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 - 2016, 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 "acevents.h"
  48
  49 #define _COMPONENT          ACPI_HARDWARE
  50 ACPI_MODULE_NAME("hwregs")
  51
  52 #if (!ACPI_REDUCED_HARDWARE)
  53 /* Local Prototypes */
  54 static u8
  55 acpi_hw_get_access_bit_width(struct acpi_generic_address *reg,
  56                              u8 max_bit_width);
  57
  58 static acpi_status
  59 acpi_hw_read_multiple(u32 *value,
  60                       struct acpi_generic_address *register_a,
  61                       struct acpi_generic_address *register_b);
  62
  63 static acpi_status
  64 acpi_hw_write_multiple(u32 value,
  65                        struct acpi_generic_address *register_a,
  66                        struct acpi_generic_address *register_b);
  67
  68 #endif                          /* !ACPI_REDUCED_HARDWARE */
  69
  70 /******************************************************************************
  71  *
  72  * FUNCTION:    acpi_hw_get_access_bit_width
  73  *
  74  * PARAMETERS:  reg                 - GAS register structure
  75  *              max_bit_width       - Max bit_width supported (32 or 64)
  76  *
  77  * RETURN:      Status
  78  *
  79  * DESCRIPTION: Obtain optimal access bit width
  80  *
  81  ******************************************************************************/
  82
  83 static u8
  84 acpi_hw_get_access_bit_width(struct acpi_generic_address *reg, u8 max_bit_width)
  85 {
  86         u64 address;
  87
  88         if (!reg->access_width) {
  89                 /*
  90                  * Detect old register descriptors where only the bit_width field
  91                  * makes senses. The target address is copied to handle possible
  92                  * alignment issues.
  93                  */
  94                 ACPI_MOVE_64_TO_64(&address, &reg->address);
  95                 if (!reg->bit_offset && reg->bit_width &&
  96                     ACPI_IS_POWER_OF_TWO(reg->bit_width) &&
  97                     ACPI_IS_ALIGNED(reg->bit_width, 8) &&
  98                     ACPI_IS_ALIGNED(address, reg->bit_width)) {
  99                         return (reg->bit_width);
 100                 } else {
 101                         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO) {
 102                                 return (32);
 103                         } else {
 104                                 return (max_bit_width);
 105                         }
 106                 }
 107         } else {
 108                 return (1 << (reg->access_width + 2));
 109         }
 110 }
 111
 112 /******************************************************************************
 113  *
 114  * FUNCTION:    acpi_hw_validate_register
 115  *
 116  * PARAMETERS:  reg                 - GAS register structure
 117  *              max_bit_width       - Max bit_width supported (32 or 64)
 118  *              address             - Pointer to where the gas->address
 119  *                                    is returned
 120  *
 121  * RETURN:      Status
 122  *
 123  * DESCRIPTION: Validate the contents of a GAS register. Checks the GAS
 124  *              pointer, Address, space_id, bit_width, and bit_offset.
 125  *
 126  ******************************************************************************/
 127
 128 acpi_status
 129 acpi_hw_validate_register(struct acpi_generic_address *reg,
 130                           u8 max_bit_width, u64 *address)
 131 {
 132         u8 bit_width;
 133         u8 access_width;
 134
 135         /* Must have a valid pointer to a GAS structure */
 136
 137         if (!reg) {
 138                 return (AE_BAD_PARAMETER);
 139         }
 140
 141         /*
 142          * Copy the target address. This handles possible alignment issues.
 143          * Address must not be null. A null address also indicates an optional
 144          * ACPI register that is not supported, so no error message.
 145          */
 146         ACPI_MOVE_64_TO_64(address, &reg->address);
 147         if (!(*address)) {
 148                 return (AE_BAD_ADDRESS);
 149         }
 150
 151         /* Validate the space_ID */
 152
 153         if ((reg->space_id != ACPI_ADR_SPACE_SYSTEM_MEMORY) &&
 154             (reg->space_id != ACPI_ADR_SPACE_SYSTEM_IO)) {
 155                 ACPI_ERROR((AE_INFO,
 156                             "Unsupported address space: 0x%X", reg->space_id));
 157                 return (AE_SUPPORT);
 158         }
 159
 160         /* Validate the access_width */
 161
 162         if (reg->access_width > 4) {
 163                 ACPI_ERROR((AE_INFO,
 164                             "Unsupported register access width: 0x%X",
 165                             reg->access_width));
 166                 return (AE_SUPPORT);
 167         }
 168
 169         /* Validate the bit_width, convert access_width into number of bits */
 170
 171         access_width = acpi_hw_get_access_bit_width(reg, max_bit_width);
 172         bit_width =
 173             ACPI_ROUND_UP(reg->bit_offset + reg->bit_width, access_width);
 174         if (max_bit_width < bit_width) {
 175                 ACPI_WARNING((AE_INFO,
 176                               "Requested bit width 0x%X is smaller than register bit width 0x%X",
 177                               max_bit_width, bit_width));
 178                 return (AE_SUPPORT);
 179         }
 180
 181         return (AE_OK);
 182 }
 183
 184 /******************************************************************************
 185  *
 186  * FUNCTION:    acpi_hw_read
 187  *
 188  * PARAMETERS:  value               - Where the value is returned
 189  *              reg                 - GAS register structure
 190  *
 191  * RETURN:      Status
 192  *
 193  * DESCRIPTION: Read from either memory or IO space. This is a 32-bit max
 194  *              version of acpi_read, used internally since the overhead of
 195  *              64-bit values is not needed.
 196  *
 197  * LIMITATIONS: <These limitations also apply to acpi_hw_write>
 198  *      bit_width must be exactly 8, 16, or 32.
 199  *      space_ID must be system_memory or system_IO.
 200  *      bit_offset and access_width are currently ignored, as there has
 201  *          not been a need to implement these.
 202  *
 203  ******************************************************************************/
 204
 205 acpi_status acpi_hw_read(u32 *value, struct acpi_generic_address *reg)
 206 {
 207         u64 address;
 208         u64 value64;
 209         acpi_status status;
 210
 211         ACPI_FUNCTION_NAME(hw_read);
 212
 213         /* Validate contents of the GAS register */
 214
 215         status = acpi_hw_validate_register(reg, 32, &address);
 216         if (ACPI_FAILURE(status)) {
 217                 return (status);
 218         }
 219
 220         /* Initialize entire 32-bit return value to zero */
 221
 222         *value = 0;
 223
 224         /*
 225          * Two address spaces supported: Memory or IO. PCI_Config is
 226          * not supported here because the GAS structure is insufficient
 227          */
 228         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 229                 status = acpi_os_read_memory((acpi_physical_address)
 230                                              address, &value64, reg->bit_width);
 231
 232                 *value = (u32)value64;
 233         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 234
 235                 status = acpi_hw_read_port((acpi_io_address)
 236                                            address, value, reg->bit_width);
 237         }
 238
 239         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 240                           "Read:  %8.8X width %2d from %8.8X%8.8X (%s)\n",
 241                           *value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 242                           acpi_ut_get_region_name(reg->space_id)));
 243
 244         return (status);
 245 }
 246
 247 /******************************************************************************
 248  *
 249  * FUNCTION:    acpi_hw_write
 250  *
 251  * PARAMETERS:  value               - Value to be written
 252  *              reg                 - GAS register structure
 253  *
 254  * RETURN:      Status
 255  *
 256  * DESCRIPTION: Write to either memory or IO space. This is a 32-bit max
 257  *              version of acpi_write, used internally since the overhead of
 258  *              64-bit values is not needed.
 259  *
 260  ******************************************************************************/
 261
 262 acpi_status acpi_hw_write(u32 value, struct acpi_generic_address *reg)
 263 {
 264         u64 address;
 265         acpi_status status;
 266
 267         ACPI_FUNCTION_NAME(hw_write);
 268
 269         /* Validate contents of the GAS register */
 270
 271         status = acpi_hw_validate_register(reg, 32, &address);
 272         if (ACPI_FAILURE(status)) {
 273                 return (status);
 274         }
 275
 276         /*
 277          * Two address spaces supported: Memory or IO. PCI_Config is
 278          * not supported here because the GAS structure is insufficient
 279          */
 280         if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {
 281                 status = acpi_os_write_memory((acpi_physical_address)
 282                                               address, (u64)value,
 283                                               reg->bit_width);
 284         } else {                /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
 285
 286                 status = acpi_hw_write_port((acpi_io_address)
 287                                             address, value, reg->bit_width);
 288         }
 289
 290         ACPI_DEBUG_PRINT((ACPI_DB_IO,
 291                           "Wrote: %8.8X width %2d   to %8.8X%8.8X (%s)\n",
 292                           value, reg->bit_width, ACPI_FORMAT_UINT64(address),
 293                           acpi_ut_get_region_name(reg->space_id)));
 294
 295         return (status);
 296 }
 297
 298 #if (!ACPI_REDUCED_HARDWARE)
 299 /*******************************************************************************
 300  *
 301  * FUNCTION:    acpi_hw_clear_acpi_status
 302  *
 303  * PARAMETERS:  None
 304  *
 305  * RETURN:      Status
 306  *
 307  * DESCRIPTION: Clears all fixed and general purpose status bits
 308  *
 309  ******************************************************************************/
 310
 311 acpi_status acpi_hw_clear_acpi_status(void)
 312 {
 313         acpi_status status;
 314         acpi_cpu_flags lock_flags = 0;
 315
 316         ACPI_FUNCTION_TRACE(hw_clear_acpi_status);
 317
 318         ACPI_DEBUG_PRINT((ACPI_DB_IO, "About to write %04X to %8.8X%8.8X\n",
 319                           ACPI_BITMASK_ALL_FIXED_STATUS,
 320                           ACPI_FORMAT_UINT64(acpi_gbl_xpm1a_status.address)));
 321
 322         lock_flags = acpi_os_acquire_lock(acpi_gbl_hardware_lock);
 323
 324         /* Clear the fixed events in PM1 A/B */
 325
 326         status = acpi_hw_register_write(ACPI_REGISTER_PM1_STATUS,
 327                                         ACPI_BITMASK_ALL_FIXED_STATUS);
 328
 329         acpi_os_release_lock(acpi_gbl_hardware_lock, lock_flags);
 330
 331         if (ACPI_FAILURE(status)) {
 332                 goto exit;
 333         }
 334
 335         /* Clear the GPE Bits in all GPE registers in all GPE blocks */
 336
 337         status = acpi_ev_walk_gpe_list(acpi_hw_clear_gpe_block, NULL);
 338
 339 exit:
 340         return_ACPI_STATUS(status);
 341 }
 342
 343 /*******************************************************************************
 344  *
 345  * FUNCTION:    acpi_hw_get_bit_register_info
 346  *
 347  * PARAMETERS:  register_id         - Index of ACPI Register to access
 348  *
 349  * RETURN:      The bitmask to be used when accessing the register
 350  *
 351  * DESCRIPTION: Map register_id into a register bitmask.
 352  *
 353  ******************************************************************************/
 354
 355 struct acpi_bit_register_info *acpi_hw_get_bit_register_info(u32 register_id)
 356 {
 357         ACPI_FUNCTION_ENTRY();
 358
 359         if (register_id > ACPI_BITREG_MAX) {
 360                 ACPI_ERROR((AE_INFO, "Invalid BitRegister ID: 0x%X",
 361                             register_id));
 362                 return (NULL);
 363         }
 364
 365         return (&acpi_gbl_bit_register_info[register_id]);
 366 }
 367
 368 /******************************************************************************
 369  *
 370  * FUNCTION:    acpi_hw_write_pm1_control
 371  *
 372  * PARAMETERS:  pm1a_control        - Value to be written to PM1A control
 373  *              pm1b_control        - Value to be written to PM1B control
 374  *
 375  * RETURN:      Status
 376  *
 377  * DESCRIPTION: Write the PM1 A/B control registers. These registers are
 378  *              different than than the PM1 A/B status and enable registers
 379  *              in that different values can be written to the A/B registers.
 380  *              Most notably, the SLP_TYP bits can be different, as per the
 381  *              values returned from the _Sx predefined methods.
 382  *
 383  ******************************************************************************/
 384
 385 acpi_status acpi_hw_write_pm1_control(u32 pm1a_control, u32 pm1b_control)
 386 {
 387         acpi_status status;
 388
 389         ACPI_FUNCTION_TRACE(hw_write_pm1_control);
 390
 391         status =
 392             acpi_hw_write(pm1a_control, &acpi_gbl_FADT.xpm1a_control_block);
 393         if (ACPI_FAILURE(status)) {
 394                 return_ACPI_STATUS(status);
 395         }
 396
 397         if (acpi_gbl_FADT.xpm1b_control_block.address) {
 398                 status =
 399                     acpi_hw_write(pm1b_control,
 400                                   &acpi_gbl_FADT.xpm1b_control_block);
 401         }
 402         return_ACPI_STATUS(status);
 403 }
 404
 405 /******************************************************************************
 406  *
 407  * FUNCTION:    acpi_hw_register_read
 408  *
 409  * PARAMETERS:  register_id         - ACPI Register ID
 410  *              return_value        - Where the register value is returned
 411  *
 412  * RETURN:      Status and the value read.
 413  *
 414  * DESCRIPTION: Read from the specified ACPI register
 415  *
 416  ******************************************************************************/
 417 acpi_status acpi_hw_register_read(u32 register_id, u32 *return_value)
 418 {
 419         u32 value = 0;
 420         acpi_status status;
 421
 422         ACPI_FUNCTION_TRACE(hw_register_read);
 423
 424         switch (register_id) {
 425         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 426
 427                 status = acpi_hw_read_multiple(&value,
 428                                                &acpi_gbl_xpm1a_status,
 429                                                &acpi_gbl_xpm1b_status);
 430                 break;
 431
 432         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 433
 434                 status = acpi_hw_read_multiple(&value,
 435                                                &acpi_gbl_xpm1a_enable,
 436                                                &acpi_gbl_xpm1b_enable);
 437                 break;
 438
 439         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 440
 441                 status = acpi_hw_read_multiple(&value,
 442                                                &acpi_gbl_FADT.
 443                                                xpm1a_control_block,
 444                                                &acpi_gbl_FADT.
 445                                                xpm1b_control_block);
 446
 447                 /*
 448                  * Zero the write-only bits. From the ACPI specification, "Hardware
 449                  * Write-Only Bits": "Upon reads to registers with write-only bits,
 450                  * software masks out all write-only bits."
 451                  */
 452                 value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 453                 break;
 454
 455         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 456
 457                 status =
 458                     acpi_hw_read(&value, &acpi_gbl_FADT.xpm2_control_block);
 459                 break;
 460
 461         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 462
 463                 status = acpi_hw_read(&value, &acpi_gbl_FADT.xpm_timer_block);
 464                 break;
 465
 466         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 467
 468                 status =
 469                     acpi_hw_read_port(acpi_gbl_FADT.smi_command, &value, 8);
 470                 break;
 471
 472         default:
 473
 474                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 475                 status = AE_BAD_PARAMETER;
 476                 break;
 477         }
 478
 479         if (ACPI_SUCCESS(status)) {
 480                 *return_value = value;
 481         }
 482
 483         return_ACPI_STATUS(status);
 484 }
 485
 486 /******************************************************************************
 487  *
 488  * FUNCTION:    acpi_hw_register_write
 489  *
 490  * PARAMETERS:  register_id         - ACPI Register ID
 491  *              value               - The value to write
 492  *
 493  * RETURN:      Status
 494  *
 495  * DESCRIPTION: Write to the specified ACPI register
 496  *
 497  * NOTE: In accordance with the ACPI specification, this function automatically
 498  * preserves the value of the following bits, meaning that these bits cannot be
 499  * changed via this interface:
 500  *
 501  * PM1_CONTROL[0] = SCI_EN
 502  * PM1_CONTROL[9]
 503  * PM1_STATUS[11]
 504  *
 505  * ACPI References:
 506  * 1) Hardware Ignored Bits: When software writes to a register with ignored
 507  *      bit fields, it preserves the ignored bit fields
 508  * 2) SCI_EN: OSPM always preserves this bit position
 509  *
 510  ******************************************************************************/
 511
 512 acpi_status acpi_hw_register_write(u32 register_id, u32 value)
 513 {
 514         acpi_status status;
 515         u32 read_value;
 516
 517         ACPI_FUNCTION_TRACE(hw_register_write);
 518
 519         switch (register_id) {
 520         case ACPI_REGISTER_PM1_STATUS:  /* PM1 A/B: 16-bit access each */
 521                 /*
 522                  * Handle the "ignored" bit in PM1 Status. According to the ACPI
 523                  * specification, ignored bits are to be preserved when writing.
 524                  * Normally, this would mean a read/modify/write sequence. However,
 525                  * preserving a bit in the status register is different. Writing a
 526                  * one clears the status, and writing a zero preserves the status.
 527                  * Therefore, we must always write zero to the ignored bit.
 528                  *
 529                  * This behavior is clarified in the ACPI 4.0 specification.
 530                  */
 531                 value &= ~ACPI_PM1_STATUS_PRESERVED_BITS;
 532
 533                 status = acpi_hw_write_multiple(value,
 534                                                 &acpi_gbl_xpm1a_status,
 535                                                 &acpi_gbl_xpm1b_status);
 536                 break;
 537
 538         case ACPI_REGISTER_PM1_ENABLE:  /* PM1 A/B: 16-bit access each */
 539
 540                 status = acpi_hw_write_multiple(value,
 541                                                 &acpi_gbl_xpm1a_enable,
 542                                                 &acpi_gbl_xpm1b_enable);
 543                 break;
 544
 545         case ACPI_REGISTER_PM1_CONTROL: /* PM1 A/B: 16-bit access each */
 546                 /*
 547                  * Perform a read first to preserve certain bits (per ACPI spec)
 548                  * Note: This includes SCI_EN, we never want to change this bit
 549                  */
 550                 status = acpi_hw_read_multiple(&read_value,
 551                                                &acpi_gbl_FADT.
 552                                                xpm1a_control_block,
 553                                                &acpi_gbl_FADT.
 554                                                xpm1b_control_block);
 555                 if (ACPI_FAILURE(status)) {
 556                         goto exit;
 557                 }
 558
 559                 /* Insert the bits to be preserved */
 560
 561                 ACPI_INSERT_BITS(value, ACPI_PM1_CONTROL_PRESERVED_BITS,
 562                                  read_value);
 563
 564                 /* Now we can write the data */
 565
 566                 status = acpi_hw_write_multiple(value,
 567                                                 &acpi_gbl_FADT.
 568                                                 xpm1a_control_block,
 569                                                 &acpi_gbl_FADT.
 570                                                 xpm1b_control_block);
 571                 break;
 572
 573         case ACPI_REGISTER_PM2_CONTROL: /* 8-bit access */
 574                 /*
 575                  * For control registers, all reserved bits must be preserved,
 576                  * as per the ACPI spec.
 577                  */
 578                 status =
 579                     acpi_hw_read(&read_value,
 580                                  &acpi_gbl_FADT.xpm2_control_block);
 581                 if (ACPI_FAILURE(status)) {
 582                         goto exit;
 583                 }
 584
 585                 /* Insert the bits to be preserved */
 586
 587                 ACPI_INSERT_BITS(value, ACPI_PM2_CONTROL_PRESERVED_BITS,
 588                                  read_value);
 589
 590                 status =
 591                     acpi_hw_write(value, &acpi_gbl_FADT.xpm2_control_block);
 592                 break;
 593
 594         case ACPI_REGISTER_PM_TIMER:    /* 32-bit access */
 595
 596                 status = acpi_hw_write(value, &acpi_gbl_FADT.xpm_timer_block);
 597                 break;
 598
 599         case ACPI_REGISTER_SMI_COMMAND_BLOCK:   /* 8-bit access */
 600
 601                 /* SMI_CMD is currently always in IO space */
 602
 603                 status =
 604                     acpi_hw_write_port(acpi_gbl_FADT.smi_command, value, 8);
 605                 break;
 606
 607         default:
 608
 609                 ACPI_ERROR((AE_INFO, "Unknown Register ID: 0x%X", register_id));
 610                 status = AE_BAD_PARAMETER;
 611                 break;
 612         }
 613
 614 exit:
 615         return_ACPI_STATUS(status);
 616 }
 617
 618 /******************************************************************************
 619  *
 620  * FUNCTION:    acpi_hw_read_multiple
 621  *
 622  * PARAMETERS:  value               - Where the register value is returned
 623  *              register_a           - First ACPI register (required)
 624  *              register_b           - Second ACPI register (optional)
 625  *
 626  * RETURN:      Status
 627  *
 628  * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
 629  *
 630  ******************************************************************************/
 631
 632 static acpi_status
 633 acpi_hw_read_multiple(u32 *value,
 634                       struct acpi_generic_address *register_a,
 635                       struct acpi_generic_address *register_b)
 636 {
 637         u32 value_a = 0;
 638         u32 value_b = 0;
 639         acpi_status status;
 640
 641         /* The first register is always required */
 642
 643         status = acpi_hw_read(&value_a, register_a);
 644         if (ACPI_FAILURE(status)) {
 645                 return (status);
 646         }
 647
 648         /* Second register is optional */
 649
 650         if (register_b->address) {
 651                 status = acpi_hw_read(&value_b, register_b);
 652                 if (ACPI_FAILURE(status)) {
 653                         return (status);
 654                 }
 655         }
 656
 657         /*
 658          * OR the two return values together. No shifting or masking is necessary,
 659          * because of how the PM1 registers are defined in the ACPI specification:
 660          *
 661          * "Although the bits can be split between the two register blocks (each
 662          * register block has a unique pointer within the FADT), the bit positions
 663          * are maintained. The register block with unimplemented bits (that is,
 664          * those implemented in the other register block) always returns zeros,
 665          * and writes have no side effects"
 666          */
 667         *value = (value_a | value_b);
 668         return (AE_OK);
 669 }
 670
 671 /******************************************************************************
 672  *
 673  * FUNCTION:    acpi_hw_write_multiple
 674  *
 675  * PARAMETERS:  value               - The value to write
 676  *              register_a           - First ACPI register (required)
 677  *              register_b           - Second ACPI register (optional)
 678  *
 679  * RETURN:      Status
 680  *
 681  * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
 682  *
 683  ******************************************************************************/
 684
 685 static acpi_status
 686 acpi_hw_write_multiple(u32 value,
 687                        struct acpi_generic_address *register_a,
 688                        struct acpi_generic_address *register_b)
 689 {
 690         acpi_status status;
 691
 692         /* The first register is always required */
 693
 694         status = acpi_hw_write(value, register_a);
 695         if (ACPI_FAILURE(status)) {
 696                 return (status);
 697         }
 698
 699         /*
 700          * Second register is optional
 701          *
 702          * No bit shifting or clearing is necessary, because of how the PM1
 703          * registers are defined in the ACPI specification:
 704          *
 705          * "Although the bits can be split between the two register blocks (each
 706          * register block has a unique pointer within the FADT), the bit positions
 707          * are maintained. The register block with unimplemented bits (that is,
 708          * those implemented in the other register block) always returns zeros,
 709          * and writes have no side effects"
 710          */
 711         if (register_b->address) {
 712                 status = acpi_hw_write(value, register_b);
 713         }
 714
 715         return (status);
 716 }
 717
 718 #endif                          /* !ACPI_REDUCED_HARDWARE */