tcl/interface: support for Raspberry Pi 5

[openocd.git] / src / target / adi_v5_swd.c

// SPDX-License-Identifier: GPL-2.0-or-later

/***************************************************************************
 *
 *   Copyright (C) 2010 by David Brownell
 ***************************************************************************/

/**
 * @file
 * Utilities to support ARM "Serial Wire Debug" (SWD), a low pin-count debug
 * link protocol used in cases where JTAG is not wanted.  This is coupled to
 * recent versions of ARM's "CoreSight" debug framework.  This specific code
 * is a transport level interface, with "target/arm_adi_v5.[hc]" code
 * understanding operation semantics, shared with the JTAG transport.
 *
 * Single DAP and multidrop-SWD support.
 *
 * for details, see "ARM IHI 0031A"
 * ARM Debug Interface v5 Architecture Specification
 * especially section 5.3 for SWD protocol
 * and "ARM IHI 0074C" ARM Debug Interface Architecture Specification ADIv6.0
 *
 * On many chips (most current Cortex-M3 parts) SWD is a run-time alternative
 * to JTAG.  Boards may support one or both.  There are also SWD-only chips,
 * (using SW-DP not SWJ-DP).
 *
 * Even boards that also support JTAG can benefit from SWD support, because
 * usually there's no way to access the SWO trace view mechanism in JTAG mode.
 * That is, trace access may require SWD support.
 *
 */

#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/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;


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 void swd_finish_read(struct adiv5_dap *dap)
{
        const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
        if (dap->last_read) {
                swd->read_reg(swd_cmd(true, false, DP_RDBUFF), dap->last_read, 0);
                dap->last_read = NULL;
        }
}

static void swd_clear_sticky_errors(struct adiv5_dap *dap)
{
        const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
        assert(swd);

        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;
}

/** Select the DP register bank */
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;

        swd->read_reg(swd_cmd(true, false, reg), data, 0);

        return check_sync(dap);
}

static int swd_queue_dp_write_inner(struct adiv5_dap *dap, unsigned int reg,
                uint32_t data)
{
        int retval = ERROR_OK;
        const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
        assert(swd);

        swd_finish_read(dap);

        if (reg == DP_SELECT) {
                dap->select = data | (dap->select & (0xffffffffull << 32));

                swd->write_reg(swd_cmd(false, false, reg), data, 0);

                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) {
                swd->write_reg(swd_cmd(false, false, reg), data, 0);

                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) */
                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 */
                swd_clear_sticky_errors(dap);

                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_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;

        swd->write_reg(swd_cmd(false, false, DP_ABORT),
                DAPABORT | STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
        return check_sync(dap);
}

static int swd_queue_dp_read(struct adiv5_dap *dap, unsigned 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;

        return swd_queue_dp_read_inner(dap, reg, data);
}

static int swd_queue_dp_write(struct adiv5_dap *dap, unsigned reg,
                uint32_t data)
{
        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;

        return swd_queue_dp_write_inner(dap, reg, data);
}

/** Select the AP register bank */
static int swd_queue_ap_bankselect(struct adiv5_ap *ap, unsigned 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);
                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));
                if (retval != ERROR_OK)
                        return retval;
        }

        return ERROR_OK;
}

static int swd_queue_ap_read(struct adiv5_ap *ap, unsigned 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;

        swd->read_reg(swd_cmd(true, true, reg), dap->last_read, ap->memaccess_tck);
        dap->last_read = data;

        return check_sync(dap);
}

static int swd_queue_ap_write(struct adiv5_ap *ap, unsigned 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;

        swd_finish_read(dap);

        retval = swd_queue_ap_bankselect(ap, reg);
        if (retval != ERROR_OK)
                return retval;

        swd->write_reg(swd_cmd(false, true, reg), data, ap->memaccess_tck);

        return check_sync(dap);
}

/** Executes all queued DAP operations. */
static int swd_run(struct adiv5_dap *dap)
{
        int retval = swd_multidrop_select(dap);
        if (retval != ERROR_OK)
                return retval;

        swd_finish_read(dap);

        retval = swd_run_inner(dap);
        if (retval != ERROR_OK) {
                /* fault response */
                dap->do_reconnect = true;
        }

        return retval;
}

/** Put the SWJ-DP back to JTAG mode */
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)
{
        /* FIXME: only place where global 'adapter_driver' is still needed */
        extern struct adapter_driver *adapter_driver;
        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 = {
        .name = "swd",
        .select = swd_select,
        .init = swd_init,
};

static void swd_constructor(void) __attribute__((constructor));
static void swd_constructor(void)
{
        transport_register(&swd_transport);
}

/** Returns true if the current debug session
 * is using SWD as its transport.
 */
bool transport_is_swd(void)
{
        return get_current_transport() == &swd_transport;
}