docs-rst: convert libata book to ReST

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+========================
+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
+=================
+
+struct 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.
+
+struct ata_port_operations
+----------------------------
+
+Disable ATA port
+~~~~~~~~~~~~~~~~
+
+::
+
+    void (*port_disable) (struct ata_port *);
+
+
+Called from 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,
+ata_port_disable() can be used as this hook.
+
+Called from ata_bus_probe() on a failed probe. Called from
+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
+ata_sff_tf_load() and 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
+ata_sff_data_xfer_noirq(), ata_sff_data_xfer(), or
+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
+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
+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 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 ata_bmdma_setup() for the bmdma_setup()
+hook. 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 exec_command() to begin the transfer.
+
+Most legacy IDE drivers use ata_bmdma_start() for the bmdma_start()
+hook. ata_bmdma_start() will write the ATA_DMA_START flag to the DMA
+Command register.
+
+Many legacy IDE drivers use ata_bmdma_stop() for the bmdma_stop()
+hook. ata_bmdma_stop() clears the ATA_DMA_START flag in the DMA
+command register.
+
+Many legacy IDE drivers use ata_bmdma_status() as the bmdma_status()
+hook.
+
+High-level taskfile hooks
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    void (*qc_prep) (struct ata_queued_cmd *qc);
+    int (*qc_issue) (struct ata_queued_cmd *qc);
+
+
+Higher-level hooks, these two hooks can potentially supercede 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. Most drivers use the standard
+ata_qc_prep() helper function, 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
+ata_qc_issue_prot() for taskfile protocol-based dispatch. More
+advanced drivers implement their own ->qc_issue.
+
+ata_qc_issue_prot() calls ->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);
+
+
+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 ata_do_eh() or 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
+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
+struct ata_host_set.
+
+Most legacy IDE drivers use 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 ata_sff_irq_clear() for the 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 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 ata_port_start() as this hook or call it from their
+own port_start() hooks. 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.txt) and ATA
+exceptions doc first.
+
+Origins of commands
+-------------------
+
+In libata, a command is represented with struct 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 ata_qc_new_init().
+    Although 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 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, 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 ata_scsi_qc_complete() while ATAPI commands use
+    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 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 ata_qc_complete() or
+time out. For commands which are handled by interrupts,
+ata_host_intr() invokes ata_qc_complete(), and, for PIO tasks,
+pio_task invokes ata_qc_complete(). In error cases, packet_task may
+also complete commands.
+
+ata_qc_complete() does the following.
+
+1. DMA memory is unmapped.
+
+2. ATA_QCFLAG_ACTIVE is cleared from qc->flags.
+
+3. qc->complete_fn() callback is invoked. If the return value of the
+   callback is not zero. Completion is short circuited and
+   ata_qc_complete() returns.
+
+4. __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 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,
+ata_qc_complete() is invoked with error status, which in turn invokes
+atapi_qc_complete() via qc->complete_fn() callback.
+
+This makes 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 ata_qc_complete()
+to return without deallocating the qc. This leads us to
+ata_scsi_error() with partially completed qc.
+
+ata_scsi_error()
+------------------
+
+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 eng_timeout() callback, the standard callback for which is
+ata_eng_timeout(). It checks if a qc is active and calls
+ata_qc_timeout() on the qc if so. Actual error handling occurs in
+ata_qc_timeout().
+
+If EH is invoked for timeout, 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 scsi_queue_insert() or finished with
+scsi_finish_command(). Here, we override qc->scsidone with
+scsi_finish_command() and calls 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
+__ata_qc_complete() explicitly. Then, internal qc for REQUEST SENSE
+is issued. Once sense data is acquired, scmd is finished by directly
+invoking 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 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 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:
+
+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.
+
+    3.2.3.3a 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).