Based on kernel version
5.8. Page generated on
2020-08-08 17:40 EST.
|1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030|
======================== libATA Developer's Guide ======================== :Author: Jeff Garzik Introduction ============ libATA is a library used inside the Linux kernel to support ATA host controllers and devices. libATA provides an ATA driver API, class transports for ATA and ATAPI devices, and SCSI<->ATA translation for ATA devices according to the T10 SAT specification. This Guide documents the libATA driver API, library functions, library internals, and a couple sample ATA low-level drivers. libata Driver API ================= :c:type:`struct ata_port_operations <ata_port_operations>` is defined for every low-level libata hardware driver, and it controls how the low-level driver interfaces with the ATA and SCSI layers. FIS-based drivers will hook into the system with ``->qc_prep()`` and ``->qc_issue()`` high-level hooks. Hardware which behaves in a manner similar to PCI IDE hardware may utilize several generic helpers, defining at a bare minimum the bus I/O addresses of the ATA shadow register blocks. :c:type:`struct ata_port_operations <ata_port_operations>` ---------------------------------------------------------- Disable ATA port ~~~~~~~~~~~~~~~~ :: void (*port_disable) (struct ata_port *); Called from :c:func:`ata_bus_probe` error path, as well as when unregistering from the SCSI module (rmmod, hot unplug). This function should do whatever needs to be done to take the port out of use. In most cases, :c:func:`ata_port_disable` can be used as this hook. Called from :c:func:`ata_bus_probe` on a failed probe. Called from :c:func:`ata_scsi_release`. Post-IDENTIFY device configuration ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: void (*dev_config) (struct ata_port *, struct ata_device *); Called after IDENTIFY [PACKET] DEVICE is issued to each device found. Typically used to apply device-specific fixups prior to issue of SET FEATURES - XFER MODE, and prior to operation. This entry may be specified as NULL in ata_port_operations. Set PIO/DMA mode ~~~~~~~~~~~~~~~~ :: void (*set_piomode) (struct ata_port *, struct ata_device *); void (*set_dmamode) (struct ata_port *, struct ata_device *); void (*post_set_mode) (struct ata_port *); unsigned int (*mode_filter) (struct ata_port *, struct ata_device *, unsigned int); Hooks called prior to the issue of SET FEATURES - XFER MODE command. The optional ``->mode_filter()`` hook is called when libata has built a mask of the possible modes. This is passed to the ``->mode_filter()`` function which should return a mask of valid modes after filtering those unsuitable due to hardware limits. It is not valid to use this interface to add modes. ``dev->pio_mode`` and ``dev->dma_mode`` are guaranteed to be valid when ``->set_piomode()`` and when ``->set_dmamode()`` is called. The timings for any other drive sharing the cable will also be valid at this point. That is the library records the decisions for the modes of each drive on a channel before it attempts to set any of them. ``->post_set_mode()`` is called unconditionally, after the SET FEATURES - XFER MODE command completes successfully. ``->set_piomode()`` is always called (if present), but ``->set_dma_mode()`` is only called if DMA is possible. Taskfile read/write ~~~~~~~~~~~~~~~~~~~ :: void (*sff_tf_load) (struct ata_port *ap, struct ata_taskfile *tf); void (*sff_tf_read) (struct ata_port *ap, struct ata_taskfile *tf); ``->tf_load()`` is called to load the given taskfile into hardware registers / DMA buffers. ``->tf_read()`` is called to read the hardware registers / DMA buffers, to obtain the current set of taskfile register values. Most drivers for taskfile-based hardware (PIO or MMIO) use :c:func:`ata_sff_tf_load` and :c:func:`ata_sff_tf_read` for these hooks. PIO data read/write ~~~~~~~~~~~~~~~~~~~ :: void (*sff_data_xfer) (struct ata_device *, unsigned char *, unsigned int, int); All bmdma-style drivers must implement this hook. This is the low-level operation that actually copies the data bytes during a PIO data transfer. Typically the driver will choose one of :c:func:`ata_sff_data_xfer`, or :c:func:`ata_sff_data_xfer32`. ATA command execute ~~~~~~~~~~~~~~~~~~~ :: void (*sff_exec_command)(struct ata_port *ap, struct ata_taskfile *tf); causes an ATA command, previously loaded with ``->tf_load()``, to be initiated in hardware. Most drivers for taskfile-based hardware use :c:func:`ata_sff_exec_command` for this hook. Per-cmd ATAPI DMA capabilities filter ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: int (*check_atapi_dma) (struct ata_queued_cmd *qc); Allow low-level driver to filter ATA PACKET commands, returning a status indicating whether or not it is OK to use DMA for the supplied PACKET command. This hook may be specified as NULL, in which case libata will assume that atapi dma can be supported. Read specific ATA shadow registers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: u8 (*sff_check_status)(struct ata_port *ap); u8 (*sff_check_altstatus)(struct ata_port *ap); Reads the Status/AltStatus ATA shadow register from hardware. On some hardware, reading the Status register has the side effect of clearing the interrupt condition. Most drivers for taskfile-based hardware use :c:func:`ata_sff_check_status` for this hook. Write specific ATA shadow register ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: void (*sff_set_devctl)(struct ata_port *ap, u8 ctl); Write the device control ATA shadow register to the hardware. Most drivers don't need to define this. Select ATA device on bus ~~~~~~~~~~~~~~~~~~~~~~~~ :: void (*sff_dev_select)(struct ata_port *ap, unsigned int device); Issues the low-level hardware command(s) that causes one of N hardware devices to be considered 'selected' (active and available for use) on the ATA bus. This generally has no meaning on FIS-based devices. Most drivers for taskfile-based hardware use :c:func:`ata_sff_dev_select` for this hook. Private tuning method ~~~~~~~~~~~~~~~~~~~~~ :: void (*set_mode) (struct ata_port *ap); By default libata performs drive and controller tuning in accordance with the ATA timing rules and also applies blacklists and cable limits. Some controllers need special handling and have custom tuning rules, typically raid controllers that use ATA commands but do not actually do drive timing. **Warning** This hook should not be used to replace the standard controller tuning logic when a controller has quirks. Replacing the default tuning logic in that case would bypass handling for drive and bridge quirks that may be important to data reliability. If a controller needs to filter the mode selection it should use the mode_filter hook instead. Control PCI IDE BMDMA engine ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: void (*bmdma_setup) (struct ata_queued_cmd *qc); void (*bmdma_start) (struct ata_queued_cmd *qc); void (*bmdma_stop) (struct ata_port *ap); u8 (*bmdma_status) (struct ata_port *ap); When setting up an IDE BMDMA transaction, these hooks arm (``->bmdma_setup``), fire (``->bmdma_start``), and halt (``->bmdma_stop``) the hardware's DMA engine. ``->bmdma_status`` is used to read the standard PCI IDE DMA Status register. These hooks are typically either no-ops, or simply not implemented, in FIS-based drivers. Most legacy IDE drivers use :c:func:`ata_bmdma_setup` for the :c:func:`bmdma_setup` hook. :c:func:`ata_bmdma_setup` will write the pointer to the PRD table to the IDE PRD Table Address register, enable DMA in the DMA Command register, and call :c:func:`exec_command` to begin the transfer. Most legacy IDE drivers use :c:func:`ata_bmdma_start` for the :c:func:`bmdma_start` hook. :c:func:`ata_bmdma_start` will write the ATA_DMA_START flag to the DMA Command register. Many legacy IDE drivers use :c:func:`ata_bmdma_stop` for the :c:func:`bmdma_stop` hook. :c:func:`ata_bmdma_stop` clears the ATA_DMA_START flag in the DMA command register. Many legacy IDE drivers use :c:func:`ata_bmdma_status` as the :c:func:`bmdma_status` hook. High-level taskfile hooks ~~~~~~~~~~~~~~~~~~~~~~~~~ :: enum ata_completion_errors (*qc_prep) (struct ata_queued_cmd *qc); int (*qc_issue) (struct ata_queued_cmd *qc); Higher-level hooks, these two hooks can potentially supersede several of the above taskfile/DMA engine hooks. ``->qc_prep`` is called after the buffers have been DMA-mapped, and is typically used to populate the hardware's DMA scatter-gather table. Some drivers use the standard :c:func:`ata_bmdma_qc_prep` and :c:func:`ata_bmdma_dumb_qc_prep` helper functions, but more advanced drivers roll their own. ``->qc_issue`` is used to make a command active, once the hardware and S/G tables have been prepared. IDE BMDMA drivers use the helper function :c:func:`ata_sff_qc_issue` for taskfile protocol-based dispatch. More advanced drivers implement their own ``->qc_issue``. :c:func:`ata_sff_qc_issue` calls ``->sff_tf_load()``, ``->bmdma_setup()``, and ``->bmdma_start()`` as necessary to initiate a transfer. Exception and probe handling (EH) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: void (*eng_timeout) (struct ata_port *ap); void (*phy_reset) (struct ata_port *ap); Deprecated. Use ``->error_handler()`` instead. :: void (*freeze) (struct ata_port *ap); void (*thaw) (struct ata_port *ap); :c:func:`ata_port_freeze` is called when HSM violations or some other condition disrupts normal operation of the port. A frozen port is not allowed to perform any operation until the port is thawed, which usually follows a successful reset. The optional ``->freeze()`` callback can be used for freezing the port hardware-wise (e.g. mask interrupt and stop DMA engine). If a port cannot be frozen hardware-wise, the interrupt handler must ack and clear interrupts unconditionally while the port is frozen. The optional ``->thaw()`` callback is called to perform the opposite of ``->freeze()``: prepare the port for normal operation once again. Unmask interrupts, start DMA engine, etc. :: void (*error_handler) (struct ata_port *ap); ``->error_handler()`` is a driver's hook into probe, hotplug, and recovery and other exceptional conditions. The primary responsibility of an implementation is to call :c:func:`ata_do_eh` or :c:func:`ata_bmdma_drive_eh` with a set of EH hooks as arguments: 'prereset' hook (may be NULL) is called during an EH reset, before any other actions are taken. 'postreset' hook (may be NULL) is called after the EH reset is performed. Based on existing conditions, severity of the problem, and hardware capabilities, Either 'softreset' (may be NULL) or 'hardreset' (may be NULL) will be called to perform the low-level EH reset. :: void (*post_internal_cmd) (struct ata_queued_cmd *qc); Perform any hardware-specific actions necessary to finish processing after executing a probe-time or EH-time command via :c:func:`ata_exec_internal`. Hardware interrupt handling ~~~~~~~~~~~~~~~~~~~~~~~~~~~ :: irqreturn_t (*irq_handler)(int, void *, struct pt_regs *); void (*irq_clear) (struct ata_port *); ``->irq_handler`` is the interrupt handling routine registered with the system, by libata. ``->irq_clear`` is called during probe just before the interrupt handler is registered, to be sure hardware is quiet. The second argument, dev_instance, should be cast to a pointer to :c:type:`struct ata_host_set <ata_host_set>`. Most legacy IDE drivers use :c:func:`ata_sff_interrupt` for the irq_handler hook, which scans all ports in the host_set, determines which queued command was active (if any), and calls ata_sff_host_intr(ap,qc). Most legacy IDE drivers use :c:func:`ata_sff_irq_clear` for the :c:func:`irq_clear` hook, which simply clears the interrupt and error flags in the DMA status register. SATA phy read/write ~~~~~~~~~~~~~~~~~~~ :: int (*scr_read) (struct ata_port *ap, unsigned int sc_reg, u32 *val); int (*scr_write) (struct ata_port *ap, unsigned int sc_reg, u32 val); Read and write standard SATA phy registers. Currently only used if ``->phy_reset`` hook called the :c:func:`sata_phy_reset` helper function. sc_reg is one of SCR_STATUS, SCR_CONTROL, SCR_ERROR, or SCR_ACTIVE. Init and shutdown ~~~~~~~~~~~~~~~~~ :: int (*port_start) (struct ata_port *ap); void (*port_stop) (struct ata_port *ap); void (*host_stop) (struct ata_host_set *host_set); ``->port_start()`` is called just after the data structures for each port are initialized. Typically this is used to alloc per-port DMA buffers / tables / rings, enable DMA engines, and similar tasks. Some drivers also use this entry point as a chance to allocate driver-private memory for ``ap->private_data``. Many drivers use :c:func:`ata_port_start` as this hook or call it from their own :c:func:`port_start` hooks. :c:func:`ata_port_start` allocates space for a legacy IDE PRD table and returns. ``->port_stop()`` is called after ``->host_stop()``. Its sole function is to release DMA/memory resources, now that they are no longer actively being used. Many drivers also free driver-private data from port at this time. ``->host_stop()`` is called after all ``->port_stop()`` calls have completed. The hook must finalize hardware shutdown, release DMA and other resources, etc. This hook may be specified as NULL, in which case it is not called. Error handling ============== This chapter describes how errors are handled under libata. Readers are advised to read SCSI EH (Documentation/scsi/scsi_eh.rst) and ATA exceptions doc first. Origins of commands ------------------- In libata, a command is represented with :c:type:`struct ata_queued_cmd <ata_queued_cmd>` or qc. qc's are preallocated during port initialization and repetitively used for command executions. Currently only one qc is allocated per port but yet-to-be-merged NCQ branch allocates one for each tag and maps each qc to NCQ tag 1-to-1. libata commands can originate from two sources - libata itself and SCSI midlayer. libata internal commands are used for initialization and error handling. All normal blk requests and commands for SCSI emulation are passed as SCSI commands through queuecommand callback of SCSI host template. How commands are issued ----------------------- Internal commands First, qc is allocated and initialized using :c:func:`ata_qc_new_init`. Although :c:func:`ata_qc_new_init` doesn't implement any wait or retry mechanism when qc is not available, internal commands are currently issued only during initialization and error recovery, so no other command is active and allocation is guaranteed to succeed. Once allocated qc's taskfile is initialized for the command to be executed. qc currently has two mechanisms to notify completion. One is via ``qc->complete_fn()`` callback and the other is completion ``qc->waiting``. ``qc->complete_fn()`` callback is the asynchronous path used by normal SCSI translated commands and ``qc->waiting`` is the synchronous (issuer sleeps in process context) path used by internal commands. Once initialization is complete, host_set lock is acquired and the qc is issued. SCSI commands All libata drivers use :c:func:`ata_scsi_queuecmd` as ``hostt->queuecommand`` callback. scmds can either be simulated or translated. No qc is involved in processing a simulated scmd. The result is computed right away and the scmd is completed. For a translated scmd, :c:func:`ata_qc_new_init` is invoked to allocate a qc and the scmd is translated into the qc. SCSI midlayer's completion notification function pointer is stored into ``qc->scsidone``. ``qc->complete_fn()`` callback is used for completion notification. ATA commands use :c:func:`ata_scsi_qc_complete` while ATAPI commands use :c:func:`atapi_qc_complete`. Both functions end up calling ``qc->scsidone`` to notify upper layer when the qc is finished. After translation is completed, the qc is issued with :c:func:`ata_qc_issue`. Note that SCSI midlayer invokes hostt->queuecommand while holding host_set lock, so all above occur while holding host_set lock. How commands are processed -------------------------- Depending on which protocol and which controller are used, commands are processed differently. For the purpose of discussion, a controller which uses taskfile interface and all standard callbacks is assumed. Currently 6 ATA command protocols are used. They can be sorted into the following four categories according to how they are processed. ATA NO DATA or DMA ATA_PROT_NODATA and ATA_PROT_DMA fall into this category. These types of commands don't require any software intervention once issued. Device will raise interrupt on completion. ATA PIO ATA_PROT_PIO is in this category. libata currently implements PIO with polling. ATA_NIEN bit is set to turn off interrupt and pio_task on ata_wq performs polling and IO. ATAPI NODATA or DMA ATA_PROT_ATAPI_NODATA and ATA_PROT_ATAPI_DMA are in this category. packet_task is used to poll BSY bit after issuing PACKET command. Once BSY is turned off by the device, packet_task transfers CDB and hands off processing to interrupt handler. ATAPI PIO ATA_PROT_ATAPI is in this category. ATA_NIEN bit is set and, as in ATAPI NODATA or DMA, packet_task submits cdb. However, after submitting cdb, further processing (data transfer) is handed off to pio_task. How commands are completed -------------------------- Once issued, all qc's are either completed with :c:func:`ata_qc_complete` or time out. For commands which are handled by interrupts, :c:func:`ata_host_intr` invokes :c:func:`ata_qc_complete`, and, for PIO tasks, pio_task invokes :c:func:`ata_qc_complete`. In error cases, packet_task may also complete commands. :c:func:`ata_qc_complete` does the following. 1. DMA memory is unmapped. 2. ATA_QCFLAG_ACTIVE is cleared from qc->flags. 3. :c:func:`qc->complete_fn` callback is invoked. If the return value of the callback is not zero. Completion is short circuited and :c:func:`ata_qc_complete` returns. 4. :c:func:`__ata_qc_complete` is called, which does 1. ``qc->flags`` is cleared to zero. 2. ``ap->active_tag`` and ``qc->tag`` are poisoned. 3. ``qc->waiting`` is cleared & completed (in that order). 4. qc is deallocated by clearing appropriate bit in ``ap->qactive``. So, it basically notifies upper layer and deallocates qc. One exception is short-circuit path in #3 which is used by :c:func:`atapi_qc_complete`. For all non-ATAPI commands, whether it fails or not, almost the same code path is taken and very little error handling takes place. A qc is completed with success status if it succeeded, with failed status otherwise. However, failed ATAPI commands require more handling as REQUEST SENSE is needed to acquire sense data. If an ATAPI command fails, :c:func:`ata_qc_complete` is invoked with error status, which in turn invokes :c:func:`atapi_qc_complete` via ``qc->complete_fn()`` callback. This makes :c:func:`atapi_qc_complete` set ``scmd->result`` to SAM_STAT_CHECK_CONDITION, complete the scmd and return 1. As the sense data is empty but ``scmd->result`` is CHECK CONDITION, SCSI midlayer will invoke EH for the scmd, and returning 1 makes :c:func:`ata_qc_complete` to return without deallocating the qc. This leads us to :c:func:`ata_scsi_error` with partially completed qc. :c:func:`ata_scsi_error` ------------------------ :c:func:`ata_scsi_error` is the current ``transportt->eh_strategy_handler()`` for libata. As discussed above, this will be entered in two cases - timeout and ATAPI error completion. This function calls low level libata driver's :c:func:`eng_timeout` callback, the standard callback for which is :c:func:`ata_eng_timeout`. It checks if a qc is active and calls :c:func:`ata_qc_timeout` on the qc if so. Actual error handling occurs in :c:func:`ata_qc_timeout`. If EH is invoked for timeout, :c:func:`ata_qc_timeout` stops BMDMA and completes the qc. Note that as we're currently in EH, we cannot call scsi_done. As described in SCSI EH doc, a recovered scmd should be either retried with :c:func:`scsi_queue_insert` or finished with :c:func:`scsi_finish_command`. Here, we override ``qc->scsidone`` with :c:func:`scsi_finish_command` and calls :c:func:`ata_qc_complete`. If EH is invoked due to a failed ATAPI qc, the qc here is completed but not deallocated. The purpose of this half-completion is to use the qc as place holder to make EH code reach this place. This is a bit hackish, but it works. Once control reaches here, the qc is deallocated by invoking :c:func:`__ata_qc_complete` explicitly. Then, internal qc for REQUEST SENSE is issued. Once sense data is acquired, scmd is finished by directly invoking :c:func:`scsi_finish_command` on the scmd. Note that as we already have completed and deallocated the qc which was associated with the scmd, we don't need to/cannot call :c:func:`ata_qc_complete` again. Problems with the current EH ---------------------------- - Error representation is too crude. Currently any and all error conditions are represented with ATA STATUS and ERROR registers. Errors which aren't ATA device errors are treated as ATA device errors by setting ATA_ERR bit. Better error descriptor which can properly represent ATA and other errors/exceptions is needed. - When handling timeouts, no action is taken to make device forget about the timed out command and ready for new commands. - EH handling via :c:func:`ata_scsi_error` is not properly protected from usual command processing. On EH entrance, the device is not in quiescent state. Timed out commands may succeed or fail any time. pio_task and atapi_task may still be running. - Too weak error recovery. Devices / controllers causing HSM mismatch errors and other errors quite often require reset to return to known state. Also, advanced error handling is necessary to support features like NCQ and hotplug. - ATA errors are directly handled in the interrupt handler and PIO errors in pio_task. This is problematic for advanced error handling for the following reasons. First, advanced error handling often requires context and internal qc execution. Second, even a simple failure (say, CRC error) needs information gathering and could trigger complex error handling (say, resetting & reconfiguring). Having multiple code paths to gather information, enter EH and trigger actions makes life painful. Third, scattered EH code makes implementing low level drivers difficult. Low level drivers override libata callbacks. If EH is scattered over several places, each affected callbacks should perform its part of error handling. This can be error prone and painful. libata Library ============== .. kernel-doc:: drivers/ata/libata-core.c :export: libata Core Internals ===================== .. kernel-doc:: drivers/ata/libata-core.c :internal: .. kernel-doc:: drivers/ata/libata-eh.c libata SCSI translation/emulation ================================= .. kernel-doc:: drivers/ata/libata-scsi.c :export: .. kernel-doc:: drivers/ata/libata-scsi.c :internal: ATA errors and exceptions ========================= This chapter tries to identify what error/exception conditions exist for ATA/ATAPI devices and describe how they should be handled in implementation-neutral way. The term 'error' is used to describe conditions where either an explicit error condition is reported from device or a command has timed out. The term 'exception' is either used to describe exceptional conditions which are not errors (say, power or hotplug events), or to describe both errors and non-error exceptional conditions. Where explicit distinction between error and exception is necessary, the term 'non-error exception' is used. Exception categories -------------------- Exceptions are described primarily with respect to legacy taskfile + bus master IDE interface. If a controller provides other better mechanism for error reporting, mapping those into categories described below shouldn't be difficult. In the following sections, two recovery actions - reset and reconfiguring transport - are mentioned. These are described further in `EH recovery actions <#exrec>`__. HSM violation ~~~~~~~~~~~~~ This error is indicated when STATUS value doesn't match HSM requirement during issuing or execution any ATA/ATAPI command. - ATA_STATUS doesn't contain !BSY && DRDY && !DRQ while trying to issue a command. - !BSY && !DRQ during PIO data transfer. - DRQ on command completion. - !BSY && ERR after CDB transfer starts but before the last byte of CDB is transferred. ATA/ATAPI standard states that "The device shall not terminate the PACKET command with an error before the last byte of the command packet has been written" in the error outputs description of PACKET command and the state diagram doesn't include such transitions. In these cases, HSM is violated and not much information regarding the error can be acquired from STATUS or ERROR register. IOW, this error can be anything - driver bug, faulty device, controller and/or cable. As HSM is violated, reset is necessary to restore known state. Reconfiguring transport for lower speed might be helpful too as transmission errors sometimes cause this kind of errors. ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ These are errors detected and reported by ATA/ATAPI devices indicating device problems. For this type of errors, STATUS and ERROR register values are valid and describe error condition. Note that some of ATA bus errors are detected by ATA/ATAPI devices and reported using the same mechanism as device errors. Those cases are described later in this section. For ATA commands, this type of errors are indicated by !BSY && ERR during command execution and on completion. For ATAPI commands, - !BSY && ERR && ABRT right after issuing PACKET indicates that PACKET command is not supported and falls in this category. - !BSY && ERR(==CHK) && !ABRT after the last byte of CDB is transferred indicates CHECK CONDITION and doesn't fall in this category. - !BSY && ERR(==CHK) && ABRT after the last byte of CDB is transferred \*probably\* indicates CHECK CONDITION and doesn't fall in this category. Of errors detected as above, the following are not ATA/ATAPI device errors but ATA bus errors and should be handled according to `ATA bus error <#excatATAbusErr>`__. CRC error during data transfer This is indicated by ICRC bit in the ERROR register and means that corruption occurred during data transfer. Up to ATA/ATAPI-7, the standard specifies that this bit is only applicable to UDMA transfers but ATA/ATAPI-8 draft revision 1f says that the bit may be applicable to multiword DMA and PIO. ABRT error during data transfer or on completion Up to ATA/ATAPI-7, the standard specifies that ABRT could be set on ICRC errors and on cases where a device is not able to complete a command. Combined with the fact that MWDMA and PIO transfer errors aren't allowed to use ICRC bit up to ATA/ATAPI-7, it seems to imply that ABRT bit alone could indicate transfer errors. However, ATA/ATAPI-8 draft revision 1f removes the part that ICRC errors can turn on ABRT. So, this is kind of gray area. Some heuristics are needed here. ATA/ATAPI device errors can be further categorized as follows. Media errors This is indicated by UNC bit in the ERROR register. ATA devices reports UNC error only after certain number of retries cannot recover the data, so there's nothing much else to do other than notifying upper layer. READ and WRITE commands report CHS or LBA of the first failed sector but ATA/ATAPI standard specifies that the amount of transferred data on error completion is indeterminate, so we cannot assume that sectors preceding the failed sector have been transferred and thus cannot complete those sectors successfully as SCSI does. Media changed / media change requested error <<TODO: fill here>> Address error This is indicated by IDNF bit in the ERROR register. Report to upper layer. Other errors This can be invalid command or parameter indicated by ABRT ERROR bit or some other error condition. Note that ABRT bit can indicate a lot of things including ICRC and Address errors. Heuristics needed. Depending on commands, not all STATUS/ERROR bits are applicable. These non-applicable bits are marked with "na" in the output descriptions but up to ATA/ATAPI-7 no definition of "na" can be found. However, ATA/ATAPI-8 draft revision 1f describes "N/A" as follows. 184.108.40.206a N/A A keyword the indicates a field has no defined value in this standard and should not be checked by the host or device. N/A fields should be cleared to zero. So, it seems reasonable to assume that "na" bits are cleared to zero by devices and thus need no explicit masking. ATAPI device CHECK CONDITION ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ATAPI device CHECK CONDITION error is indicated by set CHK bit (ERR bit) in the STATUS register after the last byte of CDB is transferred for a PACKET command. For this kind of errors, sense data should be acquired to gather information regarding the errors. REQUEST SENSE packet command should be used to acquire sense data. Once sense data is acquired, this type of errors can be handled similarly to other SCSI errors. Note that sense data may indicate ATA bus error (e.g. Sense Key 04h HARDWARE ERROR && ASC/ASCQ 47h/00h SCSI PARITY ERROR). In such cases, the error should be considered as an ATA bus error and handled according to `ATA bus error <#excatATAbusErr>`__. ATA device error (NCQ) ~~~~~~~~~~~~~~~~~~~~~~ NCQ command error is indicated by cleared BSY and set ERR bit during NCQ command phase (one or more NCQ commands outstanding). Although STATUS and ERROR registers will contain valid values describing the error, READ LOG EXT is required to clear the error condition, determine which command has failed and acquire more information. READ LOG EXT Log Page 10h reports which tag has failed and taskfile register values describing the error. With this information the failed command can be handled as a normal ATA command error as in `ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION) <#excatDevErr>`__ and all other in-flight commands must be retried. Note that this retry should not be counted - it's likely that commands retried this way would have completed normally if it were not for the failed command. Note that ATA bus errors can be reported as ATA device NCQ errors. This should be handled as described in `ATA bus error <#excatATAbusErr>`__. If READ LOG EXT Log Page 10h fails or reports NQ, we're thoroughly screwed. This condition should be treated according to `HSM violation <#excatHSMviolation>`__. ATA bus error ~~~~~~~~~~~~~ ATA bus error means that data corruption occurred during transmission over ATA bus (SATA or PATA). This type of errors can be indicated by - ICRC or ABRT error as described in `ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION) <#excatDevErr>`__. - Controller-specific error completion with error information indicating transmission error. - On some controllers, command timeout. In this case, there may be a mechanism to determine that the timeout is due to transmission error. - Unknown/random errors, timeouts and all sorts of weirdities. As described above, transmission errors can cause wide variety of symptoms ranging from device ICRC error to random device lockup, and, for many cases, there is no way to tell if an error condition is due to transmission error or not; therefore, it's necessary to employ some kind of heuristic when dealing with errors and timeouts. For example, encountering repetitive ABRT errors for known supported command is likely to indicate ATA bus error. Once it's determined that ATA bus errors have possibly occurred, lowering ATA bus transmission speed is one of actions which may alleviate the problem. See `Reconfigure transport <#exrecReconf>`__ for more information. PCI bus error ~~~~~~~~~~~~~ Data corruption or other failures during transmission over PCI (or other system bus). For standard BMDMA, this is indicated by Error bit in the BMDMA Status register. This type of errors must be logged as it indicates something is very wrong with the system. Resetting host controller is recommended. Late completion ~~~~~~~~~~~~~~~ This occurs when timeout occurs and the timeout handler finds out that the timed out command has completed successfully or with error. This is usually caused by lost interrupts. This type of errors must be logged. Resetting host controller is recommended. Unknown error (timeout) ~~~~~~~~~~~~~~~~~~~~~~~ This is when timeout occurs and the command is still processing or the host and device are in unknown state. When this occurs, HSM could be in any valid or invalid state. To bring the device to known state and make it forget about the timed out command, resetting is necessary. The timed out command may be retried. Timeouts can also be caused by transmission errors. Refer to `ATA bus error <#excatATAbusErr>`__ for more details. Hotplug and power management exceptions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <<TODO: fill here>> EH recovery actions ------------------- This section discusses several important recovery actions. Clearing error condition ~~~~~~~~~~~~~~~~~~~~~~~~ Many controllers require its error registers to be cleared by error handler. Different controllers may have different requirements. For SATA, it's strongly recommended to clear at least SError register during error handling. Reset ~~~~~ During EH, resetting is necessary in the following cases. - HSM is in unknown or invalid state - HBA is in unknown or invalid state - EH needs to make HBA/device forget about in-flight commands - HBA/device behaves weirdly Resetting during EH might be a good idea regardless of error condition to improve EH robustness. Whether to reset both or either one of HBA and device depends on situation but the following scheme is recommended. - When it's known that HBA is in ready state but ATA/ATAPI device is in unknown state, reset only device. - If HBA is in unknown state, reset both HBA and device. HBA resetting is implementation specific. For a controller complying to taskfile/BMDMA PCI IDE, stopping active DMA transaction may be sufficient iff BMDMA state is the only HBA context. But even mostly taskfile/BMDMA PCI IDE complying controllers may have implementation specific requirements and mechanism to reset themselves. This must be addressed by specific drivers. OTOH, ATA/ATAPI standard describes in detail ways to reset ATA/ATAPI devices. PATA hardware reset This is hardware initiated device reset signalled with asserted PATA RESET- signal. There is no standard way to initiate hardware reset from software although some hardware provides registers that allow driver to directly tweak the RESET- signal. Software reset This is achieved by turning CONTROL SRST bit on for at least 5us. Both PATA and SATA support it but, in case of SATA, this may require controller-specific support as the second Register FIS to clear SRST should be transmitted while BSY bit is still set. Note that on PATA, this resets both master and slave devices on a channel. EXECUTE DEVICE DIAGNOSTIC command Although ATA/ATAPI standard doesn't describe exactly, EDD implies some level of resetting, possibly similar level with software reset. Host-side EDD protocol can be handled with normal command processing and most SATA controllers should be able to handle EDD's just like other commands. As in software reset, EDD affects both devices on a PATA bus. Although EDD does reset devices, this doesn't suit error handling as EDD cannot be issued while BSY is set and it's unclear how it will act when device is in unknown/weird state. ATAPI DEVICE RESET command This is very similar to software reset except that reset can be restricted to the selected device without affecting the other device sharing the cable. SATA phy reset This is the preferred way of resetting a SATA device. In effect, it's identical to PATA hardware reset. Note that this can be done with the standard SCR Control register. As such, it's usually easier to implement than software reset. One more thing to consider when resetting devices is that resetting clears certain configuration parameters and they need to be set to their previous or newly adjusted values after reset. Parameters affected are. - CHS set up with INITIALIZE DEVICE PARAMETERS (seldom used) - Parameters set with SET FEATURES including transfer mode setting - Block count set with SET MULTIPLE MODE - Other parameters (SET MAX, MEDIA LOCK...) ATA/ATAPI standard specifies that some parameters must be maintained across hardware or software reset, but doesn't strictly specify all of them. Always reconfiguring needed parameters after reset is required for robustness. Note that this also applies when resuming from deep sleep (power-off). Also, ATA/ATAPI standard requires that IDENTIFY DEVICE / IDENTIFY PACKET DEVICE is issued after any configuration parameter is updated or a hardware reset and the result used for further operation. OS driver is required to implement revalidation mechanism to support this. Reconfigure transport ~~~~~~~~~~~~~~~~~~~~~ For both PATA and SATA, a lot of corners are cut for cheap connectors, cables or controllers and it's quite common to see high transmission error rate. This can be mitigated by lowering transmission speed. The following is a possible scheme Jeff Garzik suggested. If more than $N (3?) transmission errors happen in 15 minutes, - if SATA, decrease SATA PHY speed. if speed cannot be decreased, - decrease UDMA xfer speed. if at UDMA0, switch to PIO4, - decrease PIO xfer speed. if at PIO3, complain, but continue ata_piix Internals =================== .. kernel-doc:: drivers/ata/ata_piix.c :internal: sata_sil Internals =================== .. kernel-doc:: drivers/ata/sata_sil.c :internal: Thanks ====== The bulk of the ATA knowledge comes thanks to long conversations with Andre Hedrick (www.linux-ide.org), and long hours pondering the ATA and SCSI specifications. Thanks to Alan Cox for pointing out similarities between SATA and SCSI, and in general for motivation to hack on libata. libata's device detection method, ata_pio_devchk, and in general all the early probing was based on extensive study of Hale Landis's probe/reset code in his ATADRVR driver (www.ata-atapi.com).