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[PATCH] libata EH document update 

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>
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Tejun Heo 2005-09-26 11:28:47 +09:00 committed by Jeff Garzik
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@ -413,6 +413,362 @@ and other resources, etc.
</sect2>
</sect1>
<sect1>
<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>
<sect2><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>
</sect2>
<sect2><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>
</sect2>
<sect2><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>
</sect2>
<sect2><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>
</sect2>
<sect2><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>
</sect2>
<sect2><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>
</sect2>
</sect1>
</chapter>
<chapter id="libataExt">