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   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2
   3 /***************************************************************************
   4  *
   5  *   Copyright (C) 2010 by David Brownell
   6  ***************************************************************************/
   7
   8 /**
   9  * @file
  10  * Utilities to support ARM "Serial Wire Debug" (SWD), a low pin-count debug
  11  * link protocol used in cases where JTAG is not wanted.  This is coupled to
  12  * recent versions of ARM's "CoreSight" debug framework.  This specific code
  13  * is a transport level interface, with "target/arm_adi_v5.[hc]" code
  14  * understanding operation semantics, shared with the JTAG transport.
  15  *
  16  * Single-DAP support only.
  17  *
  18  * for details, see "ARM IHI 0031A"
  19  * ARM Debug Interface v5 Architecture Specification
  20  * especially section 5.3 for SWD protocol
  21  * and "ARM IHI 0074C" ARM Debug Interface Architecture Specification ADIv6.0
  22  *
  23  * On many chips (most current Cortex-M3 parts) SWD is a run-time alternative
  24  * to JTAG.  Boards may support one or both.  There are also SWD-only chips,
  25  * (using SW-DP not SWJ-DP).
  26  *
  27  * Even boards that also support JTAG can benefit from SWD support, because
  28  * usually there's no way to access the SWO trace view mechanism in JTAG mode.
  29  * That is, trace access may require SWD support.
  30  *
  31  */
  32
  33 #ifdef HAVE_CONFIG_H
  34 #include "config.h"
  35 #endif
  36
  37 #include "arm.h"
  38 #include "arm_adi_v5.h"
  39 #include <helper/time_support.h>
  40
  41 #include <transport/transport.h>
  42 #include <jtag/interface.h>
  43
  44 #include <jtag/swd.h>
  45
  46 /* for debug, set do_sync to true to force synchronous transfers */
  47 static bool do_sync;
  48
  49 static struct adiv5_dap *swd_multidrop_selected_dap;
  50
  51
  52 static int swd_queue_dp_write_inner(struct adiv5_dap *dap, unsigned int reg,
  53                 uint32_t data);
  54
  55
  56 static int swd_send_sequence(struct adiv5_dap *dap, enum swd_special_seq seq)
  57 {
  58         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
  59         assert(swd);
  60
  61         return swd->switch_seq(seq);
  62 }
  63
  64 static void swd_finish_read(struct adiv5_dap *dap)
  65 {
  66         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
  67         if (dap->last_read) {
  68                 swd->read_reg(swd_cmd(true, false, DP_RDBUFF), dap->last_read, 0);
  69                 dap->last_read = NULL;
  70         }
  71 }
  72
  73 static void swd_clear_sticky_errors(struct adiv5_dap *dap)
  74 {
  75         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
  76         assert(swd);
  77
  78         swd->write_reg(swd_cmd(false, false, DP_ABORT),
  79                 STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
  80 }
  81
  82 static int swd_run_inner(struct adiv5_dap *dap)
  83 {
  84         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
  85         int retval;
  86
  87         retval = swd->run();
  88
  89         if (retval != ERROR_OK) {
  90                 /* fault response */
  91                 dap->do_reconnect = true;
  92         }
  93
  94         return retval;
  95 }
  96
  97 static inline int check_sync(struct adiv5_dap *dap)
  98 {
  99         return do_sync ? swd_run_inner(dap) : ERROR_OK;
 100 }
 101
 102 /** Select the DP register bank matching bits 7:4 of reg. */
 103 static int swd_queue_dp_bankselect(struct adiv5_dap *dap, unsigned int reg)
 104 {
 105         /* Only register address 0 and 4 are banked. */
 106         if ((reg & 0xf) > 4)
 107                 return ERROR_OK;
 108
 109         uint64_t sel = (reg & 0x000000F0) >> 4;
 110         if (dap->select != DP_SELECT_INVALID)
 111                 sel |= dap->select & ~0xfULL;
 112
 113         if (sel == dap->select)
 114                 return ERROR_OK;
 115
 116         dap->select = sel;
 117
 118         int retval = swd_queue_dp_write_inner(dap, DP_SELECT, (uint32_t)sel);
 119         if (retval != ERROR_OK)
 120                 dap->select = DP_SELECT_INVALID;
 121
 122         return retval;
 123 }
 124
 125 static int swd_queue_dp_read_inner(struct adiv5_dap *dap, unsigned int reg,
 126                 uint32_t *data)
 127 {
 128         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 129         assert(swd);
 130
 131         int retval = swd_queue_dp_bankselect(dap, reg);
 132         if (retval != ERROR_OK)
 133                 return retval;
 134
 135         swd->read_reg(swd_cmd(true, false, reg), data, 0);
 136
 137         return check_sync(dap);
 138 }
 139
 140 static int swd_queue_dp_write_inner(struct adiv5_dap *dap, unsigned int reg,
 141                 uint32_t data)
 142 {
 143         int retval;
 144         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 145         assert(swd);
 146
 147         swd_finish_read(dap);
 148
 149         if (reg == DP_SELECT) {
 150                 dap->select = data & (DP_SELECT_APSEL | DP_SELECT_APBANK | DP_SELECT_DPBANK);
 151
 152                 swd->write_reg(swd_cmd(false, false, reg), data, 0);
 153
 154                 retval = check_sync(dap);
 155                 if (retval != ERROR_OK)
 156                         dap->select = DP_SELECT_INVALID;
 157
 158                 return retval;
 159         }
 160
 161         retval = swd_queue_dp_bankselect(dap, reg);
 162         if (retval != ERROR_OK)
 163                 return retval;
 164
 165         swd->write_reg(swd_cmd(false, false, reg), data, 0);
 166
 167         return check_sync(dap);
 168 }
 169
 170
 171 static int swd_multidrop_select_inner(struct adiv5_dap *dap, uint32_t *dpidr_ptr,
 172                 uint32_t *dlpidr_ptr, bool clear_sticky)
 173 {
 174         int retval;
 175         uint32_t dpidr, dlpidr;
 176
 177         assert(dap_is_multidrop(dap));
 178
 179         swd_send_sequence(dap, LINE_RESET);
 180
 181         retval = swd_queue_dp_write_inner(dap, DP_TARGETSEL, dap->multidrop_targetsel);
 182         if (retval != ERROR_OK)
 183                 return retval;
 184
 185         retval = swd_queue_dp_read_inner(dap, DP_DPIDR, &dpidr);
 186         if (retval != ERROR_OK)
 187                 return retval;
 188
 189         if (clear_sticky) {
 190                 /* Clear all sticky errors (including ORUN) */
 191                 swd_clear_sticky_errors(dap);
 192         } else {
 193                 /* Ideally just clear ORUN flag which is set by reset */
 194                 retval = swd_queue_dp_write_inner(dap, DP_ABORT, ORUNERRCLR);
 195                 if (retval != ERROR_OK)
 196                         return retval;
 197         }
 198
 199         retval = swd_queue_dp_read_inner(dap, DP_DLPIDR, &dlpidr);
 200         if (retval != ERROR_OK)
 201                 return retval;
 202
 203         retval = swd_run_inner(dap);
 204         if (retval != ERROR_OK)
 205                 return retval;
 206
 207         if ((dpidr & DP_DPIDR_VERSION_MASK) < (2UL << DP_DPIDR_VERSION_SHIFT)) {
 208                 LOG_INFO("Read DPIDR 0x%08" PRIx32
 209                                  " has version < 2. A non multidrop capable device connected?",
 210                                  dpidr);
 211                 return ERROR_FAIL;
 212         }
 213
 214         /* TODO: check TARGETID if DLIPDR is same for more than one DP */
 215         uint32_t expected_dlpidr = DP_DLPIDR_PROTVSN |
 216                         (dap->multidrop_targetsel & DP_TARGETSEL_INSTANCEID_MASK);
 217         if (dlpidr != expected_dlpidr) {
 218                 LOG_INFO("Read incorrect DLPIDR 0x%08" PRIx32
 219                                  " (possibly CTRL/STAT value)",
 220                                  dlpidr);
 221                 return ERROR_FAIL;
 222         }
 223
 224         LOG_DEBUG_IO("Selected DP_TARGETSEL 0x%08" PRIx32, dap->multidrop_targetsel);
 225         swd_multidrop_selected_dap = dap;
 226
 227         if (dpidr_ptr)
 228                 *dpidr_ptr = dpidr;
 229
 230         if (dlpidr_ptr)
 231                 *dlpidr_ptr = dlpidr;
 232
 233         return retval;
 234 }
 235
 236 static int swd_multidrop_select(struct adiv5_dap *dap)
 237 {
 238         if (!dap_is_multidrop(dap))
 239                 return ERROR_OK;
 240
 241         if (swd_multidrop_selected_dap == dap)
 242                 return ERROR_OK;
 243
 244         int retval = ERROR_OK;
 245         for (unsigned int retry = 0; ; retry++) {
 246                 bool clear_sticky = retry > 0;
 247
 248                 retval = swd_multidrop_select_inner(dap, NULL, NULL, clear_sticky);
 249                 if (retval == ERROR_OK)
 250                         break;
 251
 252                 swd_multidrop_selected_dap = NULL;
 253                 if (retry > 3) {
 254                         LOG_ERROR("Failed to select multidrop %s", adiv5_dap_name(dap));
 255                         return retval;
 256                 }
 257
 258                 LOG_DEBUG("Failed to select multidrop %s, retrying...",
 259                                   adiv5_dap_name(dap));
 260         }
 261
 262         return retval;
 263 }
 264
 265 static int swd_connect_multidrop(struct adiv5_dap *dap)
 266 {
 267         int retval;
 268         uint32_t dpidr = 0xdeadbeef;
 269         uint32_t dlpidr = 0xdeadbeef;
 270         int64_t timeout = timeval_ms() + 500;
 271
 272         do {
 273                 swd_send_sequence(dap, JTAG_TO_DORMANT);
 274                 swd_send_sequence(dap, DORMANT_TO_SWD);
 275
 276                 /* Clear link state, including the SELECT cache. */
 277                 dap->do_reconnect = false;
 278                 dap_invalidate_cache(dap);
 279                 swd_multidrop_selected_dap = NULL;
 280
 281                 retval = swd_multidrop_select_inner(dap, &dpidr, &dlpidr, true);
 282                 if (retval == ERROR_OK)
 283                         break;
 284
 285                 alive_sleep(1);
 286
 287         } while (timeval_ms() < timeout);
 288
 289         if (retval != ERROR_OK) {
 290                 swd_multidrop_selected_dap = NULL;
 291                 LOG_ERROR("Failed to connect multidrop %s", adiv5_dap_name(dap));
 292                 return retval;
 293         }
 294
 295         LOG_INFO("SWD DPIDR 0x%08" PRIx32 ", DLPIDR 0x%08" PRIx32,
 296                           dpidr, dlpidr);
 297
 298         return retval;
 299 }
 300
 301 static int swd_connect_single(struct adiv5_dap *dap)
 302 {
 303         int retval;
 304         uint32_t dpidr = 0xdeadbeef;
 305         int64_t timeout = timeval_ms() + 500;
 306
 307         do {
 308                 if (dap->switch_through_dormant) {
 309                         swd_send_sequence(dap, JTAG_TO_DORMANT);
 310                         swd_send_sequence(dap, DORMANT_TO_SWD);
 311                 } else {
 312                         swd_send_sequence(dap, JTAG_TO_SWD);
 313                 }
 314
 315                 /* Clear link state, including the SELECT cache. */
 316                 dap->do_reconnect = false;
 317                 dap_invalidate_cache(dap);
 318
 319                 /* The sequences to enter in SWD (JTAG_TO_SWD and DORMANT_TO_SWD) end
 320                  * with a SWD line reset sequence (50 clk with SWDIO high).
 321                  * From ARM IHI 0074C ADIv6.0, chapter B4.3.3 "Connection and line reset
 322                  * sequence":
 323                  * - line reset sets DP_SELECT_DPBANK to zero;
 324                  * - read of DP_DPIDR takes the connection out of reset;
 325                  * - write of DP_TARGETSEL keeps the connection in reset;
 326                  * - other accesses return protocol error (SWDIO not driven by target).
 327                  *
 328                  * Read DP_DPIDR to get out of reset. Initialize dap->select to zero to
 329                  * skip the write to DP_SELECT, avoiding the protocol error. Set again
 330                  * dap->select to DP_SELECT_INVALID because the rest of the register is
 331                  * unknown after line reset.
 332                  */
 333                 dap->select = 0;
 334                 retval = swd_queue_dp_read_inner(dap, DP_DPIDR, &dpidr);
 335                 if (retval == ERROR_OK) {
 336                         retval = swd_run_inner(dap);
 337                         if (retval == ERROR_OK)
 338                                 break;
 339                 }
 340
 341                 alive_sleep(1);
 342
 343                 dap->switch_through_dormant = !dap->switch_through_dormant;
 344         } while (timeval_ms() < timeout);
 345         dap->select = DP_SELECT_INVALID;
 346
 347         if (retval != ERROR_OK) {
 348                 LOG_ERROR("Error connecting DP: cannot read IDR");
 349                 return retval;
 350         }
 351
 352         LOG_INFO("SWD DPIDR 0x%08" PRIx32, dpidr);
 353
 354         do {
 355                 dap->do_reconnect = false;
 356
 357                 /* force clear all sticky faults */
 358                 swd_clear_sticky_errors(dap);
 359
 360                 retval = swd_run_inner(dap);
 361                 if (retval != ERROR_WAIT)
 362                         break;
 363
 364                 alive_sleep(10);
 365
 366         } while (timeval_ms() < timeout);
 367
 368         return retval;
 369 }
 370
 371 static int swd_connect(struct adiv5_dap *dap)
 372 {
 373         int status;
 374
 375         /* FIXME validate transport config ... is the
 376          * configured DAP present (check IDCODE)?
 377          */
 378
 379         /* Check if we should reset srst already when connecting, but not if reconnecting. */
 380         if (!dap->do_reconnect) {
 381                 enum reset_types jtag_reset_config = jtag_get_reset_config();
 382
 383                 if (jtag_reset_config & RESET_CNCT_UNDER_SRST) {
 384                         if (jtag_reset_config & RESET_SRST_NO_GATING)
 385                                 adapter_assert_reset();
 386                         else
 387                                 LOG_WARNING("\'srst_nogate\' reset_config option is required");
 388                 }
 389         }
 390
 391         if (dap_is_multidrop(dap))
 392                 status = swd_connect_multidrop(dap);
 393         else
 394                 status = swd_connect_single(dap);
 395
 396         /* IHI 0031E B4.3.2:
 397          * "A WAIT response must not be issued to the ...
 398          * ... writes to the ABORT register"
 399          * swd_clear_sticky_errors() writes to the ABORT register only.
 400          *
 401          * Unfortunately at least Microchip SAMD51/E53/E54 returns WAIT
 402          * in a corner case. Just try if ABORT resolves the problem.
 403          */
 404         if (status == ERROR_WAIT) {
 405                 LOG_WARNING("Connecting DP: stalled AP operation, issuing ABORT");
 406
 407                 dap->do_reconnect = false;
 408
 409                 status = swd_queue_dp_write_inner(dap, DP_ABORT,
 410                         DAPABORT | STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR);
 411
 412                 if (status == ERROR_OK)
 413                         status = swd_run_inner(dap);
 414         }
 415
 416         if (status == ERROR_OK)
 417                 status = dap_dp_init(dap);
 418
 419         return status;
 420 }
 421
 422 static int swd_check_reconnect(struct adiv5_dap *dap)
 423 {
 424         if (dap->do_reconnect)
 425                 return swd_connect(dap);
 426
 427         return ERROR_OK;
 428 }
 429
 430 static int swd_queue_ap_abort(struct adiv5_dap *dap, uint8_t *ack)
 431 {
 432         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 433         assert(swd);
 434
 435         /* TODO: Send DAPABORT in swd_multidrop_select_inner()
 436          * in the case the multidrop dap is not selected?
 437          * swd_queue_ap_abort() is not currently used anyway...
 438          */
 439         int retval = swd_multidrop_select(dap);
 440         if (retval != ERROR_OK)
 441                 return retval;
 442
 443         swd->write_reg(swd_cmd(false, false, DP_ABORT),
 444                 DAPABORT | STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
 445         return check_sync(dap);
 446 }
 447
 448 static int swd_queue_dp_read(struct adiv5_dap *dap, unsigned reg,
 449                 uint32_t *data)
 450 {
 451         int retval = swd_check_reconnect(dap);
 452         if (retval != ERROR_OK)
 453                 return retval;
 454
 455         retval = swd_multidrop_select(dap);
 456         if (retval != ERROR_OK)
 457                 return retval;
 458
 459         return swd_queue_dp_read_inner(dap, reg, data);
 460 }
 461
 462 static int swd_queue_dp_write(struct adiv5_dap *dap, unsigned reg,
 463                 uint32_t data)
 464 {
 465         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 466         assert(swd);
 467
 468         int retval = swd_check_reconnect(dap);
 469         if (retval != ERROR_OK)
 470                 return retval;
 471
 472         retval = swd_multidrop_select(dap);
 473         if (retval != ERROR_OK)
 474                 return retval;
 475
 476         return swd_queue_dp_write_inner(dap, reg, data);
 477 }
 478
 479 /** Select the AP register bank matching bits 7:4 of reg. */
 480 static int swd_queue_ap_bankselect(struct adiv5_ap *ap, unsigned reg)
 481 {
 482         int retval;
 483         struct adiv5_dap *dap = ap->dap;
 484         uint64_t sel;
 485
 486         if (is_adiv6(dap)) {
 487                 sel = ap->ap_num | (reg & 0x00000FF0);
 488                 if (sel == (dap->select & ~0xfULL))
 489                         return ERROR_OK;
 490
 491                 if (dap->select != DP_SELECT_INVALID)
 492                         sel |= dap->select & 0xf;
 493                 dap->select = sel;
 494                 LOG_DEBUG("AP BANKSEL: %" PRIx64, sel);
 495
 496                 retval = swd_queue_dp_write(dap, DP_SELECT, (uint32_t)sel);
 497
 498                 if (retval == ERROR_OK && dap->asize > 32)
 499                         retval = swd_queue_dp_write(dap, DP_SELECT1, (uint32_t)(sel >> 32));
 500
 501                 if (retval != ERROR_OK)
 502                         dap->select = DP_SELECT_INVALID;
 503
 504                 return retval;
 505         }
 506
 507         /* ADIv5 */
 508         sel = (ap->ap_num << 24) | (reg & 0x000000F0);
 509         if (dap->select != DP_SELECT_INVALID)
 510                 sel |= dap->select & DP_SELECT_DPBANK;
 511
 512         if (sel == dap->select)
 513                 return ERROR_OK;
 514
 515         dap->select = sel;
 516
 517         retval = swd_queue_dp_write_inner(dap, DP_SELECT, sel);
 518         if (retval != ERROR_OK)
 519                 dap->select = DP_SELECT_INVALID;
 520
 521         return retval;
 522 }
 523
 524 static int swd_queue_ap_read(struct adiv5_ap *ap, unsigned reg,
 525                 uint32_t *data)
 526 {
 527         struct adiv5_dap *dap = ap->dap;
 528         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 529         assert(swd);
 530
 531         int retval = swd_check_reconnect(dap);
 532         if (retval != ERROR_OK)
 533                 return retval;
 534
 535         retval = swd_multidrop_select(dap);
 536         if (retval != ERROR_OK)
 537                 return retval;
 538
 539         retval = swd_queue_ap_bankselect(ap, reg);
 540         if (retval != ERROR_OK)
 541                 return retval;
 542
 543         swd->read_reg(swd_cmd(true, true, reg), dap->last_read, ap->memaccess_tck);
 544         dap->last_read = data;
 545
 546         return check_sync(dap);
 547 }
 548
 549 static int swd_queue_ap_write(struct adiv5_ap *ap, unsigned reg,
 550                 uint32_t data)
 551 {
 552         struct adiv5_dap *dap = ap->dap;
 553         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 554         assert(swd);
 555
 556         int retval = swd_check_reconnect(dap);
 557         if (retval != ERROR_OK)
 558                 return retval;
 559
 560         retval = swd_multidrop_select(dap);
 561         if (retval != ERROR_OK)
 562                 return retval;
 563
 564         swd_finish_read(dap);
 565
 566         retval = swd_queue_ap_bankselect(ap, reg);
 567         if (retval != ERROR_OK)
 568                 return retval;
 569
 570         swd->write_reg(swd_cmd(false, true, reg), data, ap->memaccess_tck);
 571
 572         return check_sync(dap);
 573 }
 574
 575 /** Executes all queued DAP operations. */
 576 static int swd_run(struct adiv5_dap *dap)
 577 {
 578         int retval = swd_multidrop_select(dap);
 579         if (retval != ERROR_OK)
 580                 return retval;
 581
 582         swd_finish_read(dap);
 583
 584         return swd_run_inner(dap);
 585 }
 586
 587 /** Put the SWJ-DP back to JTAG mode */
 588 static void swd_quit(struct adiv5_dap *dap)
 589 {
 590         const struct swd_driver *swd = adiv5_dap_swd_driver(dap);
 591         static bool done;
 592
 593         /* There is no difference if the sequence is sent at the last
 594          * or the first swd_quit() call, send it just once */
 595         if (done)
 596                 return;
 597
 598         done = true;
 599         if (dap_is_multidrop(dap)) {
 600                 swd->switch_seq(SWD_TO_DORMANT);
 601                 /* Revisit!
 602                  * Leaving DPs in dormant state was tested and offers some safety
 603                  * against DPs mismatch in case of unintentional use of non-multidrop SWD.
 604                  * To put SWJ-DPs to power-on state issue
 605                  * swd->switch_seq(DORMANT_TO_JTAG);
 606                  */
 607         } else {
 608                 if (dap->switch_through_dormant) {
 609                         swd->switch_seq(SWD_TO_DORMANT);
 610                         swd->switch_seq(DORMANT_TO_JTAG);
 611                 } else {
 612                         swd->switch_seq(SWD_TO_JTAG);
 613                 }
 614         }
 615
 616         /* flush the queue to shift out the sequence before exit */
 617         swd->run();
 618 }
 619
 620 const struct dap_ops swd_dap_ops = {
 621         .connect = swd_connect,
 622         .send_sequence = swd_send_sequence,
 623         .queue_dp_read = swd_queue_dp_read,
 624         .queue_dp_write = swd_queue_dp_write,
 625         .queue_ap_read = swd_queue_ap_read,
 626         .queue_ap_write = swd_queue_ap_write,
 627         .queue_ap_abort = swd_queue_ap_abort,
 628         .run = swd_run,
 629         .quit = swd_quit,
 630 };
 631
 632 static const struct command_registration swd_commands[] = {
 633         {
 634                 /*
 635                  * Set up SWD and JTAG targets identically, unless/until
 636                  * infrastructure improves ...  meanwhile, ignore all
 637                  * JTAG-specific stuff like IR length for SWD.
 638                  *
 639                  * REVISIT can we verify "just one SWD DAP" here/early?
 640                  */
 641                 .name = "newdap",
 642                 .jim_handler = jim_jtag_newtap,
 643                 .mode = COMMAND_CONFIG,
 644                 .help = "declare a new SWD DAP"
 645         },
 646         COMMAND_REGISTRATION_DONE
 647 };
 648
 649 static const struct command_registration swd_handlers[] = {
 650         {
 651                 .name = "swd",
 652                 .mode = COMMAND_ANY,
 653                 .help = "SWD command group",
 654                 .chain = swd_commands,
 655                 .usage = "",
 656         },
 657         COMMAND_REGISTRATION_DONE
 658 };
 659
 660 static int swd_select(struct command_context *ctx)
 661 {
 662         /* FIXME: only place where global 'adapter_driver' is still needed */
 663         extern struct adapter_driver *adapter_driver;
 664         const struct swd_driver *swd = adapter_driver->swd_ops;
 665         int retval;
 666
 667         retval = register_commands(ctx, NULL, swd_handlers);
 668         if (retval != ERROR_OK)
 669                 return retval;
 670
 671          /* be sure driver is in SWD mode; start
 672           * with hardware default TRN (1), it can be changed later
 673           */
 674         if (!swd || !swd->read_reg || !swd->write_reg || !swd->init) {
 675                 LOG_DEBUG("no SWD driver?");
 676                 return ERROR_FAIL;
 677         }
 678
 679         retval = swd->init();
 680         if (retval != ERROR_OK) {
 681                 LOG_DEBUG("can't init SWD driver");
 682                 return retval;
 683         }
 684
 685         return retval;
 686 }
 687
 688 static int swd_init(struct command_context *ctx)
 689 {
 690         /* nothing done here, SWD is initialized
 691          * together with the DAP */
 692         return ERROR_OK;
 693 }
 694
 695 static struct transport swd_transport = {
 696         .name = "swd",
 697         .select = swd_select,
 698         .init = swd_init,
 699 };
 700
 701 static void swd_constructor(void) __attribute__((constructor));
 702 static void swd_constructor(void)
 703 {
 704         transport_register(&swd_transport);
 705 }
 706
 707 /** Returns true if the current debug session
 708  * is using SWD as its transport.
 709  */
 710 bool transport_is_swd(void)
 711 {
 712         return get_current_transport() == &swd_transport;
 713 }