: CTRL/STAT 0x%" PRIx32, ctrlstat);
                /* Check power to debug regions */
                if ((ctrlstat & (CDBGPWRUPREQ | CDBGPWRUPACK | CSYSPWRUPREQ | CSYSPWRUPACK)) !=
                                                (CDBGPWRUPREQ | CDBGPWRUPACK | CSYSPWRUPREQ | CSYSPWRUPACK)) {
                        LOG_ERROR("Debug regions are unpowered, an unexpected reset might have happened");
                        retval = ERROR_JTAG_DEVICE_ERROR;
                        goto done;
                }

                if (ctrlstat & SSTICKYERR)
                        LOG_ERROR("JTAG-DP STICKY ERROR");
                if (ctrlstat & SSTICKYORUN)
                        LOG_DEBUG("JTAG-DP STICKY OVERRUN");

                /* Clear Sticky Error Bits */
                retval = adi_jtag_scan_inout_check_u32(dap, JTAG_DP_DPACC,
                                DP_CTRL_STAT, DPAP_WRITE,
                                dap->dp_ctrl_stat | SSTICKYERR, NULL, 0);
                if (retval != ERROR_OK)
                        goto done;

                if (ctrlstat & SSTICKYERR) {
                        retval = ERROR_JTAG_DEVICE_ERROR;
                        goto done;
                }
        }

 done:
        flush_journal(&dap->cmd_journal);
        return retval;
}

/*--------------------------------------------------------------------------*/

static int jtag_dp_q_read(struct adiv5_dap *dap, unsigned reg,
                uint32_t *data)
{
        int retval =  adi_jtag_dp_scan_u32(dap, JTAG_DP_DPACC, reg,
                        DPAP_READ, 0, dap->last_read, 0, NULL);
        dap->last_read = data;
        return retval;
}

static int jtag_dp_q_write(struct adiv5_dap *dap, unsigned reg,
                uint32_t data)
{
        int retval =  adi_jtag_dp_scan_u32(dap, JTAG_DP_DPACC,
                        reg, DPAP_WRITE, data, dap->last_read, 0, NULL);
        dap->last_read = NULL;
        return retval;
}

/** Select the AP register bank matching bits 7:4 of reg. */
static int jtag_ap_q_bankselect(struct adiv5_ap *ap, unsigned reg)
{
        struct adiv5_dap *dap = ap->dap;
        uint32_t sel = ((uint32_t)ap->ap_num << 24) | (reg & 0x000000F0);

        if (sel == dap->select)
                return ERROR_OK;

        dap->select = sel;

        return jtag_dp_q_write(dap, DP_SELECT, sel);
}

static int jtag_ap_q_read(struct adiv5_ap *ap, unsigned reg,
                uint32_t *data)
{
        int retval = jtag_ap_q_bankselect(ap, reg);
        if (retval != ERROR_OK)
                return retval;

        retval =  adi_jtag_dp_scan_u32(ap->dap, JTAG_DP_APACC, reg,
                        DPAP_READ, 0, ap->dap->last_read, ap->memaccess_tck, NULL);
        ap->dap->last_read = data;

        return retval;
}

static int jtag_ap_q_write(struct adiv5_ap *ap, unsigned reg,
                uint32_t data)
{
        int retval = jtag_ap_q_bankselect(ap, reg);
        if (retval != ERROR_OK)
                return retval;

        retval =  adi_jtag_dp_scan_u32(ap->dap, JTAG_DP_APACC, reg,
                        DPAP_WRITE, data, ap->dap->last_read, ap->memaccess_tck, NULL);
        ap->dap->last_read = NULL;
        return retval;
}

static int jtag_ap_q_abort(struct adiv5_dap *dap, uint8_t *ack)
{
        /* for JTAG, this is the only valid ABORT register operation */
        int retval =  adi_jtag_dp_scan_u32(dap, JTAG_DP_ABORT,
                        0, DPAP_WRITE, 1, NULL, 0, NULL);
        if (retval != ERROR_OK)
                return retval;

        return jtag_execute_queue();
}

static int jtag_dp_run(struct adiv5_dap *dap)
{
        int retval;
        int retval2 = ERROR_OK;

        retval = adi_jtag_finish_read(dap);
        if (retval != ERROR_OK)
                goto done;
        retval2 = jtagdp_overrun_check(dap);
        retval = jtagdp_transaction_endcheck(dap);

 done:
        return (retval2 != ERROR_OK) ? retval2 : retval;
}

/* FIXME don't export ... just initialize as
 * part of DAP setup
*/
const struct dap_ops jtag_dp_ops = {
        .queue_dp_read       = jtag_dp_q_read,
        .queue_dp_write      = jtag_dp_q_write,
        .queue_ap_read       = jtag_ap_q_read,
        .queue_ap_write      = jtag_ap_q_write,
        .queue_ap_abort      = jtag_ap_q_abort,
        .run                 = jtag_dp_run,
};


static const uint8_t swd2jtag_bitseq[] = {
        /* More than 50 TCK/SWCLK cycles with TMS/SWDIO high,
         * putting both JTAG and SWD logic into reset state.
         */
        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
        /* Switching equence disables SWD and enables JTAG
         * NOTE: bits in the DP's IDCODE can expose the need for
         * the old/deprecated sequence (0xae 0xde).
         */
        0x3c, 0xe7,
        /* At least 50 TCK/SWCLK cycles with TMS/SWDIO high,
         * putting both JTAG and SWD logic into reset state.
         * NOTE:  some docs say "at least 5".
         */
        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
};

/** Put the debug link into JTAG mode, if the target supports it.
 * The link's initial mode may be either SWD or JTAG.
 *
 * @param target Enters JTAG mode (if possible).
 *
 * Note that targets implemented with SW-DP do not support JTAG, and
 * that some targets which could otherwise support it may have been
 * configured to disable JTAG signaling
 *
 * @return ERROR_OK or else a fault code.
 */
int dap_to_jtag(struct target *target)
{
        int retval;

        LOG_DEBUG("Enter JTAG mode");

        /* REVISIT it's nasty to need to make calls to a "jtag"
         * subsystem if the link isn't in JTAG mode...
         */

        retval = jtag_add_tms_seq(8 * sizeof(swd2jtag_bitseq),
                        swd2jtag_bitseq, TAP_RESET);
        if (retval == ERROR_OK)
                retval = jtag_execute_queue();

        /* REVISIT set up the DAP's ops vector for JTAG mode. */

        return retval;
}