<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="libataDevGuide">
<bookinfo>
<title>libATA Developer's Guide</title>
<authorgroup>
<author>
<firstname>Jeff</firstname>
<surname>Garzik</surname>
</author>
</authorgroup>
<copyright>
<year>2003-2006</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<->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() 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.
</para>
<para>
Called from ata_bus_probe() on a failed probe.
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>
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 (*sff_tf_load) (struct ata_port *ap, struct ata_taskfile *tf);
void (*sff_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_sff_tf_load() and ata_sff_tf_read() for these hooks.
</para>
</sect2>
<sect2><title>PIO data read/write</title>
<programlisting>
void (*sff_data_xfer) (struct ata_device *, unsigned char *, unsigned int, int);
</programlisting>
<para>
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().
</para>
</sect2>
<sect2><title>ATA command execute</title>
<programlisting>
void (*sff_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_sff_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 (*sff_check_status)(struct ata_port *ap);
u8 (*sff_check_altstatus)(struct ata_port *ap);
</programlisting>
<para>
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.
</para>
</sect2>
<sect2><title>Write specific ATA shadow register</title>
<programlisting>
void (*sff_set_devctl)(struct ata_port *ap, u8 ctl);
</programlisting>
<para>
Write the device control ATA shadow register to the hardware.
Most drivers don't need to define this.
</para>
</sect2>
<sect2><title>Select ATA device on bus</title>
<programlisting>
void (*sff_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_sff_dev_select() for this hook.
</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>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>Exception and probe handling (EH)</title>
<programlisting>
void (*eng_timeout) (struct ata_port *ap);
void (*phy_reset) (struct ata_port *ap);
</programlisting>
<para>
Deprecated. Use ->error_handler() instead.
</para>
<programlisting>
void (*freeze) (struct ata_port *ap);
void (*thaw) (struct ata_port *ap);
</programlisting>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<programlisting>
void (*error_handler) (struct ata_port *ap);
</programlisting>
<para>
->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:
</para>
<para>
'prereset' hook (may be NULL) is called during an EH reset, before any other actions
are taken.
</para>
<para>
'postreset' hook (may be NULL) is called after the EH reset is performed. Based on
existing conditions, severity of the problem, and hardware capabilities,
</para>
<para>
Either 'softreset' (may be NULL) or 'hardreset' (may be NULL) will be
called to perform the low-level EH reset.
</para>
<programlisting>
void (*post_internal_cmd) (struct ata_queued_cmd *qc);
</programlisting>
<para>
Perform any hardware-specific actions necessary to finish processing
after executing a probe-time or EH-time command via ata_exec_internal().
</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_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).
</para>
<para>
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.
</para>
</sect2>
<sect2><title>SATA phy read/write</title>
<programlisting>
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);
</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(). 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.
</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 & 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 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().
</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 & 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/ata/libata-core.c
</chapter>
<chapter id="libataInt">
<title>libata Core Internals</title>
!Idrivers/ata/libata-core.c
</chapter>
<chapter id="libataScsiInt">
<title>libata SCSI translation/emulation</title>
!Edrivers/ata/libata-scsi.c
!Idrivers/ata/libata-scsi.c
</chapter>
<chapter id="ataExceptions">
<title>ATA errors and 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 && DRDY && !DRQ while trying
to issue a command.
</para>
</listitem>
<listitem>
<para>
!BSY && !DRQ during PIO data transfer.
</para>
</listitem>
<listitem>
<para>
DRQ on command completion.
</para>
</listitem>
<listitem>
<para>
!BSY && ERR after CDB tranfer 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.
</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
&& ERR during command execution and on completion.
</para>
<para>For ATAPI commands,</para>
<itemizedlist>
<listitem>
<para>
!BSY && ERR && ABRT right after issuing PACKET
indicates that PACKET command is not supported and falls in
this category.
</para>
</listitem>
<listitem>
<para>
!BSY && ERR(==CHK) && !ABRT after the last
byte of CDB is transferred indicates CHECK CONDITION and
doesn't fall in this category.
</para>
</listitem>
<listitem>
<para>
!BSY && ERR(==CHK) && 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. 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ABRT error during data transfer or on completion</term>
<listitem>
<para>
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 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>
<<TODO: fill here>>
</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
"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.
</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 "na" 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
&& 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>
<<TODO: fill here>>
</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 is 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 (seldom 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/ata/ata_piix.c
</chapter>
<chapter id="SILInt">
<title>sata_sil Internals</title>
!Idrivers/ata/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>
