drivers/acpi/acpica/hwregs.c

   1
   2 /*******************************************************************************
   3  *
   4  * Module Name: hwregs - Read/write access functions for the various ACPI
   5  *                       control and status registers.
   6  *
   7  ******************************************************************************/
   8
   9 /*
  10  * Copyright (C) 2000 - 2011, Intel Corp.
  11  * All rights reserved.
  12  *
  13  * Redistribution and use in source and binary forms, with or without
  14  * modification, are permitted provided that the following conditions
  15  * are met:
  16  * 1. Redistributions of source code must retain the above copyright
  17  *    notice, this list of conditions, and the following disclaimer,
  18  *    without modification.
  19  * 2. Redistributions in binary form must reproduce at minimum a disclaimer
  20  *    substantially similar to the "NO WARRANTY" disclaimer below
  21  *    ("Disclaimer") and any redistribution must be conditioned upon
  22  *    including a substantially similar Disclaimer requirement for further
  23  *    binary redistribution.
  24  * 3. Neither the names of the above-listed copyright holders nor the names
  25  *    of any contributors may be used to endorse or promote products derived
  26  *    from this software without specific prior written permission.
  27  *
  28  * Alternatively, this software may be distributed under the terms of the
  29  * GNU General Public License ("GPL") version 2 as published by the Free
  30  * Software Foundation.
  31  *
  32  * NO WARRANTY
  33  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  34  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  35  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR
  36  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  37  * HOLDERS OR CONTRIBUTORS BE LIABLE FOR SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  38  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  39  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  40  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  41  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
  42  * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  43  * POSSIBILITY OF SUCH DAMAGES.
  44  */
  45
  46 #include <acpi/acpi.h>
  47 #include "accommon.h"
  48 #include "acnamesp.h"
  49 #include "acevents.h"
  50
  51 #define _COMPONENT          ACPI_HARDWARE
  52 ACPI_MODULE_NAME("hwregs")
  53
  54 /* Local Prototypes */
  55 static acpi_status
  56 acpi_hw_read_multiple(u32 *value,
  57                       struct acpi_generic_address *register_a,
  58                       struct acpi_generic_address *register_b);
  59
  60 static acpi_status
  61 acpi_hw_write_multiple(u32 value,
  62                        struct acpi_generic_address *register_a,
  63                        struct acpi_generic_address *register_b);
  64
  65 /******************************************************************************
  66  *
  67  * FUNCTION:    acpi_hw_validate_register
  68  *
  69  * PARAMETERS:  Reg                 - GAS register structure
  70  *              max_bit_width       - Max bit_width supported (32 or 64)
  71  *              Address             - Pointer to where the gas->address
  72  *                                    is returned
  73  *
  74  * RETURN:      Status
  75  *
  76  * DESCRIPTION: Validate the contents of a GAS register. Checks the GAS
  77  *              pointer, Address, space_id, bit_width, and bit_offset.
  78  *
  79  ******************************************************************************/
  80
  81 acpi_status
  82 acpi_hw_validate_register(struct acpi_generic_address *reg,
  83                           u8 max_bit_width, u64 *address)
  84 {
  85
  86         /* Must have a valid pointer to a GAS structure */
  87
  88         if (!reg) {
  89                 return (AE_BAD_PARAMETER);
  90         }
  91
  92         /*
  93          * Copy the target address. This handles possible alignment issues.
  94          * Address must not be null. A null address also indicates an optional
  95          * ACPI register that is not supported, so no error message.
  96          */
  97         ACPI_MOVE_64_TO_64(address, &reg->address);
  98         if (!(*address)) {
  99                 return (AE_BAD_ADDRESS);
 100         }
 101
 102         /* Validate the space_iD */
 103
 104         if ((reg->space_id != ACPI_ADR_SPACE_SYSTEM_MEMORY) &&
 105             (reg->space_id != ACPI_ADR_SPACE_SYSTEM_IO)) {
 106                 ACPI_ERROR((AE_INFO,
 107                             "Unsupported address space: 0x%X", reg->space_id));
 108                 return (AE_SUPPORT);
 109         }
 110
 111         /* Validate the bit_width */
 112
 113         if ((reg->bit_width != 8) &&
 114             (reg->bit_width != 16) &&
 115             (reg->bit_width != 32) && (reg->bit_width != max_bit_width)) {
 116                 ACPI_ERROR((AE_INFO,
 117                             "Unsupported register bit width: 0x%X",
 118                             reg->bit_width));
 119                 return (AE_SUPPORT);
 120         }
 121
 122         /* Validate the bit_offset. Just a warning for now. */
 123
 124         if (reg->bit_offset != 0) {
 125                 ACPI_WARNING((AE_INFO,
 126                               "Unsupported register bit offset: 0x%X",
 127                               reg->bit_offset));
 128         }
 129
 130         return (AE_OK);
 131 }
 132
 133 /******************************************************************************
 134  *
 135  * FUNCTION:    acpi_hw_read
 136  *
 137  * PARAMETERS:  Value               - Where the value is returned
 138  *              Reg                 - GAS register structure
 139  *
 140  * RETURN:      Status
 141  *
 142  * DESCRIPTION: Read from either memory or IO space. This is a 32-bit max
 143  *              version of acpi_read, used internally since the overhead of
 144  *              64-bit values is not needed.
 145  *
 146  * LIMITATIONS: <These limitations also apply to acpi_hw_write>
 147  *      bit_width must be exactly 8, 16, or 32.
 148  *      space_iD must be system_memory or system_iO.
 149  *      bit_offset and access_width are currently ignored, as there has
 150  *          not been a need to implement these.
 151  *
 152  ******************************************************************************/
 153
 154 acpi_status acpi_hw_read(u32 *value, struct acpi_generic_address *reg)
 155 {
 156         u64 address;
 157         acpi_status status;
 158
 159         ACPI_FUNCTION_NAME(hw_read);
 160
 161         /* Validate contents of the GAS register */
 162
 163         status = acpi_hw_validate_register(reg, 32, &address);
 164         if (ACPI_FAILURE(status)) {
 165                 return (status);
 166         }
 167
 168         /* Initialize entire 32-bit return value to zero */
 169
 170         *value = 0;
 171
 172         /*
 173          * Two address spaces supported: Memory or IO. PCI_Config is
 174          * not supported here because the GAS structure is insufficient
 175          */
 176         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 177                 status = acpi_os_read_memory((acpi_physical_address)
 178                                              address, value, reg->bit_width);
 179         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 180
 181                 status = acpi_hw_read_port((acpi_io_address)
 182                                            address, value, reg->bit_width);
 183         }
 184
 185         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 186                           "Read:  %8.8X width %2d from %8.8X%8.8X (%s)\n",
 187                           *value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 188                           acpi_ut_get_region_name(reg->space_id)));
 189
 190         return (status);
 191 }
 192
 193 /******************************************************************************
 194  *
 195  * FUNCTION:    acpi_hw_write
 196  *
 197  * PARAMETERS:  Value               - Value to be written
 198  *              Reg                 - GAS register structure
 199  *
 200  * RETURN:      Status
 201  *
 202  * DESCRIPTION: Write to either memory or IO space. This is a 32-bit max
 203  *              version of acpi_write, used internally since the overhead of
 204  *              64-bit values is not needed.
 205  *
 206  ******************************************************************************/
 207
 208 acpi_status acpi_hw_write(u32 value, struct acpi_generic_address *reg)
 209 {
 210         u64 address;
 211         acpi_status status;
 212
 213         ACPI_FUNCTION_NAME(hw_write);
 214
 215         /* Validate contents of the GAS register */
 216
 217         status = acpi_hw_validate_register(reg, 32, &address);
 218         if (ACPI_FAILURE(status)) {
 219                 return (status);
 220         }
 221
 222         /*
 223          * Two address spaces supported: Memory or IO. PCI_Config is
 224          * not supported here because the GAS structure is insufficient
 225          */
 226         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 227                 status = acpi_os_write_memory((acpi_physical_address)
 228                                               address, value, reg->bit_width);
 229         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 230
 231                 status = acpi_hw_write_port((acpi_io_address)
 232                                             address, value, reg->bit_width);
 233         }
 234
 235         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 236                           "Wrote: %8.8X width %2d   to %8.8X%8.8X (%s)\n",
 237                           value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 238                           acpi_ut_get_region_name(reg->space_id)));
 239
 240         return (status);
 241 }
 242
 243 /*******************************************************************************
 244  *
 245  * FUNCTION:    acpi_hw_clear_acpi_status
 246  *
 247  * PARAMETERS:  None
 248  *
 249  * RETURN:      Status
 250  *
 251  * DESCRIPTION: Clears all fixed and general purpose status bits
 252  *
 253  ******************************************************************************/
 254
 255 acpi_status acpi_hw_clear_acpi_status(void)
 256 {
 257         acpi_status status;
 258         acpi_cpu_flags lock_flags = 0;
 259
 260         ACPI_FUNCTION_TRACE(hw_clear_acpi_status);
 261
 262         ACPI_DEBUG_PRINT((ACPI_DB_IO, "About to write %04X to %8.8X%8.8X\n",
 263                           ACPI_BITMASK_ALL_FIXED_STATUS,
 264                           ACPI_FORMAT_UINT64(acpi_gbl_xpm1a_status.address)));
 265
 266         lock_flags = acpi_os_acquire_lock(acpi_gbl_hardware_lock);
 267
 268         /* Clear the fixed events in PM1 A/B */
 269
 270         status = acpi_hw_register_write(ACPI_REGISTER_PM1_STATUS,
 271                                         ACPI_BITMASK_ALL_FIXED_STATUS);
 272
 273         acpi_os_release_lock(acpi_gbl_hardware_lock, lock_flags);
 274
 275         if (ACPI_FAILURE(status))
 276                 goto exit;
 277
 278         /* Clear the GPE Bits in all GPE registers in all GPE blocks */
 279
 280         status = acpi_ev_walk_gpe_list(acpi_hw_clear_gpe_block, NULL);
 281
 282 exit:
 283         return_ACPI_STATUS(status);
 284 }
 285
 286 /*******************************************************************************
 287  *
 288  * FUNCTION:    acpi_hw_get_register_bit_mask
 289  *
 290  * PARAMETERS:  register_id         - Index of ACPI Register to access
 291  *
 292  * RETURN:      The bitmask to be used when accessing the register
 293  *
 294  * DESCRIPTION: Map register_id into a register bitmask.
 295  *
 296  ******************************************************************************/
 297
 298 struct acpi_bit_register_info *acpi_hw_get_bit_register_info(u32 register_id)
 299 {
 300         ACPI_FUNCTION_ENTRY();
 301
 302         if (register_id > ACPI_BITREG_MAX) {
 303                 ACPI_ERROR((AE_INFO, "Invalid BitRegister ID: 0x%X",
 304                             register_id));
 305                 return (NULL);
 306         }
 307
 308         return (&acpi_gbl_bit_register_info[register_id]);
 309 }
 310
 311 /******************************************************************************
 312  *
 313  * FUNCTION:    acpi_hw_write_pm1_control
 314  *
 315  * PARAMETERS:  pm1a_control        - Value to be written to PM1A control
 316  *              pm1b_control        - Value to be written to PM1B control
 317  *
 318  * RETURN:      Status
 319  *
 320  * DESCRIPTION: Write the PM1 A/B control registers. These registers are
 321  *              different than than the PM1 A/B status and enable registers
 322  *              in that different values can be written to the A/B registers.
 323  *              Most notably, the SLP_TYP bits can be different, as per the
 324  *              values returned from the _Sx predefined methods.
 325  *
 326  ******************************************************************************/
 327
 328 acpi_status acpi_hw_write_pm1_control(u32 pm1a_control, u32 pm1b_control)
 329 {
 330         acpi_status status;
 331
 332         ACPI_FUNCTION_TRACE(hw_write_pm1_control);
 333
 334         status =
 335             acpi_hw_write(pm1a_control, &acpi_gbl_FADT.xpm1a_control_block);
 336         if (ACPI_FAILURE(status)) {
 337                 return_ACPI_STATUS(status);
 338         }
 339
 340         if (acpi_gbl_FADT.xpm1b_control_block.address) {
 341                 status =
 342                     acpi_hw_write(pm1b_control,
 343                                   &acpi_gbl_FADT.xpm1b_control_block);
 344         }
 345         return_ACPI_STATUS(status);
 346 }
 347
 348 /******************************************************************************
 349  *
 350  * FUNCTION:    acpi_hw_register_read
 351  *
 352  * PARAMETERS:  register_id         - ACPI Register ID
 353  *              return_value        - Where the register value is returned
 354  *
 355  * RETURN:      Status and the value read.
 356  *
 357  * DESCRIPTION: Read from the specified ACPI register
 358  *
 359  ******************************************************************************/
 360 acpi_status
 361 acpi_hw_register_read(u32 register_id, u32 * return_value)
 362 {
 363         u32 value = 0;
 364         acpi_status status;
 365
 366         ACPI_FUNCTION_TRACE(hw_register_read);
 367
 368         switch (register_id) {
 369         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 370
 371                 status = acpi_hw_read_multiple(&value,
 372                                                &acpi_gbl_xpm1a_status,
 373                                                &acpi_gbl_xpm1b_status);
 374                 break;
 375
 376         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 377
 378                 status = acpi_hw_read_multiple(&value,
 379                                                &acpi_gbl_xpm1a_enable,
 380                                                &acpi_gbl_xpm1b_enable);
 381                 break;
 382
 383         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 384
 385                 status = acpi_hw_read_multiple(&value,
 386                                                &acpi_gbl_FADT.
 387                                                xpm1a_control_block,
 388                                                &acpi_gbl_FADT.
 389                                                xpm1b_control_block);
 390
 391                 /*
 392                  * Zero the write-only bits. From the ACPI specification, "Hardware
 393                  * Write-Only Bits": "Upon reads to registers with write-only bits,
 394                  * software masks out all write-only bits."
 395                  */
 396                 value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 397                 break;
 398
 399         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 400
 401                 status =
 402                     acpi_hw_read(&value, &acpi_gbl_FADT.xpm2_control_block);
 403                 break;
 404
 405         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 406
 407                 status = acpi_hw_read(&value, &acpi_gbl_FADT.xpm_timer_block);
 408                 break;
 409
 410         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 411
 412                 status =
 413                     acpi_hw_read_port(acpi_gbl_FADT.smi_command, &value, 8);
 414                 break;
 415
 416         default:
 417                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 418                 status = AE_BAD_PARAMETER;
 419                 break;
 420         }
 421
 422         if (ACPI_SUCCESS(status)) {
 423                 *return_value = value;
 424         }
 425
 426         return_ACPI_STATUS(status);
 427 }
 428
 429 /******************************************************************************
 430  *
 431  * FUNCTION:    acpi_hw_register_write
 432  *
 433  * PARAMETERS:  register_id         - ACPI Register ID
 434  *              Value               - The value to write
 435  *
 436  * RETURN:      Status
 437  *
 438  * DESCRIPTION: Write to the specified ACPI register
 439  *
 440  * NOTE: In accordance with the ACPI specification, this function automatically
 441  * preserves the value of the following bits, meaning that these bits cannot be
 442  * changed via this interface:
 443  *
 444  * PM1_CONTROL[0] = SCI_EN
 445  * PM1_CONTROL[9]
 446  * PM1_STATUS[11]
 447  *
 448  * ACPI References:
 449  * 1) Hardware Ignored Bits: When software writes to a register with ignored
 450  *      bit fields, it preserves the ignored bit fields
 451  * 2) SCI_EN: OSPM always preserves this bit position
 452  *
 453  ******************************************************************************/
 454
 455 acpi_status acpi_hw_register_write(u32 register_id, u32 value)
 456 {
 457         acpi_status status;
 458         u32 read_value;
 459
 460         ACPI_FUNCTION_TRACE(hw_register_write);
 461
 462         switch (register_id) {
 463         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 464                 /*
 465                  * Handle the "ignored" bit in PM1 Status. According to the ACPI
 466                  * specification, ignored bits are to be preserved when writing.
 467                  * Normally, this would mean a read/modify/write sequence. However,
 468                  * preserving a bit in the status register is different. Writing a
 469                  * one clears the status, and writing a zero preserves the status.
 470                  * Therefore, we must always write zero to the ignored bit.
 471                  *
 472                  * This behavior is clarified in the ACPI 4.0 specification.
 473                  */
 474                 value &= ~ACPI_PM1_STATUS_PRESERVED_BITS;
 475
 476                 status = acpi_hw_write_multiple(value,
 477                                                 &acpi_gbl_xpm1a_status,
 478                                                 &acpi_gbl_xpm1b_status);
 479                 break;
 480
 481         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access */
 482
 483                 status = acpi_hw_write_multiple(value,
 484                                                 &acpi_gbl_xpm1a_enable,
 485                                                 &acpi_gbl_xpm1b_enable);
 486                 break;
 487
 488         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 489
 490                 /*
 491                  * Perform a read first to preserve certain bits (per ACPI spec)
 492                  * Note: This includes SCI_EN, we never want to change this bit
 493                  */
 494                 status = acpi_hw_read_multiple(&read_value,
 495                                                &acpi_gbl_FADT.
 496                                                xpm1a_control_block,
 497                                                &acpi_gbl_FADT.
 498                                                xpm1b_control_block);
 499                 if (ACPI_FAILURE(status)) {
 500                         goto exit;
 501                 }
 502
 503                 /* Insert the bits to be preserved */
 504
 505                 ACPI_INSERT_BITS(value, ACPI_PM1_CONTROL_PRESERVED_BITS,
 506                                  read_value);
 507
 508                 /* Now we can write the data */
 509
 510                 status = acpi_hw_write_multiple(value,
 511                                                 &acpi_gbl_FADT.
 512                                                 xpm1a_control_block,
 513                                                 &acpi_gbl_FADT.
 514                                                 xpm1b_control_block);
 515                 break;
 516
 517         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 518
 519                 /*
 520                  * For control registers, all reserved bits must be preserved,
 521                  * as per the ACPI spec.
 522                  */
 523                 status =
 524                     acpi_hw_read(&read_value,
 525                                  &acpi_gbl_FADT.xpm2_control_block);
 526                 if (ACPI_FAILURE(status)) {
 527                         goto exit;
 528                 }
 529
 530                 /* Insert the bits to be preserved */
 531
 532                 ACPI_INSERT_BITS(value, ACPI_PM2_CONTROL_PRESERVED_BITS,
 533                                  read_value);
 534
 535                 status =
 536                     acpi_hw_write(value, &acpi_gbl_FADT.xpm2_control_block);
 537                 break;
 538
 539         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 540
 541                 status = acpi_hw_write(value, &acpi_gbl_FADT.xpm_timer_block);
 542                 break;
 543
 544         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 545
 546                 /* SMI_CMD is currently always in IO space */
 547
 548                 status =
 549                     acpi_hw_write_port(acpi_gbl_FADT.smi_command, value, 8);
 550                 break;
 551
 552         default:
 553                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 554                 status = AE_BAD_PARAMETER;
 555                 break;
 556         }
 557
 558       exit:
 559         return_ACPI_STATUS(status);
 560 }
 561
 562 /******************************************************************************
 563  *
 564  * FUNCTION:    acpi_hw_read_multiple
 565  *
 566  * PARAMETERS:  Value               - Where the register value is returned
 567  *              register_a           - First ACPI register (required)
 568  *              register_b           - Second ACPI register (optional)
 569  *
 570  * RETURN:      Status
 571  *
 572  * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
 573  *
 574  ******************************************************************************/
 575
 576 static acpi_status
 577 acpi_hw_read_multiple(u32 *value,
 578                       struct acpi_generic_address *register_a,
 579                       struct acpi_generic_address *register_b)
 580 {
 581         u32 value_a = 0;
 582         u32 value_b = 0;
 583         acpi_status status;
 584
 585         /* The first register is always required */
 586
 587         status = acpi_hw_read(&value_a, register_a);
 588         if (ACPI_FAILURE(status)) {
 589                 return (status);
 590         }
 591
 592         /* Second register is optional */
 593
 594         if (register_b->address) {
 595                 status = acpi_hw_read(&value_b, register_b);
 596                 if (ACPI_FAILURE(status)) {
 597                         return (status);
 598                 }
 599         }
 600
 601         /*
 602          * OR the two return values together. No shifting or masking is necessary,
 603          * because of how the PM1 registers are defined in the ACPI specification:
 604          *
 605          * "Although the bits can be split between the two register blocks (each
 606          * register block has a unique pointer within the FADT), the bit positions
 607          * are maintained. The register block with unimplemented bits (that is,
 608          * those implemented in the other register block) always returns zeros,
 609          * and writes have no side effects"
 610          */
 611         *value = (value_a | value_b);
 612         return (AE_OK);
 613 }
 614
 615 /******************************************************************************
 616  *
 617  * FUNCTION:    acpi_hw_write_multiple
 618  *
 619  * PARAMETERS:  Value               - The value to write
 620  *              register_a           - First ACPI register (required)
 621  *              register_b           - Second ACPI register (optional)
 622  *
 623  * RETURN:      Status
 624  *
 625  * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
 626  *
 627  ******************************************************************************/
 628
 629 static acpi_status
 630 acpi_hw_write_multiple(u32 value,
 631                        struct acpi_generic_address *register_a,
 632                        struct acpi_generic_address *register_b)
 633 {
 634         acpi_status status;
 635
 636         /* The first register is always required */
 637
 638         status = acpi_hw_write(value, register_a);
 639         if (ACPI_FAILURE(status)) {
 640                 return (status);
 641         }
 642
 643         /*
 644          * Second register is optional
 645          *
 646          * No bit shifting or clearing is necessary, because of how the PM1
 647          * registers are defined in the ACPI specification:
 648          *
 649          * "Although the bits can be split between the two register blocks (each
 650          * register block has a unique pointer within the FADT), the bit positions
 651          * are maintained. The register block with unimplemented bits (that is,
 652          * those implemented in the other register block) always returns zeros,
 653          * and writes have no side effects"
 654          */
 655         if (register_b->address) {
 656                 status = acpi_hw_write(value, register_b);
 657         }
 658
 659         return (status);
 660 }