drivers/acpi/acpica/hwregs.c

   1 /*******************************************************************************
   2  *
   3  * Module Name: hwregs - Read/write access functions for the various ACPI
   4  *                       control and status registers.
   5  *
   6  ******************************************************************************/
   7
   8 /*
   9  * Copyright (C) 2000 - 2012, Intel Corp.
  10  * All rights reserved.
  11  *
  12  * Redistribution and use in source and binary forms, with or without
  13  * modification, are permitted provided that the following conditions
  14  * are met:
  15  * 1. Redistributions of source code must retain the above copyright
  16  *    notice, this list of conditions, and the following disclaimer,
  17  *    without modification.
  18  * 2. Redistributions in binary form must reproduce at minimum a disclaimer
  19  *    substantially similar to the "NO WARRANTY" disclaimer below
  20  *    ("Disclaimer") and any redistribution must be conditioned upon
  21  *    including a substantially similar Disclaimer requirement for further
  22  *    binary redistribution.
  23  * 3. Neither the names of the above-listed copyright holders nor the names
  24  *    of any contributors may be used to endorse or promote products derived
  25  *    from this software without specific prior written permission.
  26  *
  27  * Alternatively, this software may be distributed under the terms of the
  28  * GNU General Public License ("GPL") version 2 as published by the Free
  29  * Software Foundation.
  30  *
  31  * NO WARRANTY
  32  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  33  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  34  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR
  35  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  36  * HOLDERS OR CONTRIBUTORS BE LIABLE FOR SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  37  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  38  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  39  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  40  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
  41  * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  42  * POSSIBILITY OF SUCH DAMAGES.
  43  */
  44
  45 #include <acpi/acpi.h>
  46 #include "accommon.h"
  47 #include "acnamesp.h"
  48 #include "acevents.h"
  49
  50 #define _COMPONENT          ACPI_HARDWARE
  51 ACPI_MODULE_NAME("hwregs")
  52
  53 #if (!ACPI_REDUCED_HARDWARE)
  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 #endif                          /* !ACPI_REDUCED_HARDWARE */
  66
  67 /******************************************************************************
  68  *
  69  * FUNCTION:    acpi_hw_validate_register
  70  *
  71  * PARAMETERS:  reg                 - GAS register structure
  72  *              max_bit_width       - Max bit_width supported (32 or 64)
  73  *              address             - Pointer to where the gas->address
  74  *                                    is returned
  75  *
  76  * RETURN:      Status
  77  *
  78  * DESCRIPTION: Validate the contents of a GAS register. Checks the GAS
  79  *              pointer, Address, space_id, bit_width, and bit_offset.
  80  *
  81  ******************************************************************************/
  82
  83 acpi_status
  84 acpi_hw_validate_register(struct acpi_generic_address *reg,
  85                           u8 max_bit_width, u64 *address)
  86 {
  87
  88         /* Must have a valid pointer to a GAS structure */
  89
  90         if (!reg) {
  91                 return (AE_BAD_PARAMETER);
  92         }
  93
  94         /*
  95          * Copy the target address. This handles possible alignment issues.
  96          * Address must not be null. A null address also indicates an optional
  97          * ACPI register that is not supported, so no error message.
  98          */
  99         ACPI_MOVE_64_TO_64(address, &reg->address);
 100         if (!(*address)) {
 101                 return (AE_BAD_ADDRESS);
 102         }
 103
 104         /* Validate the space_ID */
 105
 106         if ((reg->space_id != ACPI_ADR_SPACE_SYSTEM_MEMORY) &&
 107             (reg->space_id != ACPI_ADR_SPACE_SYSTEM_IO)) {
 108                 ACPI_ERROR((AE_INFO,
 109                             "Unsupported address space: 0x%X", reg->space_id));
 110                 return (AE_SUPPORT);
 111         }
 112
 113         /* Validate the bit_width */
 114
 115         if ((reg->bit_width != 8) &&
 116             (reg->bit_width != 16) &&
 117             (reg->bit_width != 32) && (reg->bit_width != max_bit_width)) {
 118                 ACPI_ERROR((AE_INFO,
 119                             "Unsupported register bit width: 0x%X",
 120                             reg->bit_width));
 121                 return (AE_SUPPORT);
 122         }
 123
 124         /* Validate the bit_offset. Just a warning for now. */
 125
 126         if (reg->bit_offset != 0) {
 127                 ACPI_WARNING((AE_INFO,
 128                               "Unsupported register bit offset: 0x%X",
 129                               reg->bit_offset));
 130         }
 131
 132         return (AE_OK);
 133 }
 134
 135 /******************************************************************************
 136  *
 137  * FUNCTION:    acpi_hw_read
 138  *
 139  * PARAMETERS:  value               - Where the value is returned
 140  *              reg                 - GAS register structure
 141  *
 142  * RETURN:      Status
 143  *
 144  * DESCRIPTION: Read from either memory or IO space. This is a 32-bit max
 145  *              version of acpi_read, used internally since the overhead of
 146  *              64-bit values is not needed.
 147  *
 148  * LIMITATIONS: <These limitations also apply to acpi_hw_write>
 149  *      bit_width must be exactly 8, 16, or 32.
 150  *      space_ID must be system_memory or system_IO.
 151  *      bit_offset and access_width are currently ignored, as there has
 152  *          not been a need to implement these.
 153  *
 154  ******************************************************************************/
 155
 156 acpi_status acpi_hw_read(u32 *value, struct acpi_generic_address *reg)
 157 {
 158         u64 address;
 159         u64 value64;
 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, &value64, reg->bit_width);
 182
 183                 *value = (u32)value64;
 184         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 185
 186                 status = acpi_hw_read_port((acpi_io_address)
 187                                            address, value, reg->bit_width);
 188         }
 189
 190         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 191                           "Read:  %8.8X width %2d from %8.8X%8.8X (%s)\n",
 192                           *value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 193                           acpi_ut_get_region_name(reg->space_id)));
 194
 195         return (status);
 196 }
 197
 198 /******************************************************************************
 199  *
 200  * FUNCTION:    acpi_hw_write
 201  *
 202  * PARAMETERS:  value               - Value to be written
 203  *              reg                 - GAS register structure
 204  *
 205  * RETURN:      Status
 206  *
 207  * DESCRIPTION: Write to either memory or IO space. This is a 32-bit max
 208  *              version of acpi_write, used internally since the overhead of
 209  *              64-bit values is not needed.
 210  *
 211  ******************************************************************************/
 212
 213 acpi_status acpi_hw_write(u32 value, struct acpi_generic_address *reg)
 214 {
 215         u64 address;
 216         acpi_status status;
 217
 218         ACPI_FUNCTION_NAME(hw_write);
 219
 220         /* Validate contents of the GAS register */
 221
 222         status = acpi_hw_validate_register(reg, 32, &address);
 223         if (ACPI_FAILURE(status)) {
 224                 return (status);
 225         }
 226
 227         /*
 228          * Two address spaces supported: Memory or IO. PCI_Config is
 229          * not supported here because the GAS structure is insufficient
 230          */
 231         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 232                 status = acpi_os_write_memory((acpi_physical_address)
 233                                               address, (u64)value,
 234                                               reg->bit_width);
 235         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 236
 237                 status = acpi_hw_write_port((acpi_io_address)
 238                                             address, value, reg->bit_width);
 239         }
 240
 241         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 242                           "Wrote: %8.8X width %2d   to %8.8X%8.8X (%s)\n",
 243                           value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 244                           acpi_ut_get_region_name(reg->space_id)));
 245
 246         return (status);
 247 }
 248
 249 #if (!ACPI_REDUCED_HARDWARE)
 250 /*******************************************************************************
 251  *
 252  * FUNCTION:    acpi_hw_clear_acpi_status
 253  *
 254  * PARAMETERS:  None
 255  *
 256  * RETURN:      Status
 257  *
 258  * DESCRIPTION: Clears all fixed and general purpose status bits
 259  *
 260  ******************************************************************************/
 261
 262 acpi_status acpi_hw_clear_acpi_status(void)
 263 {
 264         acpi_status status;
 265         acpi_cpu_flags lock_flags = 0;
 266
 267         ACPI_FUNCTION_TRACE(hw_clear_acpi_status);
 268
 269         ACPI_DEBUG_PRINT((ACPI_DB_IO, "About to write %04X to %8.8X%8.8X\n",
 270                           ACPI_BITMASK_ALL_FIXED_STATUS,
 271                           ACPI_FORMAT_UINT64(acpi_gbl_xpm1a_status.address)));
 272
 273         lock_flags = acpi_os_acquire_lock(acpi_gbl_hardware_lock);
 274
 275         /* Clear the fixed events in PM1 A/B */
 276
 277         status = acpi_hw_register_write(ACPI_REGISTER_PM1_STATUS,
 278                                         ACPI_BITMASK_ALL_FIXED_STATUS);
 279
 280         acpi_os_release_lock(acpi_gbl_hardware_lock, lock_flags);
 281
 282         if (ACPI_FAILURE(status))
 283                 goto exit;
 284
 285         /* Clear the GPE Bits in all GPE registers in all GPE blocks */
 286
 287         status = acpi_ev_walk_gpe_list(acpi_hw_clear_gpe_block, NULL);
 288
 289 exit:
 290         return_ACPI_STATUS(status);
 291 }
 292
 293 /*******************************************************************************
 294  *
 295  * FUNCTION:    acpi_hw_get_bit_register_info
 296  *
 297  * PARAMETERS:  register_id         - Index of ACPI Register to access
 298  *
 299  * RETURN:      The bitmask to be used when accessing the register
 300  *
 301  * DESCRIPTION: Map register_id into a register bitmask.
 302  *
 303  ******************************************************************************/
 304
 305 struct acpi_bit_register_info *acpi_hw_get_bit_register_info(u32 register_id)
 306 {
 307         ACPI_FUNCTION_ENTRY();
 308
 309         if (register_id > ACPI_BITREG_MAX) {
 310                 ACPI_ERROR((AE_INFO, "Invalid BitRegister ID: 0x%X",
 311                             register_id));
 312                 return (NULL);
 313         }
 314
 315         return (&acpi_gbl_bit_register_info[register_id]);
 316 }
 317
 318 /******************************************************************************
 319  *
 320  * FUNCTION:    acpi_hw_write_pm1_control
 321  *
 322  * PARAMETERS:  pm1a_control        - Value to be written to PM1A control
 323  *              pm1b_control        - Value to be written to PM1B control
 324  *
 325  * RETURN:      Status
 326  *
 327  * DESCRIPTION: Write the PM1 A/B control registers. These registers are
 328  *              different than than the PM1 A/B status and enable registers
 329  *              in that different values can be written to the A/B registers.
 330  *              Most notably, the SLP_TYP bits can be different, as per the
 331  *              values returned from the _Sx predefined methods.
 332  *
 333  ******************************************************************************/
 334
 335 acpi_status acpi_hw_write_pm1_control(u32 pm1a_control, u32 pm1b_control)
 336 {
 337         acpi_status status;
 338
 339         ACPI_FUNCTION_TRACE(hw_write_pm1_control);
 340
 341         status =
 342             acpi_hw_write(pm1a_control, &acpi_gbl_FADT.xpm1a_control_block);
 343         if (ACPI_FAILURE(status)) {
 344                 return_ACPI_STATUS(status);
 345         }
 346
 347         if (acpi_gbl_FADT.xpm1b_control_block.address) {
 348                 status =
 349                     acpi_hw_write(pm1b_control,
 350                                   &acpi_gbl_FADT.xpm1b_control_block);
 351         }
 352         return_ACPI_STATUS(status);
 353 }
 354
 355 /******************************************************************************
 356  *
 357  * FUNCTION:    acpi_hw_register_read
 358  *
 359  * PARAMETERS:  register_id         - ACPI Register ID
 360  *              return_value        - Where the register value is returned
 361  *
 362  * RETURN:      Status and the value read.
 363  *
 364  * DESCRIPTION: Read from the specified ACPI register
 365  *
 366  ******************************************************************************/
 367 acpi_status
 368 acpi_hw_register_read(u32 register_id, u32 * return_value)
 369 {
 370         u32 value = 0;
 371         acpi_status status;
 372
 373         ACPI_FUNCTION_TRACE(hw_register_read);
 374
 375         switch (register_id) {
 376         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 377
 378                 status = acpi_hw_read_multiple(&value,
 379                                                &acpi_gbl_xpm1a_status,
 380                                                &acpi_gbl_xpm1b_status);
 381                 break;
 382
 383         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 384
 385                 status = acpi_hw_read_multiple(&value,
 386                                                &acpi_gbl_xpm1a_enable,
 387                                                &acpi_gbl_xpm1b_enable);
 388                 break;
 389
 390         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 391
 392                 status = acpi_hw_read_multiple(&value,
 393                                                &acpi_gbl_FADT.
 394                                                xpm1a_control_block,
 395                                                &acpi_gbl_FADT.
 396                                                xpm1b_control_block);
 397
 398                 /*
 399                  * Zero the write-only bits. From the ACPI specification, "Hardware
 400                  * Write-Only Bits": "Upon reads to registers with write-only bits,
 401                  * software masks out all write-only bits."
 402                  */
 403                 value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 404                 break;
 405
 406         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 407
 408                 status =
 409                     acpi_hw_read(&value, &acpi_gbl_FADT.xpm2_control_block);
 410                 break;
 411
 412         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 413
 414                 status = acpi_hw_read(&value, &acpi_gbl_FADT.xpm_timer_block);
 415                 break;
 416
 417         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 418
 419                 status =
 420                     acpi_hw_read_port(acpi_gbl_FADT.smi_command, &value, 8);
 421                 break;
 422
 423         default:
 424                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 425                 status = AE_BAD_PARAMETER;
 426                 break;
 427         }
 428
 429         if (ACPI_SUCCESS(status)) {
 430                 *return_value = value;
 431         }
 432
 433         return_ACPI_STATUS(status);
 434 }
 435
 436 /******************************************************************************
 437  *
 438  * FUNCTION:    acpi_hw_register_write
 439  *
 440  * PARAMETERS:  register_id         - ACPI Register ID
 441  *              value               - The value to write
 442  *
 443  * RETURN:      Status
 444  *
 445  * DESCRIPTION: Write to the specified ACPI register
 446  *
 447  * NOTE: In accordance with the ACPI specification, this function automatically
 448  * preserves the value of the following bits, meaning that these bits cannot be
 449  * changed via this interface:
 450  *
 451  * PM1_CONTROL[0] = SCI_EN
 452  * PM1_CONTROL[9]
 453  * PM1_STATUS[11]
 454  *
 455  * ACPI References:
 456  * 1) Hardware Ignored Bits: When software writes to a register with ignored
 457  *      bit fields, it preserves the ignored bit fields
 458  * 2) SCI_EN: OSPM always preserves this bit position
 459  *
 460  ******************************************************************************/
 461
 462 acpi_status acpi_hw_register_write(u32 register_id, u32 value)
 463 {
 464         acpi_status status;
 465         u32 read_value;
 466
 467         ACPI_FUNCTION_TRACE(hw_register_write);
 468
 469         switch (register_id) {
 470         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 471                 /*
 472                  * Handle the "ignored" bit in PM1 Status. According to the ACPI
 473                  * specification, ignored bits are to be preserved when writing.
 474                  * Normally, this would mean a read/modify/write sequence. However,
 475                  * preserving a bit in the status register is different. Writing a
 476                  * one clears the status, and writing a zero preserves the status.
 477                  * Therefore, we must always write zero to the ignored bit.
 478                  *
 479                  * This behavior is clarified in the ACPI 4.0 specification.
 480                  */
 481                 value &= ~ACPI_PM1_STATUS_PRESERVED_BITS;
 482
 483                 status = acpi_hw_write_multiple(value,
 484                                                 &acpi_gbl_xpm1a_status,
 485                                                 &acpi_gbl_xpm1b_status);
 486                 break;
 487
 488         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access */
 489
 490                 status = acpi_hw_write_multiple(value,
 491                                                 &acpi_gbl_xpm1a_enable,
 492                                                 &acpi_gbl_xpm1b_enable);
 493                 break;
 494
 495         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 496
 497                 /*
 498                  * Perform a read first to preserve certain bits (per ACPI spec)
 499                  * Note: This includes SCI_EN, we never want to change this bit
 500                  */
 501                 status = acpi_hw_read_multiple(&read_value,
 502                                                &acpi_gbl_FADT.
 503                                                xpm1a_control_block,
 504                                                &acpi_gbl_FADT.
 505                                                xpm1b_control_block);
 506                 if (ACPI_FAILURE(status)) {
 507                         goto exit;
 508                 }
 509
 510                 /* Insert the bits to be preserved */
 511
 512                 ACPI_INSERT_BITS(value, ACPI_PM1_CONTROL_PRESERVED_BITS,
 513                                  read_value);
 514
 515                 /* Now we can write the data */
 516
 517                 status = acpi_hw_write_multiple(value,
 518                                                 &acpi_gbl_FADT.
 519                                                 xpm1a_control_block,
 520                                                 &acpi_gbl_FADT.
 521                                                 xpm1b_control_block);
 522                 break;
 523
 524         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 525
 526                 /*
 527                  * For control registers, all reserved bits must be preserved,
 528                  * as per the ACPI spec.
 529                  */
 530                 status =
 531                     acpi_hw_read(&read_value,
 532                                  &acpi_gbl_FADT.xpm2_control_block);
 533                 if (ACPI_FAILURE(status)) {
 534                         goto exit;
 535                 }
 536
 537                 /* Insert the bits to be preserved */
 538
 539                 ACPI_INSERT_BITS(value, ACPI_PM2_CONTROL_PRESERVED_BITS,
 540                                  read_value);
 541
 542                 status =
 543                     acpi_hw_write(value, &acpi_gbl_FADT.xpm2_control_block);
 544                 break;
 545
 546         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 547
 548                 status = acpi_hw_write(value, &acpi_gbl_FADT.xpm_timer_block);
 549                 break;
 550
 551         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 552
 553                 /* SMI_CMD is currently always in IO space */
 554
 555                 status =
 556                     acpi_hw_write_port(acpi_gbl_FADT.smi_command, value, 8);
 557                 break;
 558
 559         default:
 560                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 561                 status = AE_BAD_PARAMETER;
 562                 break;
 563         }
 564
 565       exit:
 566         return_ACPI_STATUS(status);
 567 }
 568
 569 /******************************************************************************
 570  *
 571  * FUNCTION:    acpi_hw_read_multiple
 572  *
 573  * PARAMETERS:  value               - Where the register value is returned
 574  *              register_a           - First ACPI register (required)
 575  *              register_b           - Second ACPI register (optional)
 576  *
 577  * RETURN:      Status
 578  *
 579  * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
 580  *
 581  ******************************************************************************/
 582
 583 static acpi_status
 584 acpi_hw_read_multiple(u32 *value,
 585                       struct acpi_generic_address *register_a,
 586                       struct acpi_generic_address *register_b)
 587 {
 588         u32 value_a = 0;
 589         u32 value_b = 0;
 590         acpi_status status;
 591
 592         /* The first register is always required */
 593
 594         status = acpi_hw_read(&value_a, register_a);
 595         if (ACPI_FAILURE(status)) {
 596                 return (status);
 597         }
 598
 599         /* Second register is optional */
 600
 601         if (register_b->address) {
 602                 status = acpi_hw_read(&value_b, register_b);
 603                 if (ACPI_FAILURE(status)) {
 604                         return (status);
 605                 }
 606         }
 607
 608         /*
 609          * OR the two return values together. No shifting or masking is necessary,
 610          * because of how the PM1 registers are defined in the ACPI specification:
 611          *
 612          * "Although the bits can be split between the two register blocks (each
 613          * register block has a unique pointer within the FADT), the bit positions
 614          * are maintained. The register block with unimplemented bits (that is,
 615          * those implemented in the other register block) always returns zeros,
 616          * and writes have no side effects"
 617          */
 618         *value = (value_a | value_b);
 619         return (AE_OK);
 620 }
 621
 622 /******************************************************************************
 623  *
 624  * FUNCTION:    acpi_hw_write_multiple
 625  *
 626  * PARAMETERS:  value               - The value to write
 627  *              register_a           - First ACPI register (required)
 628  *              register_b           - Second ACPI register (optional)
 629  *
 630  * RETURN:      Status
 631  *
 632  * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
 633  *
 634  ******************************************************************************/
 635
 636 static acpi_status
 637 acpi_hw_write_multiple(u32 value,
 638                        struct acpi_generic_address *register_a,
 639                        struct acpi_generic_address *register_b)
 640 {
 641         acpi_status status;
 642
 643         /* The first register is always required */
 644
 645         status = acpi_hw_write(value, register_a);
 646         if (ACPI_FAILURE(status)) {
 647                 return (status);
 648         }
 649
 650         /*
 651          * Second register is optional
 652          *
 653          * No bit shifting or clearing is necessary, because of how the PM1
 654          * registers are defined in the ACPI specification:
 655          *
 656          * "Although the bits can be split between the two register blocks (each
 657          * register block has a unique pointer within the FADT), the bit positions
 658          * are maintained. The register block with unimplemented bits (that is,
 659          * those implemented in the other register block) always returns zeros,
 660          * and writes have no side effects"
 661          */
 662         if (register_b->address) {
 663                 status = acpi_hw_write(value, register_b);
 664         }
 665
 666         return (status);
 667 }
 668
 669 #endif                          /* !ACPI_REDUCED_HARDWARE */