Documentation/DocBook/libata.tmpl

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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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<book id="libataDevGuide">
 <bookinfo>
  <title>libATA Developer's Guide</title>
  
  <authorgroup>
   <author>
    <firstname>Jeff</firstname>
    <surname>Garzik</surname>
   </author>
  </authorgroup>

  <copyright>
   <year>2003-2005</year>
   <holder>Jeff Garzik</holder>
  </copyright>

  <legalnotice>
   <para>
   The contents of this file are subject to the Open
   Software License version 1.1 that can be found at
   <ulink url="http://www.opensource.org/licenses/osl-1.1.txt">http://www.opensource.org/licenses/osl-1.1.txt</ulink> and is included herein
   by reference.
   </para>

   <para>
   Alternatively, the contents of this file may be used under the terms
   of the GNU General Public License version 2 (the "GPL") as distributed
   in the kernel source COPYING file, in which case the provisions of
   the GPL are applicable instead of the above.  If you wish to allow
   the use of your version of this file only under the terms of the
   GPL and not to allow others to use your version of this file under
   the OSL, indicate your decision by deleting the provisions above and
   replace them with the notice and other provisions required by the GPL.
   If you do not delete the provisions above, a recipient may use your
   version of this file under either the OSL or the GPL.
   </para>

  </legalnotice>
 </bookinfo>

<toc></toc>

  <chapter id="libataIntroduction">
     <title>Introduction</title>
  <para>
  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&lt;-&gt;ATA translation
  for ATA devices according to the T10 SAT specification.
  </para>
  <para>
  This Guide documents the libATA driver API, library functions, library
  internals, and a couple sample ATA low-level drivers.
  </para>
  </chapter>

  <chapter id="libataDriverApi">
     <title>libata Driver API</title>
     <para>
     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.
     </para>
     <para>
     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.
     </para>
     <sect1>
        <title>struct ata_port_operations</title>

	<sect2><title>Disable ATA port</title>
	<programlisting>
void (*port_disable) (struct ata_port *);
	</programlisting>

	<para>
	Called from ata_bus_probe() and ata_bus_reset() error paths,
	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.
	</para>
	<para>
	Called from ata_bus_probe() on a failed probe.
	Called from ata_bus_reset() on a failed bus reset.
	Called from ata_scsi_release().
	</para>

	</sect2>

	<sect2><title>Post-IDENTIFY device configuration</title>
	<programlisting>
void (*dev_config) (struct ata_port *, struct ata_device *);
	</programlisting>

	<para>
	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.
	</para>
	<para>
	Called by ata_device_add() after ata_dev_identify() determines
	a device is present.
	</para>
	<para>
	This entry may be specified as NULL in ata_port_operations.
	</para>

	</sect2>

	<sect2><title>Set PIO/DMA mode</title>
	<programlisting>
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);
	</programlisting>

	<para>
	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.
	</para>
	<para>
	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.
	</para>
	<para>
	->post_set_mode() is
	called unconditionally, after the SET FEATURES - XFER MODE
	command completes successfully.
	</para>

	<para>
	->set_piomode() is always called (if present), but
	->set_dma_mode() is only called if DMA is possible.
	</para>

	</sect2>

	<sect2><title>Taskfile read/write</title>
	<programlisting>
void (*tf_load) (struct ata_port *ap, struct ata_taskfile *tf);
void (*tf_read) (struct ata_port *ap, struct ata_taskfile *tf);
	</programlisting>

	<para>
	->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_tf_load() and ata_tf_read() for these hooks.
	</para>

	</sect2>

	<sect2><title>ATA command execute</title>
	<programlisting>
void (*exec_command)(struct ata_port *ap, struct ata_taskfile *tf);
	</programlisting>

	<para>
	causes an ATA command, previously loaded with
	->tf_load(), to be initiated in hardware.
	Most drivers for taskfile-based hardware use ata_exec_command()
	for this hook.
	</para>

	</sect2>

	<sect2><title>Per-cmd ATAPI DMA capabilities filter</title>
	<programlisting>
int (*check_atapi_dma) (struct ata_queued_cmd *qc);
	</programlisting>

	<para>
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.
	</para>
	<para>
	This hook may be specified as NULL, in which case libata will
	assume that atapi dma can be supported.
	</para>

	</sect2>

	<sect2><title>Read specific ATA shadow registers</title>
	<programlisting>
u8   (*check_status)(struct ata_port *ap);
u8   (*check_altstatus)(struct ata_port *ap);
u8   (*check_err)(struct ata_port *ap);
	</programlisting>

	<para>
	Reads the Status/AltStatus/Error 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_check_status() for this hook.
	</para>
	<para>
	Note that because this is called from ata_device_add(), at
	least a dummy function that clears device interrupts must be
	provided for all drivers, even if the controller doesn't
	actually have a taskfile status register.
	</para>

	</sect2>

	<sect2><title>Select ATA device on bus</title>
	<programlisting>
void (*dev_select)(struct ata_port *ap, unsigned int device);
	</programlisting>

	<para>
	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.
	</para>
	<para>
	Most drivers for taskfile-based hardware use
	ata_std_dev_select() for this hook.  Controllers which do not
	support second drives on a port (such as SATA contollers) will
	use ata_noop_dev_select().
	</para>

	</sect2>

	<sect2><title>Private tuning method</title>
	<programlisting>
void (*set_mode) (struct ata_port *ap);
	</programlisting>

	<para>
	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.
	</para>

	<warning>
	<para>
	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.
	</para>
	</warning>

	</sect2>

	<sect2><title>Reset ATA bus</title>
	<programlisting>
void (*phy_reset) (struct ata_port *ap);
	</programlisting>

	<para>
	The very first step in the probe phase.  Actions vary depending
	on the bus type, typically.  After waking up the device and probing
	for device presence (PATA and SATA), typically a soft reset
	(SRST) will be performed.  Drivers typically use the helper
	functions ata_bus_reset() or sata_phy_reset() for this hook.
	Many SATA drivers use sata_phy_reset() or call it from within
	their own phy_reset() functions.
	</para>

	</sect2>

	<sect2><title>Control PCI IDE BMDMA engine</title>
	<programlisting>
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);
	</programlisting>

	<para>
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.
	</para>

	<para>
These hooks are typically either no-ops, or simply not implemented, in
FIS-based drivers.
	</para>
	<para>
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.
	</para>
	<para>
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.
	</para>
	<para>
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.
	</para>
	<para>
Many legacy IDE drivers use ata_bmdma_status() as the bmdma_status() hook.
	</para>

	</sect2>

	<sect2><title>High-level taskfile hooks</title>
	<programlisting>
void (*qc_prep) (struct ata_queued_cmd *qc);
int (*qc_issue) (struct ata_queued_cmd *qc);
	</programlisting>

	<para>
	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.
	</para>
	<para>
	->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.
	</para>
	<para>
	ata_qc_issue_prot() calls ->tf_load(), ->bmdma_setup(), and
	->bmdma_start() as necessary to initiate a transfer.
	</para>

	</sect2>

	<sect2><title>Timeout (error) handling</title>
	<programlisting>
void (*eng_timeout) (struct ata_port *ap);
	</programlisting>

	<para>
This is a high level error handling function, called from the
error handling thread, when a command times out.  Most newer
hardware will implement its own error handling code here.  IDE BMDMA
drivers may use the helper function ata_eng_timeout().
	</para>

	</sect2>

	<sect2><title>Hardware interrupt handling</title>
	<programlisting>
irqreturn_t (*irq_handler)(int, void *, struct pt_regs *);
void (*irq_clear) (struct ata_port *);
	</programlisting>

	<para>
	->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.
	</para>
	<para>
	The second argument, dev_instance, should be cast to a pointer
	to struct ata_host_set.
	</para>
	<para>
	Most legacy IDE drivers use ata_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_host_intr(ap,qc).
	</para>
	<para>
	Most legacy IDE drivers use ata_bmdma_irq_clear() for the
	irq_clear() hook, which simply clears the interrupt and error
	flags in the DMA status register.
	</para>

	</sect2>

	<sect2><title>SATA phy read/write</title>
	<programlisting>
u32 (*scr_read) (struct ata_port *ap, unsigned int sc_reg);
void (*scr_write) (struct ata_port *ap, unsigned int sc_reg,
                   u32 val);
	</programlisting>

	<para>
	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.
	</para>

	</sect2>

	<sect2><title>Init and shutdown</title>
	<programlisting>
int (*port_start) (struct ata_port *ap);
void (*port_stop) (struct ata_port *ap);
void (*host_stop) (struct ata_host_set *host_set);
	</programlisting>

	<para>
	->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.
	</para>
	<para>
	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.
	</para>
	<para>
	->port_stop() is called after ->host_stop().  It's 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.
	</para>
	<para>
	Many drivers use ata_port_stop() as this hook, which frees the
	PRD table.
	</para>
	<para>
	->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.
	</para>

	</sect2>

     </sect1>
  </chapter>

  <chapter id="libataEH">
        <title>Error handling</title>

	<para>
	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.
	</para>

	<sect1><title>Origins of commands</title>
	<para>
	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.
	</para>
	<para>
	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.
	</para>
	</sect1>

	<sect1><title>How commands are issued</title>

	<variablelist>

	<varlistentry><term>Internal commands</term>
	<listitem>
	<para>
	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.
	</para>
	<para>
	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.
	</para>
	<para>
	Once initialization is complete, host_set lock is acquired
	and the qc is issued.
	</para>
	</listitem>
	</varlistentry>

	<varlistentry><term>SCSI commands</term>
	<listitem>
	<para>
	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.
	</para>
	<para>
	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.
	</para>
	<para>
	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().
	</para>
	<para>
	Note that SCSI midlayer invokes hostt->queuecommand while
	holding host_set lock, so all above occur while holding
	host_set lock.
	</para>
	</listitem>
	</varlistentry>

	</variablelist>
	</sect1>

	<sect1><title>How commands are processed</title>
	<para>
	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.
	</para>
	<para>
	Currently 6 ATA command protocols are used.  They can be
	sorted into the following four categories according to how
	they are processed.
	</para>

	<variablelist>
	   <varlistentry><term>ATA NO DATA or DMA</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>ATA PIO</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>ATAPI NODATA or DMA</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>ATAPI PIO</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>
	</variablelist>
        </sect1>

	<sect1><title>How commands are completed</title>
	<para>
	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.
	</para>
	<para>
	ata_qc_complete() does the following.
	</para>

	<orderedlist>

	<listitem>
	<para>
	DMA memory is unmapped.
	</para>
	</listitem>

	<listitem>
	<para>
	ATA_QCFLAG_ACTIVE is clared from qc->flags.
	</para>
	</listitem>

	<listitem>
	<para>
	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.
	</para>
	</listitem>

	<listitem>
	<para>
	__ata_qc_complete() is called, which does
	   <orderedlist>

	   <listitem>
	   <para>
	   qc->flags is cleared to zero.
	   </para>
	   </listitem>

	   <listitem>
	   <para>
	   ap->active_tag and qc->tag are poisoned.
	   </para>
	   </listitem>

	   <listitem>
	   <para>
	   qc->waiting is claread &amp; completed (in that order).
	   </para>
	   </listitem>

	   <listitem>
	   <para>
	   qc is deallocated by clearing appropriate bit in ap->qactive.
	   </para>
	   </listitem>

	   </orderedlist>
	</para>
	</listitem>

	</orderedlist>

	<para>
	So, it basically notifies upper layer and deallocates qc.  One
	exception is short-circuit path in #3 which is used by
	atapi_qc_complete().
	</para>
	<para>
	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.
	</para>
	<para>
	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.
	</para>
	<para>
	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.
	</para>

	</sect1>

	<sect1><title>ata_scsi_error()</title>
	<para>
	ata_scsi_error() is the current hostt->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().
	</para>
	<para>
	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().
	</para>
	<para>
	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.
	</para>
	<para>
	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.
	</para>

	</sect1>

	<sect1><title>Problems with the current EH</title>

	<itemizedlist>

	<listitem>
	<para>
	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.
	</para>
	</listitem>

	<listitem>
	<para>
	When handling timeouts, no action is taken to make device
	forget about the timed out command and ready for new commands.
	</para>
	</listitem>

	<listitem>
	<para>
	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.
	</para>
	</listitem>

	<listitem>
	<para>
	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.
	</para>
	</listitem>

	<listitem>
	<para>
	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.
	</para>
	<para>
	First, advanced error handling often requires context and
	internal qc execution.
	</para>
	<para>
	Second, even a simple failure (say, CRC error) needs
	information gathering and could trigger complex error handling
	(say, resetting &amp; reconfiguring).  Having multiple code
	paths to gather information, enter EH and trigger actions
	makes life painful.
	</para>
	<para>
	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.
	</para>
	</listitem>

	</itemizedlist>
	</sect1>
  </chapter>

  <chapter id="libataExt">
     <title>libata Library</title>
!Edrivers/scsi/libata-core.c
  </chapter>

  <chapter id="libataInt">
     <title>libata Core Internals</title>
!Idrivers/scsi/libata-core.c
  </chapter>

  <chapter id="libataScsiInt">
     <title>libata SCSI translation/emulation</title>
!Edrivers/scsi/libata-scsi.c
!Idrivers/scsi/libata-scsi.c
  </chapter>

  <chapter id="ataExceptions">
     <title>ATA errors &amp; exceptions</title>

  <para>
  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.
  </para>

  <para>
  The term 'error' is used to describe conditions where either an
  explicit error condition is reported from device or a command has
  timed out.
  </para>

  <para>
  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.
  </para>

  <sect1 id="excat">
     <title>Exception categories</title>
     <para>
     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.
     </para>

     <para>
     In the following sections, two recovery actions - reset and
     reconfiguring transport - are mentioned.  These are described
     further in <xref linkend="exrec"/>.
     </para>

     <sect2 id="excatHSMviolation">
        <title>HSM violation</title>
        <para>
        This error is indicated when STATUS value doesn't match HSM
        requirement during issuing or excution any ATA/ATAPI command.
        </para>

	<itemizedlist>
	<title>Examples</title>

        <listitem>
	<para>
	ATA_STATUS doesn't contain !BSY &amp;&amp; DRDY &amp;&amp; !DRQ while trying
	to issue a command.
        </para>
	</listitem>

        <listitem>
	<para>
	!BSY &amp;&amp; !DRQ during PIO data transfer.
        </para>
	</listitem>

        <listitem>
	<para>
	DRQ on command completion.
        </para>
	</listitem>

        <listitem>
	<para>
	!BSY &amp;&amp; ERR after CDB tranfer starts but before the
        last byte of CDB is transferred.  ATA/ATAPI standard states
        that &quot;The device shall not terminate the PACKET command
        with an error before the last byte of the command packet has
        been written&quot; in the error outputs description of PACKET
        command and the state diagram doesn't include such
        transitions.
	</para>
	</listitem>

	</itemizedlist>

	<para>
	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.
	</para>

	<para>
	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.
	</para>
     </sect2>
     
     <sect2 id="excatDevErr">
        <title>ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION)</title>

	<para>
	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.
	</para>

	<para>
	For ATA commands, this type of errors are indicated by !BSY
	&amp;&amp; ERR during command execution and on completion.
	</para>

	<para>For ATAPI commands,</para>

	<itemizedlist>

	<listitem>
	<para>
	!BSY &amp;&amp; ERR &amp;&amp; ABRT right after issuing PACKET
	indicates that PACKET command is not supported and falls in
	this category.
	</para>
	</listitem>

	<listitem>
	<para>
	!BSY &amp;&amp; ERR(==CHK) &amp;&amp; !ABRT after the last
	byte of CDB is transferred indicates CHECK CONDITION and
	doesn't fall in this category.
	</para>
	</listitem>

	<listitem>
	<para>
	!BSY &amp;&amp; ERR(==CHK) &amp;&amp; ABRT after the last byte
        of CDB is transferred *probably* indicates CHECK CONDITION and
        doesn't fall in this category.
	</para>
	</listitem>

	</itemizedlist>

	<para>
	Of errors detected as above, the followings are not ATA/ATAPI
	device errors but ATA bus errors and should be handled
	according to <xref linkend="excatATAbusErr"/>.
	</para>

	<variablelist>

	   <varlistentry>
	   <term>CRC error during data transfer</term>
	   <listitem>
	   <para>
	   This is indicated by ICRC bit in the ERROR register and
	   means that corruption occurred during data transfer.  Upto
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry>
	   <term>ABRT error during data transfer or on completion</term>
	   <listitem>
	   <para>
	   Upto 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 upto
	   ATA/ATAPI-7, it seems to imply that ABRT bit alone could
	   indicate tranfer errors.
	   </para>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	</variablelist>

	<para>
	ATA/ATAPI device errors can be further categorized as follows.
	</para>

	<variablelist>

	   <varlistentry>
	   <term>Media errors</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry>
	   <term>Media changed / media change requested error</term>
	   <listitem>
	   <para>
	   &lt;&lt;TODO: fill here&gt;&gt;
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>Address error</term>
	   <listitem>
	   <para>
	   This is indicated by IDNF bit in the ERROR register.
	   Report to upper layer.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>Other errors</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	</variablelist>

	<para>
	Depending on commands, not all STATUS/ERROR bits are
	applicable.  These non-applicable bits are marked with
	&quot;na&quot; in the output descriptions but upto ATA/ATAPI-7
	no definition of &quot;na&quot; can be found.  However,
	ATA/ATAPI-8 draft revision 1f describes &quot;N/A&quot; as
	follows.
	</para>

	<blockquote>
	<variablelist>
	   <varlistentry><term>3.2.3.3a N/A</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>
	</variablelist>
	</blockquote>

	<para>
	So, it seems reasonable to assume that &quot;na&quot; bits are
	cleared to zero by devices and thus need no explicit masking.
	</para>

     </sect2>

     <sect2 id="excatATAPIcc">
        <title>ATAPI device CHECK CONDITION</title>

	<para>
	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.
	</para>

	<para>
	Once sense data is acquired, this type of errors can be
	handled similary to other SCSI errors.  Note that sense data
	may indicate ATA bus error (e.g. Sense Key 04h HARDWARE ERROR
	&amp;&amp; ASC/ASCQ 47h/00h SCSI PARITY ERROR).  In such
	cases, the error should be considered as an ATA bus error and
	handled according to <xref linkend="excatATAbusErr"/>.
	</para>

     </sect2>

     <sect2 id="excatNCQerr">
        <title>ATA device error (NCQ)</title>

	<para>
	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.
	</para>

	<para>
	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 <xref linkend="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.
	</para>

	<para>
	Note that ATA bus errors can be reported as ATA device NCQ
	errors.  This should be handled as described in <xref
	linkend="excatATAbusErr"/>.
	</para>

	<para>
	If READ LOG EXT Log Page 10h fails or reports NQ, we're
	thoroughly screwed.  This condition should be treated
	according to <xref linkend="excatHSMviolation"/>.
	</para>

     </sect2>

     <sect2 id="excatATAbusErr">
        <title>ATA bus error</title>

	<para>
	ATA bus error means that data corruption occurred during
	transmission over ATA bus (SATA or PATA).  This type of errors
	can be indicated by
	</para>

	<itemizedlist>

	<listitem>
	<para>
	ICRC or ABRT error as described in <xref linkend="excatDevErr"/>.
	</para>
	</listitem>

	<listitem>
	<para>
	Controller-specific error completion with error information
	indicating transmission error.
	</para>
	</listitem>

	<listitem>
	<para>
	On some controllers, command timeout.  In this case, there may
	be a mechanism to determine that the timeout is due to
	transmission error.
	</para>
	</listitem>

	<listitem>
	<para>
	Unknown/random errors, timeouts and all sorts of weirdities.
	</para>
	</listitem>

	</itemizedlist>

	<para>
	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.
	</para>

	<para>
	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 <xref
	linkend="exrecReconf"/> for more information.
	</para>

     </sect2>

     <sect2 id="excatPCIbusErr">
        <title>PCI bus error</title>

	<para>
	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.
	</para>

     </sect2>

     <sect2 id="excatLateCompletion">
        <title>Late completion</title>

	<para>
	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.
	</para>

     </sect2>

     <sect2 id="excatUnknown">
        <title>Unknown error (timeout)</title>

	<para>
	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.
	</para>

	<para>
	Timeouts can also be caused by transmission errors.  Refer to
	<xref linkend="excatATAbusErr"/> for more details.
	</para>

     </sect2>

     <sect2 id="excatHoplugPM">
        <title>Hotplug and power management exceptions</title>

	<para>
	&lt;&lt;TODO: fill here&gt;&gt;
	</para>

     </sect2>

  </sect1>

  <sect1 id="exrec">
     <title>EH recovery actions</title>

     <para>
     This section discusses several important recovery actions.
     </para>

     <sect2 id="exrecClr">
        <title>Clearing error condition</title>

	<para>
	Many controllers require its error registers to be cleared by
	error handler.  Different controllers may have different
	requirements.
	</para>

	<para>
	For SATA, it's strongly recommended to clear at least SError
	register during error handling.
	</para>
     </sect2>

     <sect2 id="exrecRst">
        <title>Reset</title>

	<para>
	During EH, resetting is necessary in the following cases.
	</para>

	<itemizedlist>

	<listitem>
	<para>
	HSM is in unknown or invalid state
	</para>
	</listitem>

	<listitem>
	<para>
	HBA is in unknown or invalid state
	</para>
	</listitem>

	<listitem>
	<para>
	EH needs to make HBA/device forget about in-flight commands
	</para>
	</listitem>

	<listitem>
	<para>
	HBA/device behaves weirdly
	</para>
	</listitem>

	</itemizedlist>

	<para>
	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.
	</para>

	<itemizedlist>

	<listitem>
	<para>
	When it's known that HBA is in ready state but ATA/ATAPI
	device in in unknown state, reset only device.
	</para>
	</listitem>

	<listitem>
	<para>
	If HBA is in unknown state, reset both HBA and device.
	</para>
	</listitem>

	</itemizedlist>

	<para>
	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.
	</para>

	<para>
	OTOH, ATA/ATAPI standard describes in detail ways to reset
	ATA/ATAPI devices.
	</para>

	<variablelist>

	   <varlistentry><term>PATA hardware reset</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>Software reset</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>EXECUTE DEVICE DIAGNOSTIC command</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>ATAPI DEVICE RESET command</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	   <varlistentry><term>SATA phy reset</term>
	   <listitem>
	   <para>
	   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.
	   </para>
	   </listitem>
	   </varlistentry>

	</variablelist>

	<para>
	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.
	</para>

	<para>
	Parameters affected are.
	</para>

	<itemizedlist>

	<listitem>
	<para>
	CHS set up with INITIALIZE DEVICE PARAMETERS (seldomly used)
	</para>
	</listitem>

	<listitem>
	<para>
	Parameters set with SET FEATURES including transfer mode setting
	</para>
	</listitem>

	<listitem>
	<para>
	Block count set with SET MULTIPLE MODE
	</para>
	</listitem>

	<listitem>
	<para>
	Other parameters (SET MAX, MEDIA LOCK...)
	</para>
	</listitem>

	</itemizedlist>

	<para>
	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).
	</para>

	<para>
	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.
	</para>

     </sect2>

     <sect2 id="exrecReconf">
        <title>Reconfigure transport</title>

	<para>
	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.
	</para>

	<para>
	The following is a possible scheme Jeff Garzik suggested.
	</para>

	<blockquote>
	<para>
	If more than $N (3?) transmission errors happen in 15 minutes,
	</para>	
	<itemizedlist>
	<listitem>
	<para>
	if SATA, decrease SATA PHY speed.  if speed cannot be decreased,
	</para>
	</listitem>
	<listitem>
	<para>
	decrease UDMA xfer speed.  if at UDMA0, switch to PIO4,
	</para>
	</listitem>
	<listitem>
	<para>
	decrease PIO xfer speed.  if at PIO3, complain, but continue
	</para>
	</listitem>
	</itemizedlist>
	</blockquote>

     </sect2>

  </sect1>

  </chapter>

  <chapter id="PiixInt">
     <title>ata_piix Internals</title>
!Idrivers/scsi/ata_piix.c
  </chapter>

  <chapter id="SILInt">
     <title>sata_sil Internals</title>
!Idrivers/scsi/sata_sil.c
  </chapter>

  <chapter id="libataThanks">
     <title>Thanks</title>
  <para>
  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.
  </para>
  <para>
  Thanks to Alan Cox for pointing out similarities 
  between SATA and SCSI, and in general for motivation to hack on
  libata.
  </para>
  <para>
  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).
  </para>
  </chapter>

</book>