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diff --git a/Documentation/DocBook/libata.tmpl b/Documentation/DocBook/libata.tmpl index 375ae760dc1..fcbb58fc0fe 100644 --- a/Documentation/DocBook/libata.tmpl +++ b/Documentation/DocBook/libata.tmpl @@ -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 & 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 & 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"> |