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 - 2008, 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         if (ACPI_FAILURE(status)) {
 273                 goto unlock_and_exit;
 274         }
 275
 276         /* Clear the GPE Bits in all GPE registers in all GPE blocks */
 277
 278         status = acpi_ev_walk_gpe_list(acpi_hw_clear_gpe_block, NULL);
 279
 280       unlock_and_exit:
 281         acpi_os_release_lock(acpi_gbl_hardware_lock, lock_flags);
 282         return_ACPI_STATUS(status);
 283 }
 284
 285 /*******************************************************************************
 286  *
 287  * FUNCTION:    acpi_hw_get_register_bit_mask
 288  *
 289  * PARAMETERS:  register_id         - Index of ACPI Register to access
 290  *
 291  * RETURN:      The bitmask to be used when accessing the register
 292  *
 293  * DESCRIPTION: Map register_id into a register bitmask.
 294  *
 295  ******************************************************************************/
 296
 297 struct acpi_bit_register_info *acpi_hw_get_bit_register_info(u32 register_id)
 298 {
 299         ACPI_FUNCTION_ENTRY();
 300
 301         if (register_id > ACPI_BITREG_MAX) {
 302                 ACPI_ERROR((AE_INFO, "Invalid BitRegister ID: %X",
 303                             register_id));
 304                 return (NULL);
 305         }
 306
 307         return (&acpi_gbl_bit_register_info[register_id]);
 308 }
 309
 310 /******************************************************************************
 311  *
 312  * FUNCTION:    acpi_hw_write_pm1_control
 313  *
 314  * PARAMETERS:  pm1a_control        - Value to be written to PM1A control
 315  *              pm1b_control        - Value to be written to PM1B control
 316  *
 317  * RETURN:      Status
 318  *
 319  * DESCRIPTION: Write the PM1 A/B control registers. These registers are
 320  *              different than than the PM1 A/B status and enable registers
 321  *              in that different values can be written to the A/B registers.
 322  *              Most notably, the SLP_TYP bits can be different, as per the
 323  *              values returned from the _Sx predefined methods.
 324  *
 325  ******************************************************************************/
 326
 327 acpi_status acpi_hw_write_pm1_control(u32 pm1a_control, u32 pm1b_control)
 328 {
 329         acpi_status status;
 330
 331         ACPI_FUNCTION_TRACE(hw_write_pm1_control);
 332
 333         status =
 334             acpi_hw_write(pm1a_control, &acpi_gbl_FADT.xpm1a_control_block);
 335         if (ACPI_FAILURE(status)) {
 336                 return_ACPI_STATUS(status);
 337         }
 338
 339         if (acpi_gbl_FADT.xpm1b_control_block.address) {
 340                 status =
 341                     acpi_hw_write(pm1b_control,
 342                                   &acpi_gbl_FADT.xpm1b_control_block);
 343         }
 344         return_ACPI_STATUS(status);
 345 }
 346
 347 /******************************************************************************
 348  *
 349  * FUNCTION:    acpi_hw_register_read
 350  *
 351  * PARAMETERS:  register_id         - ACPI Register ID
 352  *              return_value        - Where the register value is returned
 353  *
 354  * RETURN:      Status and the value read.
 355  *
 356  * DESCRIPTION: Read from the specified ACPI register
 357  *
 358  ******************************************************************************/
 359 acpi_status
 360 acpi_hw_register_read(u32 register_id, u32 * return_value)
 361 {
 362         u32 value = 0;
 363         acpi_status status;
 364
 365         ACPI_FUNCTION_TRACE(hw_register_read);
 366
 367         switch (register_id) {
 368         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 369
 370                 status = acpi_hw_read_multiple(&value,
 371                                                &acpi_gbl_xpm1a_status,
 372                                                &acpi_gbl_xpm1b_status);
 373                 break;
 374
 375         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 376
 377                 status = acpi_hw_read_multiple(&value,
 378                                                &acpi_gbl_xpm1a_enable,
 379                                                &acpi_gbl_xpm1b_enable);
 380                 break;
 381
 382         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 383
 384                 status = acpi_hw_read_multiple(&value,
 385                                                &acpi_gbl_FADT.
 386                                                xpm1a_control_block,
 387                                                &acpi_gbl_FADT.
 388                                                xpm1b_control_block);
 389
 390                 /*
 391                  * Zero the write-only bits. From the ACPI specification, "Hardware
 392                  * Write-Only Bits": "Upon reads to registers with write-only bits,
 393                  * software masks out all write-only bits."
 394                  */
 395                 value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 396                 break;
 397
 398         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 399
 400                 status =
 401                     acpi_hw_read(&value, &acpi_gbl_FADT.xpm2_control_block);
 402                 break;
 403
 404         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 405
 406                 status = acpi_hw_read(&value, &acpi_gbl_FADT.xpm_timer_block);
 407                 break;
 408
 409         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 410
 411                 status =
 412                     acpi_hw_read_port(acpi_gbl_FADT.smi_command, &value, 8);
 413                 break;
 414
 415         default:
 416                 ACPI_ERROR((AE_INFO, "Unknown Register ID: %X", register_id));
 417                 status = AE_BAD_PARAMETER;
 418                 break;
 419         }
 420
 421         if (ACPI_SUCCESS(status)) {
 422                 *return_value = value;
 423         }
 424
 425         return_ACPI_STATUS(status);
 426 }
 427
 428 /******************************************************************************
 429  *
 430  * FUNCTION:    acpi_hw_register_write
 431  *
 432  * PARAMETERS:  register_id         - ACPI Register ID
 433  *              Value               - The value to write
 434  *
 435  * RETURN:      Status
 436  *
 437  * DESCRIPTION: Write to the specified ACPI register
 438  *
 439  * NOTE: In accordance with the ACPI specification, this function automatically
 440  * preserves the value of the following bits, meaning that these bits cannot be
 441  * changed via this interface:
 442  *
 443  * PM1_CONTROL[0] = SCI_EN
 444  * PM1_CONTROL[9]
 445  * PM1_STATUS[11]
 446  *
 447  * ACPI References:
 448  * 1) Hardware Ignored Bits: When software writes to a register with ignored
 449  *      bit fields, it preserves the ignored bit fields
 450  * 2) SCI_EN: OSPM always preserves this bit position
 451  *
 452  ******************************************************************************/
 453
 454 acpi_status acpi_hw_register_write(u32 register_id, u32 value)
 455 {
 456         acpi_status status;
 457         u32 read_value;
 458
 459         ACPI_FUNCTION_TRACE(hw_register_write);
 460
 461         switch (register_id) {
 462         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 463                 /*
 464                  * Handle the "ignored" bit in PM1 Status. According to the ACPI
 465                  * specification, ignored bits are to be preserved when writing.
 466                  * Normally, this would mean a read/modify/write sequence. However,
 467                  * preserving a bit in the status register is different. Writing a
 468                  * one clears the status, and writing a zero preserves the status.
 469                  * Therefore, we must always write zero to the ignored bit.
 470                  *
 471                  * This behavior is clarified in the ACPI 4.0 specification.
 472                  */
 473                 value &= ~ACPI_PM1_STATUS_PRESERVED_BITS;
 474
 475                 status = acpi_hw_write_multiple(value,
 476                                                 &acpi_gbl_xpm1a_status,
 477                                                 &acpi_gbl_xpm1b_status);
 478                 break;
 479
 480         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access */
 481
 482                 status = acpi_hw_write_multiple(value,
 483                                                 &acpi_gbl_xpm1a_enable,
 484                                                 &acpi_gbl_xpm1b_enable);
 485                 break;
 486
 487         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 488
 489                 /*
 490                  * Perform a read first to preserve certain bits (per ACPI spec)
 491                  * Note: This includes SCI_EN, we never want to change this bit
 492                  */
 493                 status = acpi_hw_read_multiple(&read_value,
 494                                                &acpi_gbl_FADT.
 495                                                xpm1a_control_block,
 496                                                &acpi_gbl_FADT.
 497                                                xpm1b_control_block);
 498                 if (ACPI_FAILURE(status)) {
 499                         goto exit;
 500                 }
 501
 502                 /* Insert the bits to be preserved */
 503
 504                 ACPI_INSERT_BITS(value, ACPI_PM1_CONTROL_PRESERVED_BITS,
 505                                  read_value);
 506
 507                 /* Now we can write the data */
 508
 509                 status = acpi_hw_write_multiple(value,
 510                                                 &acpi_gbl_FADT.
 511                                                 xpm1a_control_block,
 512                                                 &acpi_gbl_FADT.
 513                                                 xpm1b_control_block);
 514                 break;
 515
 516         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 517
 518                 /*
 519                  * For control registers, all reserved bits must be preserved,
 520                  * as per the ACPI spec.
 521                  */
 522                 status =
 523                     acpi_hw_read(&read_value,
 524                                  &acpi_gbl_FADT.xpm2_control_block);
 525                 if (ACPI_FAILURE(status)) {
 526                         goto exit;
 527                 }
 528
 529                 /* Insert the bits to be preserved */
 530
 531                 ACPI_INSERT_BITS(value, ACPI_PM2_CONTROL_PRESERVED_BITS,
 532                                  read_value);
 533
 534                 status =
 535                     acpi_hw_write(value, &acpi_gbl_FADT.xpm2_control_block);
 536                 break;
 537
 538         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 539
 540                 status = acpi_hw_write(value, &acpi_gbl_FADT.xpm_timer_block);
 541                 break;
 542
 543         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 544
 545                 /* SMI_CMD is currently always in IO space */
 546
 547                 status =
 548                     acpi_hw_write_port(acpi_gbl_FADT.smi_command, value, 8);
 549                 break;
 550
 551         default:
 552                 ACPI_ERROR((AE_INFO, "Unknown Register ID: %X", register_id));
 553                 status = AE_BAD_PARAMETER;
 554                 break;
 555         }
 556
 557       exit:
 558         return_ACPI_STATUS(status);
 559 }
 560
 561 /******************************************************************************
 562  *
 563  * FUNCTION:    acpi_hw_read_multiple
 564  *
 565  * PARAMETERS:  Value               - Where the register value is returned
 566  *              register_a           - First ACPI register (required)
 567  *              register_b           - Second ACPI register (optional)
 568  *
 569  * RETURN:      Status
 570  *
 571  * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
 572  *
 573  ******************************************************************************/
 574
 575 static acpi_status
 576 acpi_hw_read_multiple(u32 *value,
 577                       struct acpi_generic_address *register_a,
 578                       struct acpi_generic_address *register_b)
 579 {
 580         u32 value_a = 0;
 581         u32 value_b = 0;
 582         acpi_status status;
 583
 584         /* The first register is always required */
 585
 586         status = acpi_hw_read(&value_a, register_a);
 587         if (ACPI_FAILURE(status)) {
 588                 return (status);
 589         }
 590
 591         /* Second register is optional */
 592
 593         if (register_b->address) {
 594                 status = acpi_hw_read(&value_b, register_b);
 595                 if (ACPI_FAILURE(status)) {
 596                         return (status);
 597                 }
 598         }
 599
 600         /*
 601          * OR the two return values together. No shifting or masking is necessary,
 602          * because of how the PM1 registers are defined in the ACPI specification:
 603          *
 604          * "Although the bits can be split between the two register blocks (each
 605          * register block has a unique pointer within the FADT), the bit positions
 606          * are maintained. The register block with unimplemented bits (that is,
 607          * those implemented in the other register block) always returns zeros,
 608          * and writes have no side effects"
 609          */
 610         *value = (value_a | value_b);
 611         return (AE_OK);
 612 }
 613
 614 /******************************************************************************
 615  *
 616  * FUNCTION:    acpi_hw_write_multiple
 617  *
 618  * PARAMETERS:  Value               - The value to write
 619  *              register_a           - First ACPI register (required)
 620  *              register_b           - Second ACPI register (optional)
 621  *
 622  * RETURN:      Status
 623  *
 624  * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
 625  *
 626  ******************************************************************************/
 627
 628 static acpi_status
 629 acpi_hw_write_multiple(u32 value,
 630                        struct acpi_generic_address *register_a,
 631                        struct acpi_generic_address *register_b)
 632 {
 633         acpi_status status;
 634
 635         /* The first register is always required */
 636
 637         status = acpi_hw_write(value, register_a);
 638         if (ACPI_FAILURE(status)) {
 639                 return (status);
 640         }
 641
 642         /*
 643          * Second register is optional
 644          *
 645          * No bit shifting or clearing is necessary, because of how the PM1
 646          * registers are defined in the ACPI specification:
 647          *
 648          * "Although the bits can be split between the two register blocks (each
 649          * register block has a unique pointer within the FADT), the bit positions
 650          * are maintained. The register block with unimplemented bits (that is,
 651          * those implemented in the other register block) always returns zeros,
 652          * and writes have no side effects"
 653          */
 654         if (register_b->address) {
 655                 status = acpi_hw_write(value, register_b);
 656         }
 657
 658         return (status);
 659 }