xref: /linux/Documentation/driver-api/libata.rst (revision 6f47c7ae8c7afaf9ad291d39f0d3974f191a7946)

========================
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>`
----------------------------------------------------------

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 (*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.
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
    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.

    ``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:expr:`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 will check if a qc is active
and has not failed yet. Such a qc will be marked with AC_ERR_TIMEOUT such that
EH will know to handle it later. Then it calls low level libata driver's
:c:func:`error_handler` callback.

When the :c:func:`error_handler` callback is invoked it 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.

    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).