adi_v5_swd.c

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// SPDX-License-Identifier: GPL-2.0-or-later
 
/***************************************************************************
 *
 *   Copyright (C) 2010 by David Brownell
 ***************************************************************************/
 
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
 
#include "arm.h"
#include "arm_adi_v5.h"
#include <helper/time_support.h>
 
#include <transport/transport.h>
#include <jtag/adapter.h>
#include <jtag/interface.h>
 
#include <jtag/swd.h>
 
/* for debug, set do_sync to true to force synchronous transfers */
static bool do_sync;
 
static struct adiv5_dap *swd_multidrop_selected_dap;
 
static bool swd_multidrop_in_swd_state;
 
enum swd_recovery_mode {
    SWD_RECOVERY_QUEUE,
    SWD_RECOVERY_RUN
};
 
static int swd_queue_dp_write_inner(struct adiv5_dap *dap, unsigned int reg,
        uint32_t data);
 
 
static int swd_send_sequence(struct adiv5_dap *dap, enum swd_special_seq seq)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    return swd->switch_seq(seq);
}
 
static int swd_finish_read(struct adiv5_dap *dap)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    if (!dap->last_read)
        return ERROR_OK;
 
    int retval = swd->read_reg(swd_cmd(true, false, DP_RDBUFF),
                               dap->last_read, 0);
    dap->last_read = NULL;
    return retval;
}
 
static int swd_clear_sticky_errors(struct adiv5_dap *dap)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    return swd->write_reg(swd_cmd(false, false, DP_ABORT),
        STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
}
 
static int swd_run_inner(struct adiv5_dap *dap)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 
    return swd->run();
}
 
static inline int check_sync(struct adiv5_dap *dap)
{
    return do_sync ? swd_run_inner(dap) : ERROR_OK;
}
 
static int swd_queue_dp_bankselect(struct adiv5_dap *dap, unsigned int reg)
{
    /* Only register address 0 (ADIv6 only) and 4 are banked. */
    if (is_adiv6(dap) ? (reg & 0xf) > 4 : (reg & 0xf) != 4)
        return ERROR_OK;
 
    uint32_t sel = (reg >> 4) & DP_SELECT_DPBANK;
 
    /* ADIv6 ensures DPBANKSEL = 0 after line reset */
    if ((dap->select_valid || (is_adiv6(dap) && dap->select_dpbanksel_valid))
            && (sel == (dap->select & DP_SELECT_DPBANK)))
        return ERROR_OK;
 
    /* Use the AP part of dap->select regardless of dap->select_valid:
     * if !dap->select_valid
     * dap->select contains a speculative value likely going to be used
     * in the following swd_queue_ap_bankselect() */
    sel |= (uint32_t)(dap->select & SELECT_AP_MASK);
 
    LOG_DEBUG_IO("DP BANK SELECT: %" PRIx32, sel);
 
    /* dap->select cache gets updated in the following call */
    return swd_queue_dp_write_inner(dap, DP_SELECT, sel);
}
 
static int swd_queue_dp_read_inner(struct adiv5_dap *dap, unsigned int reg,
        uint32_t *data)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    int retval = swd_queue_dp_bankselect(dap, reg);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd->read_reg(swd_cmd(true, false, reg), data, 0);
    if (retval != ERROR_OK)
        return retval;
 
    return check_sync(dap);
}
 
static int swd_queue_dp_write_inner(struct adiv5_dap *dap, unsigned int reg,
        uint32_t data)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    int retval = swd_finish_read(dap);
    if (retval != ERROR_OK)
        return retval;
 
 
    if (reg == DP_SELECT) {
        dap->select = data | (dap->select & (0xffffffffull << 32));
 
        retval = swd->write_reg(swd_cmd(false, false, reg), data, 0);
        if (retval != ERROR_OK)
            return retval;
 
        retval = check_sync(dap);
        dap->select_valid = (retval == ERROR_OK);
        dap->select_dpbanksel_valid = dap->select_valid;
 
        return retval;
    }
 
    if (reg == DP_SELECT1)
        dap->select = ((uint64_t)data << 32) | (dap->select & 0xffffffffull);
 
    /* DP_ABORT write is not banked.
     * Prevent writing DP_SELECT before as it would fail on locked up DP */
    if (reg != DP_ABORT)
        retval = swd_queue_dp_bankselect(dap, reg);
 
    if (retval == ERROR_OK) {
        retval = swd->write_reg(swd_cmd(false, false, reg), data, 0);
        if (retval != ERROR_OK)
            return retval;
 
        retval = check_sync(dap);
    }
 
    if (reg == DP_SELECT1)
        dap->select1_valid = (retval == ERROR_OK);
 
    return retval;
}
 
 
static int swd_multidrop_select_inner(struct adiv5_dap *dap, uint32_t *dpidr_ptr,
        uint32_t *dlpidr_ptr, bool clear_sticky)
{
    int retval;
    uint32_t dpidr, dlpidr;
 
    assert(dap_is_multidrop(dap));
 
    /* Send JTAG_TO_DORMANT and DORMANT_TO_SWD just once
     * and then use shorter LINE_RESET until communication fails */
    if (!swd_multidrop_in_swd_state) {
        swd_send_sequence(dap, JTAG_TO_DORMANT);
        swd_send_sequence(dap, DORMANT_TO_SWD);
    } else {
        swd_send_sequence(dap, LINE_RESET);
    }
 
    /*
     * Zero dap->select and set dap->select_dpbanksel_valid
     * to skip the write to DP_SELECT before DPIDR read, avoiding
     * the protocol error.
     * Clear the other validity flags because the rest of the DP
     * SELECT and SELECT1 registers is unknown after line reset.
     */
    dap->select = 0;
    dap->select_dpbanksel_valid = true;
    dap->select_valid = false;
    dap->select1_valid = false;
 
    retval = swd_queue_dp_write_inner(dap, DP_TARGETSEL, dap->multidrop_targetsel);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_dp_read_inner(dap, DP_DPIDR, &dpidr);
    if (retval != ERROR_OK)
        return retval;
 
    if (clear_sticky) {
        /* Clear all sticky errors (including ORUN) */
        retval = swd_clear_sticky_errors(dap);
    } else {
        /* Ideally just clear ORUN flag which is set by reset */
        retval = swd_queue_dp_write_inner(dap, DP_ABORT, ORUNERRCLR);
    }
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_dp_read_inner(dap, DP_DLPIDR, &dlpidr);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_run_inner(dap);
    if (retval != ERROR_OK)
        return retval;
 
    if ((dpidr & DP_DPIDR_VERSION_MASK) < (2UL << DP_DPIDR_VERSION_SHIFT)) {
        LOG_INFO("Read DPIDR 0x%08" PRIx32
                 " has version < 2. A non multidrop capable device connected?",
                 dpidr);
        return ERROR_FAIL;
    }
 
    /* TODO: check TARGETID if DLIPDR is same for more than one DP */
    uint32_t expected_dlpidr = DP_DLPIDR_PROTVSN |
            (dap->multidrop_targetsel & DP_TARGETSEL_INSTANCEID_MASK);
    if (dlpidr != expected_dlpidr) {
        LOG_INFO("Read incorrect DLPIDR 0x%08" PRIx32
                 " (possibly CTRL/STAT value)",
                 dlpidr);
        return ERROR_FAIL;
    }
 
    LOG_DEBUG_IO("Selected DP_TARGETSEL 0x%08" PRIx32, dap->multidrop_targetsel);
    swd_multidrop_selected_dap = dap;
    swd_multidrop_in_swd_state = true;
 
    if (dpidr_ptr)
        *dpidr_ptr = dpidr;
 
    if (dlpidr_ptr)
        *dlpidr_ptr = dlpidr;
 
    return retval;
}
 
static int swd_multidrop_select(struct adiv5_dap *dap)
{
    if (!dap_is_multidrop(dap))
        return ERROR_OK;
 
    if (swd_multidrop_selected_dap == dap)
        return ERROR_OK;
 
    int retval = ERROR_OK;
    for (unsigned int retry = 0; ; retry++) {
        bool clear_sticky = retry > 0;
 
        retval = swd_multidrop_select_inner(dap, NULL, NULL, clear_sticky);
        if (retval == ERROR_OK)
            break;
 
        swd_multidrop_selected_dap = NULL;
        if (retry > 3) {
            LOG_ERROR("Failed to select multidrop %s", adiv5_dap_name(dap));
            dap->do_reconnect = true;
            return retval;
        }
 
        LOG_DEBUG("Failed to select multidrop %s, retrying...",
                  adiv5_dap_name(dap));
    }
 
    dap->do_reconnect = false;
    return retval;
}
 
static int swd_connect_multidrop(struct adiv5_dap *dap)
{
    int retval;
    uint32_t dpidr = 0xdeadbeef;
    uint32_t dlpidr = 0xdeadbeef;
    int64_t timeout = timeval_ms() + 500;
 
    do {
        /* Do not make any assumptions about SWD state in case of reconnect */
        if (dap->do_reconnect)
            swd_multidrop_in_swd_state = false;
 
        /* Clear link state, including the SELECT cache. */
        dap->do_reconnect = false;
        dap_invalidate_cache(dap);
        swd_multidrop_selected_dap = NULL;
 
        retval = swd_multidrop_select_inner(dap, &dpidr, &dlpidr, true);
        if (retval == ERROR_OK)
            break;
 
        swd_multidrop_in_swd_state = false;
        alive_sleep(1);
 
    } while (timeval_ms() < timeout);
 
    if (retval != ERROR_OK) {
        swd_multidrop_selected_dap = NULL;
        LOG_ERROR("Failed to connect multidrop %s", adiv5_dap_name(dap));
        return retval;
    }
 
    swd_multidrop_in_swd_state = true;
    LOG_INFO("SWD DPIDR 0x%08" PRIx32 ", DLPIDR 0x%08" PRIx32,
              dpidr, dlpidr);
 
    return retval;
}
 
static int swd_connect_single(struct adiv5_dap *dap)
{
    int retval;
    uint32_t dpidr = 0xdeadbeef;
    int64_t timeout = timeval_ms() + 500;
 
    do {
        if (dap->switch_through_dormant) {
            swd_send_sequence(dap, JTAG_TO_DORMANT);
            swd_send_sequence(dap, DORMANT_TO_SWD);
        } else {
            swd_send_sequence(dap, JTAG_TO_SWD);
        }
 
        /* Clear link state, including the SELECT cache. */
        dap->do_reconnect = false;
        dap_invalidate_cache(dap);
 
        /* The sequences to enter in SWD (JTAG_TO_SWD and DORMANT_TO_SWD) end
         * with a SWD line reset sequence (50 clk with SWDIO high).
         * From ARM IHI 0031F ADIv5.2 and ARM IHI 0074C ADIv6.0,
         * chapter B4.3.3 "Connection and line reset sequence":
         * - DPv3 (ADIv6) only: line reset sets DP_SELECT_DPBANK to zero;
         * - read of DP_DPIDR takes the connection out of reset;
         * - write of DP_TARGETSEL keeps the connection in reset;
         * - other accesses return protocol error (SWDIO not driven by target).
         *
         * dap_invalidate_cache() sets dap->select to zero and all validity
         * flags to invalid. Set dap->select_dpbanksel_valid only
         * to skip the write to DP_SELECT, avoiding the protocol error.
         * Read DP_DPIDR to get out of reset.
         */
        dap->select_dpbanksel_valid = true;
 
        retval = swd_queue_dp_read_inner(dap, DP_DPIDR, &dpidr);
        if (retval == ERROR_OK) {
            retval = swd_run_inner(dap);
            if (retval == ERROR_OK)
                break;
        }
 
        alive_sleep(1);
 
        dap->switch_through_dormant = !dap->switch_through_dormant;
    } while (timeval_ms() < timeout);
 
    if (retval != ERROR_OK) {
        LOG_ERROR("Error connecting DP: cannot read IDR");
        return retval;
    }
 
    LOG_INFO("SWD DPIDR 0x%08" PRIx32, dpidr);
 
    do {
        dap->do_reconnect = false;
 
        /* force clear all sticky faults */
        retval = swd_clear_sticky_errors(dap);
 
        if (retval == ERROR_OK)
            retval = swd_run_inner(dap);
 
        if (retval != ERROR_WAIT)
            break;
 
        alive_sleep(10);
 
    } while (timeval_ms() < timeout);
 
    return retval;
}
 
static int swd_pre_connect(struct adiv5_dap *dap)
{
    swd_multidrop_in_swd_state = false;
 
    return ERROR_OK;
}
 
static int swd_connect(struct adiv5_dap *dap)
{
    int status;
 
    /* FIXME validate transport config ... is the
     * configured DAP present (check IDCODE)?
     */
 
    /* Check if we should reset srst already when connecting, but not if reconnecting. */
    if (!dap->do_reconnect) {
        enum reset_types jtag_reset_config = jtag_get_reset_config();
 
        if (jtag_reset_config & RESET_CNCT_UNDER_SRST) {
            if (jtag_reset_config & RESET_SRST_NO_GATING)
                adapter_assert_reset();
            else
                LOG_WARNING("\'srst_nogate\' reset_config option is required");
        }
    }
 
    if (dap_is_multidrop(dap))
        status = swd_connect_multidrop(dap);
    else
        status = swd_connect_single(dap);
 
    /* IHI 0031E B4.3.2:
     * "A WAIT response must not be issued to the ...
     * ... writes to the ABORT register"
     * swd_clear_sticky_errors() writes to the ABORT register only.
     *
     * Unfortunately at least Microchip SAMD51/E53/E54 returns WAIT
     * in a corner case. Just try if ABORT resolves the problem.
     */
    if (status == ERROR_WAIT) {
        LOG_WARNING("Connecting DP: stalled AP operation, issuing ABORT");
 
        dap->do_reconnect = false;
 
        status = swd_queue_dp_write_inner(dap, DP_ABORT,
            DAPABORT | STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR);
 
        if (status == ERROR_OK)
            status = swd_run_inner(dap);
    }
 
    if (status == ERROR_OK)
        status = dap_dp_init(dap);
 
    return status;
}
 
static int swd_error_recovery(struct adiv5_dap *dap, int err,
                              enum swd_recovery_mode mode)
{
    /* TODO: do not reconnect on all errors, just clear sticky
     * after SWD FAULT */
    if (err != ERROR_OK) {
        dap->do_reconnect = true;
        LOG_DEBUG_IO("Reconnect on next SWD op");
    }
    return err;
}
 
static int swd_check_reconnect(struct adiv5_dap *dap)
{
    if (dap->do_reconnect)
        return swd_connect(dap);
 
    return ERROR_OK;
}
 
static int swd_queue_ap_abort(struct adiv5_dap *dap, uint8_t *ack)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    /* TODO: Send DAPABORT in swd_multidrop_select_inner()
     * in the case the multidrop dap is not selected?
     * swd_queue_ap_abort() is not currently used anyway...
     */
    int retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd->write_reg(swd_cmd(false, false, DP_ABORT),
        DAPABORT | STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
    if (retval != ERROR_OK)
        return retval;
 
    return check_sync(dap);
}
 
static int swd_queue_dp_read(struct adiv5_dap *dap, unsigned int reg,
        uint32_t *data)
{
    int retval = swd_check_reconnect(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_dp_read_inner(dap, reg, data);
    return swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
}
 
static int swd_queue_dp_write(struct adiv5_dap *dap, unsigned int reg,
        uint32_t data)
{
    int retval = swd_check_reconnect(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_dp_write_inner(dap, reg, data);
    return swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
}
 
static int swd_queue_ap_bankselect(struct adiv5_ap *ap, unsigned int reg)
{
    int retval;
    struct adiv5_dap *dap = ap->dap;
    uint64_t sel;
 
    if (is_adiv6(dap))
        sel = ap->ap_num | (reg & 0x00000FF0);
    else
        sel = (ap->ap_num << 24) | (reg & ADIV5_DP_SELECT_APBANK);
 
    uint64_t sel_diff = (sel ^ dap->select) & SELECT_AP_MASK;
 
    bool set_select = !dap->select_valid || (sel_diff & 0xffffffffull);
    bool set_select1 = is_adiv6(dap) && dap->asize > 32
                        && (!dap->select1_valid
                            || sel_diff & (0xffffffffull << 32));
 
    if (set_select && set_select1) {
        /* Prepare DP bank for DP_SELECT1 now to save one write */
        sel |= (DP_SELECT1 & 0x000000f0) >> 4;
    } else {
        /* Use the DP part of dap->select regardless of dap->select_valid:
         * if !dap->select_valid
         * dap->select contains a speculative value likely going to be used
         * in the following swd_queue_dp_bankselect().
         * Moreover dap->select_valid should never be false here as a DP bank
         * is always selected before selecting an AP bank */
        sel |= dap->select & DP_SELECT_DPBANK;
    }
 
    if (set_select) {
        LOG_DEBUG_IO("AP BANK SELECT: %" PRIx32, (uint32_t)sel);
 
        retval = swd_queue_dp_write(dap, DP_SELECT, (uint32_t)sel);
        retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
        if (retval != ERROR_OK)
            return retval;
    }
 
    if (set_select1) {
        LOG_DEBUG_IO("AP BANK SELECT1: %" PRIx32, (uint32_t)(sel >> 32));
 
        retval = swd_queue_dp_write(dap, DP_SELECT1, (uint32_t)(sel >> 32));
        retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
        if (retval != ERROR_OK)
            return retval;
    }
 
    return ERROR_OK;
}
 
static int swd_queue_ap_read(struct adiv5_ap *ap, unsigned int reg,
        uint32_t *data)
{
    struct adiv5_dap *dap = ap->dap;
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    int retval = swd_check_reconnect(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_ap_bankselect(ap, reg);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd->read_reg(swd_cmd(true, true, reg),
                           dap->last_read, ap->memaccess_tck);
    retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
    if (retval != ERROR_OK)
        return retval;
 
    dap->last_read = data;
 
    return check_sync(dap);
}
 
static int swd_queue_ap_write(struct adiv5_ap *ap, unsigned int reg,
        uint32_t data)
{
    struct adiv5_dap *dap = ap->dap;
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    assert(swd);
 
    int retval = swd_check_reconnect(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_finish_read(dap);
    retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_queue_ap_bankselect(ap, reg);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd->write_reg(swd_cmd(false, true, reg), data, ap->memaccess_tck);
    retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
    if (retval != ERROR_OK)
        return retval;
 
    return check_sync(dap);
}
 
static int swd_run(struct adiv5_dap *dap)
{
    int retval = swd_multidrop_select(dap);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_finish_read(dap);
    retval = swd_error_recovery(dap, retval, SWD_RECOVERY_QUEUE);
    if (retval != ERROR_OK)
        return retval;
 
    retval = swd_run_inner(dap);
    return swd_error_recovery(dap, retval, SWD_RECOVERY_RUN);
}
 
static void swd_quit(struct adiv5_dap *dap)
{
    const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
    static bool done;
 
    /* There is no difference if the sequence is sent at the last
     * or the first swd_quit() call, send it just once */
    if (done)
        return;
 
    done = true;
    if (dap_is_multidrop(dap)) {
        /* Emit the switch seq to dormant state regardless the state mirrored
         * in swd_multidrop_in_swd_state. Doing so ensures robust operation
         * in the case the variable is out of sync.
         * Sending SWD_TO_DORMANT makes no change if the DP is already dormant. */
        swd->switch_seq(SWD_TO_DORMANT);
        swd_multidrop_in_swd_state = false;
        /* Revisit!
         * Leaving DPs in dormant state was tested and offers some safety
         * against DPs mismatch in case of unintentional use of non-multidrop SWD.
         * To put SWJ-DPs to power-on state issue
         * swd->switch_seq(DORMANT_TO_JTAG);
         */
    } else {
        if (dap->switch_through_dormant) {
            swd->switch_seq(SWD_TO_DORMANT);
            swd->switch_seq(DORMANT_TO_JTAG);
        } else {
            swd->switch_seq(SWD_TO_JTAG);
        }
    }
 
    /* flush the queue to shift out the sequence before exit */
    swd->run();
}
 
const struct dap_ops swd_dap_ops = {
    .pre_connect_init = swd_pre_connect,
    .connect = swd_connect,
    .send_sequence = swd_send_sequence,
    .queue_dp_read = swd_queue_dp_read,
    .queue_dp_write = swd_queue_dp_write,
    .queue_ap_read = swd_queue_ap_read,
    .queue_ap_write = swd_queue_ap_write,
    .queue_ap_abort = swd_queue_ap_abort,
    .run = swd_run,
    .quit = swd_quit,
};
 
static const struct command_registration swd_commands[] = {
    {
        /*
         * Set up SWD and JTAG targets identically, unless/until
         * infrastructure improves ...  meanwhile, ignore all
         * JTAG-specific stuff like IR length for SWD.
         *
         * REVISIT can we verify "just one SWD DAP" here/early?
         */
        .name = "newdap",
        .handler = handle_jtag_newtap,
        .mode = COMMAND_CONFIG,
        .help = "declare a new SWD DAP",
        .usage = "basename dap_type ['-irlen' count] "
            "['-enable'|'-disable'] "
            "['-expected-id' number] "
            "['-ignore-version'] "
            "['-ignore-bypass'] "
            "['-ircapture' number] "
            "['-ir-bypass' number] "
            "['-mask' number]",
    },
    COMMAND_REGISTRATION_DONE
};
 
static const struct command_registration swd_handlers[] = {
    {
        .name = "swd",
        .mode = COMMAND_ANY,
        .help = "SWD command group",
        .chain = swd_commands,
        .usage = "",
    },
    COMMAND_REGISTRATION_DONE
};
 
static int swd_select(struct command_context *ctx)
{
    const struct swd_driver *swd = adapter_driver->swd_ops;
    int retval;
 
    retval = register_commands(ctx, NULL, swd_handlers);
    if (retval != ERROR_OK)
        return retval;
 
     /* be sure driver is in SWD mode; start
      * with hardware default TRN (1), it can be changed later
      */
    if (!swd || !swd->read_reg || !swd->write_reg || !swd->init) {
        LOG_DEBUG("no SWD driver?");
        return ERROR_FAIL;
    }
 
    retval = swd->init();
    if (retval != ERROR_OK) {
        LOG_DEBUG("can't init SWD driver");
        return retval;
    }
 
    return retval;
}
 
static int swd_init(struct command_context *ctx)
{
    /* nothing done here, SWD is initialized
     * together with the DAP */
    return ERROR_OK;
}
 
static struct transport swd_transport = {
    .id = TRANSPORT_SWD,
    .select = swd_select,
    .init = swd_init,
};
 
static void swd_constructor(void) __attribute__((constructor));
static void swd_constructor(void)
{
    transport_register(&swd_transport);
}
 
bool transport_is_swd(void)
{
    return get_current_transport() == &swd_transport;
}