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 - 2012, 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 #if (!ACPI_REDUCED_HARDWARE)
  55 /* Local Prototypes */
  56 static acpi_status
  57 acpi_hw_read_multiple(u32 *value,
  58                       struct acpi_generic_address *register_a,
  59                       struct acpi_generic_address *register_b);
  60
  61 static acpi_status
  62 acpi_hw_write_multiple(u32 value,
  63                        struct acpi_generic_address *register_a,
  64                        struct acpi_generic_address *register_b);
  65
  66 #endif                          /* !ACPI_REDUCED_HARDWARE */
  67
  68 /******************************************************************************
  69  *
  70  * FUNCTION:    acpi_hw_validate_register
  71  *
  72  * PARAMETERS:  Reg                 - GAS register structure
  73  *              max_bit_width       - Max bit_width supported (32 or 64)
  74  *              Address             - Pointer to where the gas->address
  75  *                                    is returned
  76  *
  77  * RETURN:      Status
  78  *
  79  * DESCRIPTION: Validate the contents of a GAS register. Checks the GAS
  80  *              pointer, Address, space_id, bit_width, and bit_offset.
  81  *
  82  ******************************************************************************/
  83
  84 acpi_status
  85 acpi_hw_validate_register(struct acpi_generic_address *reg,
  86                           u8 max_bit_width, u64 *address)
  87 {
  88
  89         /* Must have a valid pointer to a GAS structure */
  90
  91         if (!reg) {
  92                 return (AE_BAD_PARAMETER);
  93         }
  94
  95         /*
  96          * Copy the target address. This handles possible alignment issues.
  97          * Address must not be null. A null address also indicates an optional
  98          * ACPI register that is not supported, so no error message.
  99          */
 100         ACPI_MOVE_64_TO_64(address, &reg->address);
 101         if (!(*address)) {
 102                 return (AE_BAD_ADDRESS);
 103         }
 104
 105         /* Validate the space_iD */
 106
 107         if ((reg->space_id != ACPI_ADR_SPACE_SYSTEM_MEMORY) &&
 108             (reg->space_id != ACPI_ADR_SPACE_SYSTEM_IO)) {
 109                 ACPI_ERROR((AE_INFO,
 110                             "Unsupported address space: 0x%X", reg->space_id));
 111                 return (AE_SUPPORT);
 112         }
 113
 114         /* Validate the bit_width */
 115
 116         if ((reg->bit_width != 8) &&
 117             (reg->bit_width != 16) &&
 118             (reg->bit_width != 32) && (reg->bit_width != max_bit_width)) {
 119                 ACPI_ERROR((AE_INFO,
 120                             "Unsupported register bit width: 0x%X",
 121                             reg->bit_width));
 122                 return (AE_SUPPORT);
 123         }
 124
 125         /* Validate the bit_offset. Just a warning for now. */
 126
 127         if (reg->bit_offset != 0) {
 128                 ACPI_WARNING((AE_INFO,
 129                               "Unsupported register bit offset: 0x%X",
 130                               reg->bit_offset));
 131         }
 132
 133         return (AE_OK);
 134 }
 135
 136 /******************************************************************************
 137  *
 138  * FUNCTION:    acpi_hw_read
 139  *
 140  * PARAMETERS:  Value               - Where the value is returned
 141  *              Reg                 - GAS register structure
 142  *
 143  * RETURN:      Status
 144  *
 145  * DESCRIPTION: Read from either memory or IO space. This is a 32-bit max
 146  *              version of acpi_read, used internally since the overhead of
 147  *              64-bit values is not needed.
 148  *
 149  * LIMITATIONS: <These limitations also apply to acpi_hw_write>
 150  *      bit_width must be exactly 8, 16, or 32.
 151  *      space_iD must be system_memory or system_iO.
 152  *      bit_offset and access_width are currently ignored, as there has
 153  *          not been a need to implement these.
 154  *
 155  ******************************************************************************/
 156
 157 acpi_status acpi_hw_read(u32 *value, struct acpi_generic_address *reg)
 158 {
 159         u64 address;
 160         acpi_status status;
 161
 162         ACPI_FUNCTION_NAME(hw_read);
 163
 164         /* Validate contents of the GAS register */
 165
 166         status = acpi_hw_validate_register(reg, 32, &address);
 167         if (ACPI_FAILURE(status)) {
 168                 return (status);
 169         }
 170
 171         /* Initialize entire 32-bit return value to zero */
 172
 173         *value = 0;
 174
 175         /*
 176          * Two address spaces supported: Memory or IO. PCI_Config is
 177          * not supported here because the GAS structure is insufficient
 178          */
 179         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 180                 status = acpi_os_read_memory((acpi_physical_address)
 181                                              address, value, reg->bit_width);
 182         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 183
 184                 status = acpi_hw_read_port((acpi_io_address)
 185                                            address, value, reg->bit_width);
 186         }
 187
 188         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 189                           "Read:  %8.8X width %2d from %8.8X%8.8X (%s)\n",
 190                           *value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 191                           acpi_ut_get_region_name(reg->space_id)));
 192
 193         return (status);
 194 }
 195
 196 /******************************************************************************
 197  *
 198  * FUNCTION:    acpi_hw_write
 199  *
 200  * PARAMETERS:  Value               - Value to be written
 201  *              Reg                 - GAS register structure
 202  *
 203  * RETURN:      Status
 204  *
 205  * DESCRIPTION: Write to either memory or IO space. This is a 32-bit max
 206  *              version of acpi_write, used internally since the overhead of
 207  *              64-bit values is not needed.
 208  *
 209  ******************************************************************************/
 210
 211 acpi_status acpi_hw_write(u32 value, struct acpi_generic_address *reg)
 212 {
 213         u64 address;
 214         acpi_status status;
 215
 216         ACPI_FUNCTION_NAME(hw_write);
 217
 218         /* Validate contents of the GAS register */
 219
 220         status = acpi_hw_validate_register(reg, 32, &address);
 221         if (ACPI_FAILURE(status)) {
 222                 return (status);
 223         }
 224
 225         /*
 226          * Two address spaces supported: Memory or IO. PCI_Config is
 227          * not supported here because the GAS structure is insufficient
 228          */
 229         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 230                 status = acpi_os_write_memory((acpi_physical_address)
 231                                               address, value, reg->bit_width);
 232         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 233
 234                 status = acpi_hw_write_port((acpi_io_address)
 235                                             address, value, reg->bit_width);
 236         }
 237
 238         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 239                           "Wrote: %8.8X width %2d   to %8.8X%8.8X (%s)\n",
 240                           value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 241                           acpi_ut_get_region_name(reg->space_id)));
 242
 243         return (status);
 244 }
 245
 246 #if (!ACPI_REDUCED_HARDWARE)
 247 /*******************************************************************************
 248  *
 249  * FUNCTION:    acpi_hw_clear_acpi_status
 250  *
 251  * PARAMETERS:  None
 252  *
 253  * RETURN:      Status
 254  *
 255  * DESCRIPTION: Clears all fixed and general purpose status bits
 256  *
 257  ******************************************************************************/
 258
 259 acpi_status acpi_hw_clear_acpi_status(void)
 260 {
 261         acpi_status status;
 262         acpi_cpu_flags lock_flags = 0;
 263
 264         ACPI_FUNCTION_TRACE(hw_clear_acpi_status);
 265
 266         ACPI_DEBUG_PRINT((ACPI_DB_IO, "About to write %04X to %8.8X%8.8X\n",
 267                           ACPI_BITMASK_ALL_FIXED_STATUS,
 268                           ACPI_FORMAT_UINT64(acpi_gbl_xpm1a_status.address)));
 269
 270         lock_flags = acpi_os_acquire_lock(acpi_gbl_hardware_lock);
 271
 272         /* Clear the fixed events in PM1 A/B */
 273
 274         status = acpi_hw_register_write(ACPI_REGISTER_PM1_STATUS,
 275                                         ACPI_BITMASK_ALL_FIXED_STATUS);
 276
 277         acpi_os_release_lock(acpi_gbl_hardware_lock, lock_flags);
 278
 279         if (ACPI_FAILURE(status))
 280                 goto exit;
 281
 282         /* Clear the GPE Bits in all GPE registers in all GPE blocks */
 283
 284         status = acpi_ev_walk_gpe_list(acpi_hw_clear_gpe_block, NULL);
 285
 286 exit:
 287         return_ACPI_STATUS(status);
 288 }
 289
 290 /*******************************************************************************
 291  *
 292  * FUNCTION:    acpi_hw_get_bit_register_info
 293  *
 294  * PARAMETERS:  register_id         - Index of ACPI Register to access
 295  *
 296  * RETURN:      The bitmask to be used when accessing the register
 297  *
 298  * DESCRIPTION: Map register_id into a register bitmask.
 299  *
 300  ******************************************************************************/
 301
 302 struct acpi_bit_register_info *acpi_hw_get_bit_register_info(u32 register_id)
 303 {
 304         ACPI_FUNCTION_ENTRY();
 305
 306         if (register_id > ACPI_BITREG_MAX) {
 307                 ACPI_ERROR((AE_INFO, "Invalid BitRegister ID: 0x%X",
 308                             register_id));
 309                 return (NULL);
 310         }
 311
 312         return (&acpi_gbl_bit_register_info[register_id]);
 313 }
 314
 315 /******************************************************************************
 316  *
 317  * FUNCTION:    acpi_hw_write_pm1_control
 318  *
 319  * PARAMETERS:  pm1a_control        - Value to be written to PM1A control
 320  *              pm1b_control        - Value to be written to PM1B control
 321  *
 322  * RETURN:      Status
 323  *
 324  * DESCRIPTION: Write the PM1 A/B control registers. These registers are
 325  *              different than than the PM1 A/B status and enable registers
 326  *              in that different values can be written to the A/B registers.
 327  *              Most notably, the SLP_TYP bits can be different, as per the
 328  *              values returned from the _Sx predefined methods.
 329  *
 330  ******************************************************************************/
 331
 332 acpi_status acpi_hw_write_pm1_control(u32 pm1a_control, u32 pm1b_control)
 333 {
 334         acpi_status status;
 335
 336         ACPI_FUNCTION_TRACE(hw_write_pm1_control);
 337
 338         status =
 339             acpi_hw_write(pm1a_control, &acpi_gbl_FADT.xpm1a_control_block);
 340         if (ACPI_FAILURE(status)) {
 341                 return_ACPI_STATUS(status);
 342         }
 343
 344         if (acpi_gbl_FADT.xpm1b_control_block.address) {
 345                 status =
 346                     acpi_hw_write(pm1b_control,
 347                                   &acpi_gbl_FADT.xpm1b_control_block);
 348         }
 349         return_ACPI_STATUS(status);
 350 }
 351
 352 /******************************************************************************
 353  *
 354  * FUNCTION:    acpi_hw_register_read
 355  *
 356  * PARAMETERS:  register_id         - ACPI Register ID
 357  *              return_value        - Where the register value is returned
 358  *
 359  * RETURN:      Status and the value read.
 360  *
 361  * DESCRIPTION: Read from the specified ACPI register
 362  *
 363  ******************************************************************************/
 364 acpi_status
 365 acpi_hw_register_read(u32 register_id, u32 * return_value)
 366 {
 367         u32 value = 0;
 368         acpi_status status;
 369
 370         ACPI_FUNCTION_TRACE(hw_register_read);
 371
 372         switch (register_id) {
 373         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 374
 375                 status = acpi_hw_read_multiple(&value,
 376                                                &acpi_gbl_xpm1a_status,
 377                                                &acpi_gbl_xpm1b_status);
 378                 break;
 379
 380         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 381
 382                 status = acpi_hw_read_multiple(&value,
 383                                                &acpi_gbl_xpm1a_enable,
 384                                                &acpi_gbl_xpm1b_enable);
 385                 break;
 386
 387         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 388
 389                 status = acpi_hw_read_multiple(&value,
 390                                                &acpi_gbl_FADT.
 391                                                xpm1a_control_block,
 392                                                &acpi_gbl_FADT.
 393                                                xpm1b_control_block);
 394
 395                 /*
 396                  * Zero the write-only bits. From the ACPI specification, "Hardware
 397                  * Write-Only Bits": "Upon reads to registers with write-only bits,
 398                  * software masks out all write-only bits."
 399                  */
 400                 value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 401                 break;
 402
 403         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 404
 405                 status =
 406                     acpi_hw_read(&value, &acpi_gbl_FADT.xpm2_control_block);
 407                 break;
 408
 409         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 410
 411                 status = acpi_hw_read(&value, &acpi_gbl_FADT.xpm_timer_block);
 412                 break;
 413
 414         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 415
 416                 status =
 417                     acpi_hw_read_port(acpi_gbl_FADT.smi_command, &value, 8);
 418                 break;
 419
 420         default:
 421                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 422                 status = AE_BAD_PARAMETER;
 423                 break;
 424         }
 425
 426         if (ACPI_SUCCESS(status)) {
 427                 *return_value = value;
 428         }
 429
 430         return_ACPI_STATUS(status);
 431 }
 432
 433 /******************************************************************************
 434  *
 435  * FUNCTION:    acpi_hw_register_write
 436  *
 437  * PARAMETERS:  register_id         - ACPI Register ID
 438  *              Value               - The value to write
 439  *
 440  * RETURN:      Status
 441  *
 442  * DESCRIPTION: Write to the specified ACPI register
 443  *
 444  * NOTE: In accordance with the ACPI specification, this function automatically
 445  * preserves the value of the following bits, meaning that these bits cannot be
 446  * changed via this interface:
 447  *
 448  * PM1_CONTROL[0] = SCI_EN
 449  * PM1_CONTROL[9]
 450  * PM1_STATUS[11]
 451  *
 452  * ACPI References:
 453  * 1) Hardware Ignored Bits: When software writes to a register with ignored
 454  *      bit fields, it preserves the ignored bit fields
 455  * 2) SCI_EN: OSPM always preserves this bit position
 456  *
 457  ******************************************************************************/
 458
 459 acpi_status acpi_hw_register_write(u32 register_id, u32 value)
 460 {
 461         acpi_status status;
 462         u32 read_value;
 463
 464         ACPI_FUNCTION_TRACE(hw_register_write);
 465
 466         switch (register_id) {
 467         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 468                 /*
 469                  * Handle the "ignored" bit in PM1 Status. According to the ACPI
 470                  * specification, ignored bits are to be preserved when writing.
 471                  * Normally, this would mean a read/modify/write sequence. However,
 472                  * preserving a bit in the status register is different. Writing a
 473                  * one clears the status, and writing a zero preserves the status.
 474                  * Therefore, we must always write zero to the ignored bit.
 475                  *
 476                  * This behavior is clarified in the ACPI 4.0 specification.
 477                  */
 478                 value &= ~ACPI_PM1_STATUS_PRESERVED_BITS;
 479
 480                 status = acpi_hw_write_multiple(value,
 481                                                 &acpi_gbl_xpm1a_status,
 482                                                 &acpi_gbl_xpm1b_status);
 483                 break;
 484
 485         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access */
 486
 487                 status = acpi_hw_write_multiple(value,
 488                                                 &acpi_gbl_xpm1a_enable,
 489                                                 &acpi_gbl_xpm1b_enable);
 490                 break;
 491
 492         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 493
 494                 /*
 495                  * Perform a read first to preserve certain bits (per ACPI spec)
 496                  * Note: This includes SCI_EN, we never want to change this bit
 497                  */
 498                 status = acpi_hw_read_multiple(&read_value,
 499                                                &acpi_gbl_FADT.
 500                                                xpm1a_control_block,
 501                                                &acpi_gbl_FADT.
 502                                                xpm1b_control_block);
 503                 if (ACPI_FAILURE(status)) {
 504                         goto exit;
 505                 }
 506
 507                 /* Insert the bits to be preserved */
 508
 509                 ACPI_INSERT_BITS(value, ACPI_PM1_CONTROL_PRESERVED_BITS,
 510                                  read_value);
 511
 512                 /* Now we can write the data */
 513
 514                 status = acpi_hw_write_multiple(value,
 515                                                 &acpi_gbl_FADT.
 516                                                 xpm1a_control_block,
 517                                                 &acpi_gbl_FADT.
 518                                                 xpm1b_control_block);
 519                 break;
 520
 521         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 522
 523                 /*
 524                  * For control registers, all reserved bits must be preserved,
 525                  * as per the ACPI spec.
 526                  */
 527                 status =
 528                     acpi_hw_read(&read_value,
 529                                  &acpi_gbl_FADT.xpm2_control_block);
 530                 if (ACPI_FAILURE(status)) {
 531                         goto exit;
 532                 }
 533
 534                 /* Insert the bits to be preserved */
 535
 536                 ACPI_INSERT_BITS(value, ACPI_PM2_CONTROL_PRESERVED_BITS,
 537                                  read_value);
 538
 539                 status =
 540                     acpi_hw_write(value, &acpi_gbl_FADT.xpm2_control_block);
 541                 break;
 542
 543         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 544
 545                 status = acpi_hw_write(value, &acpi_gbl_FADT.xpm_timer_block);
 546                 break;
 547
 548         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 549
 550                 /* SMI_CMD is currently always in IO space */
 551
 552                 status =
 553                     acpi_hw_write_port(acpi_gbl_FADT.smi_command, value, 8);
 554                 break;
 555
 556         default:
 557                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 558                 status = AE_BAD_PARAMETER;
 559                 break;
 560         }
 561
 562       exit:
 563         return_ACPI_STATUS(status);
 564 }
 565
 566 /******************************************************************************
 567  *
 568  * FUNCTION:    acpi_hw_read_multiple
 569  *
 570  * PARAMETERS:  Value               - Where the register value is returned
 571  *              register_a           - First ACPI register (required)
 572  *              register_b           - Second ACPI register (optional)
 573  *
 574  * RETURN:      Status
 575  *
 576  * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
 577  *
 578  ******************************************************************************/
 579
 580 static acpi_status
 581 acpi_hw_read_multiple(u32 *value,
 582                       struct acpi_generic_address *register_a,
 583                       struct acpi_generic_address *register_b)
 584 {
 585         u32 value_a = 0;
 586         u32 value_b = 0;
 587         acpi_status status;
 588
 589         /* The first register is always required */
 590
 591         status = acpi_hw_read(&value_a, register_a);
 592         if (ACPI_FAILURE(status)) {
 593                 return (status);
 594         }
 595
 596         /* Second register is optional */
 597
 598         if (register_b->address) {
 599                 status = acpi_hw_read(&value_b, register_b);
 600                 if (ACPI_FAILURE(status)) {
 601                         return (status);
 602                 }
 603         }
 604
 605         /*
 606          * OR the two return values together. No shifting or masking is necessary,
 607          * because of how the PM1 registers are defined in the ACPI specification:
 608          *
 609          * "Although the bits can be split between the two register blocks (each
 610          * register block has a unique pointer within the FADT), the bit positions
 611          * are maintained. The register block with unimplemented bits (that is,
 612          * those implemented in the other register block) always returns zeros,
 613          * and writes have no side effects"
 614          */
 615         *value = (value_a | value_b);
 616         return (AE_OK);
 617 }
 618
 619 /******************************************************************************
 620  *
 621  * FUNCTION:    acpi_hw_write_multiple
 622  *
 623  * PARAMETERS:  Value               - The value to write
 624  *              register_a           - First ACPI register (required)
 625  *              register_b           - Second ACPI register (optional)
 626  *
 627  * RETURN:      Status
 628  *
 629  * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
 630  *
 631  ******************************************************************************/
 632
 633 static acpi_status
 634 acpi_hw_write_multiple(u32 value,
 635                        struct acpi_generic_address *register_a,
 636                        struct acpi_generic_address *register_b)
 637 {
 638         acpi_status status;
 639
 640         /* The first register is always required */
 641
 642         status = acpi_hw_write(value, register_a);
 643         if (ACPI_FAILURE(status)) {
 644                 return (status);
 645         }
 646
 647         /*
 648          * Second register is optional
 649          *
 650          * No bit shifting or clearing is necessary, because of how the PM1
 651          * registers are defined in the ACPI specification:
 652          *
 653          * "Although the bits can be split between the two register blocks (each
 654          * register block has a unique pointer within the FADT), the bit positions
 655          * are maintained. The register block with unimplemented bits (that is,
 656          * those implemented in the other register block) always returns zeros,
 657          * and writes have no side effects"
 658          */
 659         if (register_b->address) {
 660                 status = acpi_hw_write(value, register_b);
 661         }
 662
 663         return (status);
 664 }
 665
 666 #endif                          /* !ACPI_REDUCED_HARDWARE */