linux - Documentation/driver-api/libata.rst

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1	========================
2	libATA Developer's Guide
3	========================
4
5	:Author: Jeff Garzik
6
7	Introduction
8	============
9
10	libATA is a library used inside the Linux kernel to support ATA host
11	controllers and devices. libATA provides an ATA driver API, class
12	transports for ATA and ATAPI devices, and SCSI<->ATA translation for ATA
13	devices according to the T10 SAT specification.
14
15	This Guide documents the libATA driver API, library functions, library
16	internals, and a couple sample ATA low-level drivers.
17
18	libata Driver API
19	=================
20
21	:c:type:`struct ata_port_operations <ata_port_operations>`
22	is defined for every low-level libata
23	hardware driver, and it controls how the low-level driver interfaces
24	with the ATA and SCSI layers.
25
26	FIS-based drivers will hook into the system with ``->qc_prep()`` and
27	``->qc_issue()`` high-level hooks. Hardware which behaves in a manner
28	similar to PCI IDE hardware may utilize several generic helpers,
29	defining at a bare minimum the bus I/O addresses of the ATA shadow
30	register blocks.
31
32	:c:type:`struct ata_port_operations <ata_port_operations>`
33	----------------------------------------------------------
34
35	Post-IDENTIFY device configuration
36	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
37
38	::
39
40	void (dev_config) (struct ata_port , struct ata_device *);
41
42
43	Called after IDENTIFY [PACKET] DEVICE is issued to each device found.
44	Typically used to apply device-specific fixups prior to issue of SET
45	FEATURES - XFER MODE, and prior to operation.
46
47	This entry may be specified as NULL in ata_port_operations.
48
49	Set PIO/DMA mode
50	~~~~~~~~~~~~~~~~
51
52	::
53
54	void (set_piomode) (struct ata_port , struct ata_device *);
55	void (set_dmamode) (struct ata_port , struct ata_device *);
56	void (post_set_mode) (struct ata_port );
57	unsigned int (mode_filter) (struct ata_port , struct ata_device *, unsigned int);
58
59
60	Hooks called prior to the issue of SET FEATURES - XFER MODE command. The
61	optional ``->mode_filter()`` hook is called when libata has built a mask of
62	the possible modes. This is passed to the ``->mode_filter()`` function
63	which should return a mask of valid modes after filtering those
64	unsuitable due to hardware limits. It is not valid to use this interface
65	to add modes.
66
67	``dev->pio_mode`` and ``dev->dma_mode`` are guaranteed to be valid when
68	``->set_piomode()`` and when ``->set_dmamode()`` is called. The timings for
69	any other drive sharing the cable will also be valid at this point. That
70	is the library records the decisions for the modes of each drive on a
71	channel before it attempts to set any of them.
72
73	``->post_set_mode()`` is called unconditionally, after the SET FEATURES -
74	XFER MODE command completes successfully.
75
76	``->set_piomode()`` is always called (if present), but ``->set_dma_mode()``
77	is only called if DMA is possible.
78
79	Taskfile read/write
80	~~~~~~~~~~~~~~~~~~~
81
82	::
83
84	void (sff_tf_load) (struct ata_port ap, struct ata_taskfile *tf);
85	void (sff_tf_read) (struct ata_port ap, struct ata_taskfile *tf);
86
87
88	``->tf_load()`` is called to load the given taskfile into hardware
89	registers / DMA buffers. ``->tf_read()`` is called to read the hardware
90	registers / DMA buffers, to obtain the current set of taskfile register
91	values. Most drivers for taskfile-based hardware (PIO or MMIO) use
92	:c:func:`ata_sff_tf_load` and :c:func:`ata_sff_tf_read` for these hooks.
93
94	PIO data read/write
95	~~~~~~~~~~~~~~~~~~~
96
97	::
98
99	void (sff_data_xfer) (struct ata_device , unsigned char *, unsigned int, int);
100
101
102	All bmdma-style drivers must implement this hook. This is the low-level
103	operation that actually copies the data bytes during a PIO data
104	transfer. Typically the driver will choose one of
105	:c:func:`ata_sff_data_xfer`, or :c:func:`ata_sff_data_xfer32`.
106
107	ATA command execute
108	~~~~~~~~~~~~~~~~~~~
109
110	::
111
112	void (sff_exec_command)(struct ata_port ap, struct ata_taskfile *tf);
113
114
115	causes an ATA command, previously loaded with ``->tf_load()``, to be
116	initiated in hardware. Most drivers for taskfile-based hardware use
117	:c:func:`ata_sff_exec_command` for this hook.
118
119	Per-cmd ATAPI DMA capabilities filter
120	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
121
122	::
123
124	int (check_atapi_dma) (struct ata_queued_cmd qc);
125
126
127	Allow low-level driver to filter ATA PACKET commands, returning a status
128	indicating whether or not it is OK to use DMA for the supplied PACKET
129	command.
130
131	This hook may be specified as NULL, in which case libata will assume
132	that atapi dma can be supported.
133
134	Read specific ATA shadow registers
135	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
136
137	::
138
139	u8 (sff_check_status)(struct ata_port ap);
140	u8 (sff_check_altstatus)(struct ata_port ap);
141
142
143	Reads the Status/AltStatus ATA shadow register from hardware. On some
144	hardware, reading the Status register has the side effect of clearing
145	the interrupt condition. Most drivers for taskfile-based hardware use
146	:c:func:`ata_sff_check_status` for this hook.
147
148	Write specific ATA shadow register
149	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150
151	::
152
153	void (sff_set_devctl)(struct ata_port ap, u8 ctl);
154
155
156	Write the device control ATA shadow register to the hardware. Most
157	drivers don't need to define this.
158
159	Select ATA device on bus
160	~~~~~~~~~~~~~~~~~~~~~~~~
161
162	::
163
164	void (sff_dev_select)(struct ata_port ap, unsigned int device);
165
166
167	Issues the low-level hardware command(s) that causes one of N hardware
168	devices to be considered 'selected' (active and available for use) on
169	the ATA bus. This generally has no meaning on FIS-based devices.
170
171	Most drivers for taskfile-based hardware use :c:func:`ata_sff_dev_select` for
172	this hook.
173
174	Private tuning method
175	~~~~~~~~~~~~~~~~~~~~~
176
177	::
178
179	void (set_mode) (struct ata_port ap);
180
181
182	By default libata performs drive and controller tuning in accordance
183	with the ATA timing rules and also applies blacklists and cable limits.
184	Some controllers need special handling and have custom tuning rules,
185	typically raid controllers that use ATA commands but do not actually do
186	drive timing.
187
188	Warning
189
190	This hook should not be used to replace the standard controller
191	tuning logic when a controller has quirks. Replacing the default
192	tuning logic in that case would bypass handling for drive and bridge
193	quirks that may be important to data reliability. If a controller
194	needs to filter the mode selection it should use the mode_filter
195	hook instead.
196
197	Control PCI IDE BMDMA engine
198	~~~~~~~~~~~~~~~~~~~~~~~~~~~~
199
200	::
201
202	void (bmdma_setup) (struct ata_queued_cmd qc);
203	void (bmdma_start) (struct ata_queued_cmd qc);
204	void (bmdma_stop) (struct ata_port ap);
205	u8 (bmdma_status) (struct ata_port ap);
206
207
208	When setting up an IDE BMDMA transaction, these hooks arm
209	(``->bmdma_setup``), fire (``->bmdma_start``), and halt (``->bmdma_stop``) the
210	hardware's DMA engine. ``->bmdma_status`` is used to read the standard PCI
211	IDE DMA Status register.
212
213	These hooks are typically either no-ops, or simply not implemented, in
214	FIS-based drivers.
215
216	Most legacy IDE drivers use :c:func:`ata_bmdma_setup` for the
217	:c:func:`bmdma_setup` hook. :c:func:`ata_bmdma_setup` will write the pointer
218	to the PRD table to the IDE PRD Table Address register, enable DMA in the DMA
219	Command register, and call :c:func:`exec_command` to begin the transfer.
220
221	Most legacy IDE drivers use :c:func:`ata_bmdma_start` for the
222	:c:func:`bmdma_start` hook. :c:func:`ata_bmdma_start` will write the
223	ATA_DMA_START flag to the DMA Command register.
224
225	Many legacy IDE drivers use :c:func:`ata_bmdma_stop` for the
226	:c:func:`bmdma_stop` hook. :c:func:`ata_bmdma_stop` clears the ATA_DMA_START
227	flag in the DMA command register.
228
229	Many legacy IDE drivers use :c:func:`ata_bmdma_status` as the
230	:c:func:`bmdma_status` hook.
231
232	High-level taskfile hooks
233	~~~~~~~~~~~~~~~~~~~~~~~~~
234
235	::
236
237	enum ata_completion_errors (qc_prep) (struct ata_queued_cmd qc);
238	int (qc_issue) (struct ata_queued_cmd qc);
239
240
241	Higher-level hooks, these two hooks can potentially supersede several of
242	the above taskfile/DMA engine hooks. ``->qc_prep`` is called after the
243	buffers have been DMA-mapped, and is typically used to populate the
244	hardware's DMA scatter-gather table. Some drivers use the standard
245	:c:func:`ata_bmdma_qc_prep` and :c:func:`ata_bmdma_dumb_qc_prep` helper
246	functions, but more advanced drivers roll their own.
247
248	``->qc_issue`` is used to make a command active, once the hardware and S/G
249	tables have been prepared. IDE BMDMA drivers use the helper function
250	:c:func:`ata_sff_qc_issue` for taskfile protocol-based dispatch. More
251	advanced drivers implement their own ``->qc_issue``.
252
253	:c:func:`ata_sff_qc_issue` calls ``->sff_tf_load()``, ``->bmdma_setup()``, and
254	``->bmdma_start()`` as necessary to initiate a transfer.
255
256	Exception and probe handling (EH)
257	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
258
259	::
260
261	void (freeze) (struct ata_port ap);
262	void (thaw) (struct ata_port ap);
263
264
265	:c:func:`ata_port_freeze` is called when HSM violations or some other
266	condition disrupts normal operation of the port. A frozen port is not
267	allowed to perform any operation until the port is thawed, which usually
268	follows a successful reset.
269
270	The optional ``->freeze()`` callback can be used for freezing the port
271	hardware-wise (e.g. mask interrupt and stop DMA engine). If a port
272	cannot be frozen hardware-wise, the interrupt handler must ack and clear
273	interrupts unconditionally while the port is frozen.
274
275	The optional ``->thaw()`` callback is called to perform the opposite of
276	``->freeze()``: prepare the port for normal operation once again. Unmask
277	interrupts, start DMA engine, etc.
278
279	::
280
281	void (error_handler) (struct ata_port ap);
282
283
284	``->error_handler()`` is a driver's hook into probe, hotplug, and recovery
285	and other exceptional conditions. The primary responsibility of an
286	implementation is to call :c:func:`ata_std_error_handler`.
287
288	:c:func:`ata_std_error_handler` will perform a standard error handling sequence
289	to resurrect failed devices, detach lost devices and add new devices (if any).
290	This function will call the various reset operations for a port, as needed.
291	These operations are as follows.
292
293	* The 'prereset' operation (which may be NULL) is called during an EH reset,
294	before any other action is taken.
295
296	* The 'postreset' hook (which may be NULL) is called after the EH reset is
297	performed. Based on existing conditions, severity of the problem, and hardware
298	capabilities,
299
300	* Either the 'softreset' operation or the 'hardreset' operation will be called
301	to perform the low-level EH reset. If both operations are defined,
302	'hardreset' is preferred and used. If both are not defined, no low-level reset
303	is performed and EH assumes that an ATA class device is connected through the
304	link.
305
306	::
307
308	void (post_internal_cmd) (struct ata_queued_cmd qc);
309
310
311	Perform any hardware-specific actions necessary to finish processing
312	after executing a probe-time or EH-time command via
313	:c:func:`ata_exec_internal`.
314
315	Hardware interrupt handling
316	~~~~~~~~~~~~~~~~~~~~~~~~~~~
317
318	::
319
320	irqreturn_t (irq_handler)(int, void , struct pt_regs *);
321	void (irq_clear) (struct ata_port );
322
323
324	``->irq_handler`` is the interrupt handling routine registered with the
325	system, by libata. ``->irq_clear`` is called during probe just before the
326	interrupt handler is registered, to be sure hardware is quiet.
327
328	The second argument, dev_instance, should be cast to a pointer to
329	:c:type:`struct ata_host_set <ata_host_set>`.
330
331	Most legacy IDE drivers use :c:func:`ata_sff_interrupt` for the irq_handler
332	hook, which scans all ports in the host_set, determines which queued
333	command was active (if any), and calls ata_sff_host_intr(ap,qc).
334
335	Most legacy IDE drivers use :c:func:`ata_sff_irq_clear` for the
336	:c:func:`irq_clear` hook, which simply clears the interrupt and error flags
337	in the DMA status register.
338
339	SATA phy read/write
340	~~~~~~~~~~~~~~~~~~~
341
342	::
343
344	int (scr_read) (struct ata_port ap, unsigned int sc_reg,
345	u32 *val);
346	int (scr_write) (struct ata_port ap, unsigned int sc_reg,
347	u32 val);
348
349
350	Read and write standard SATA phy registers.
351	sc_reg is one of SCR_STATUS, SCR_CONTROL, SCR_ERROR, or SCR_ACTIVE.
352
353	Init and shutdown
354	~~~~~~~~~~~~~~~~~
355
356	::
357
358	int (port_start) (struct ata_port ap);
359	void (port_stop) (struct ata_port ap);
360	void (host_stop) (struct ata_host_set host_set);
361
362
363	``->port_start()`` is called just after the data structures for each port
364	are initialized. Typically this is used to alloc per-port DMA buffers /
365	tables / rings, enable DMA engines, and similar tasks. Some drivers also
366	use this entry point as a chance to allocate driver-private memory for
367	``ap->private_data``.
368
369	Many drivers use :c:func:`ata_port_start` as this hook or call it from their
370	own :c:func:`port_start` hooks. :c:func:`ata_port_start` allocates space for
371	a legacy IDE PRD table and returns.
372
373	``->port_stop()`` is called after ``->host_stop()``. Its sole function is to
374	release DMA/memory resources, now that they are no longer actively being
375	used. Many drivers also free driver-private data from port at this time.
376
377	``->host_stop()`` is called after all ``->port_stop()`` calls have completed.
378	The hook must finalize hardware shutdown, release DMA and other
379	resources, etc. This hook may be specified as NULL, in which case it is
380	not called.
381
382	Error handling
383	==============
384
385	This chapter describes how errors are handled under libata. Readers are
386	advised to read SCSI EH (Documentation/scsi/scsi_eh.rst) and ATA
387	exceptions doc first.
388
389	Origins of commands
390	-------------------
391
392	In libata, a command is represented with
393	:c:type:`struct ata_queued_cmd <ata_queued_cmd>` or qc.
394	qc's are preallocated during port initialization and repetitively used
395	for command executions. Currently only one qc is allocated per port but
396	yet-to-be-merged NCQ branch allocates one for each tag and maps each qc
397	to NCQ tag 1-to-1.
398
399	libata commands can originate from two sources - libata itself and SCSI
400	midlayer. libata internal commands are used for initialization and error
401	handling. All normal blk requests and commands for SCSI emulation are
402	passed as SCSI commands through queuecommand callback of SCSI host
403	template.
404
405	How commands are issued
406	-----------------------
407
408	Internal commands
409	Once allocated qc's taskfile is initialized for the command to be
410	executed. qc currently has two mechanisms to notify completion. One
411	is via ``qc->complete_fn()`` callback and the other is completion
412	``qc->waiting``. ``qc->complete_fn()`` callback is the asynchronous path
413	used by normal SCSI translated commands and ``qc->waiting`` is the
414	synchronous (issuer sleeps in process context) path used by internal
415	commands.
416
417	Once initialization is complete, host_set lock is acquired and the
418	qc is issued.
419
420	SCSI commands
421	All libata drivers use :c:func:`ata_scsi_queuecmd` as
422	``hostt->queuecommand`` callback. scmds can either be simulated or
423	translated. No qc is involved in processing a simulated scmd. The
424	result is computed right away and the scmd is completed.
425
426	``qc->complete_fn()`` callback is used for completion notification. ATA
427	commands use :c:func:`ata_scsi_qc_complete` while ATAPI commands use
428	:c:func:`atapi_qc_complete`. Both functions end up calling ``qc->scsidone``
429	to notify upper layer when the qc is finished. After translation is
430	completed, the qc is issued with :c:func:`ata_qc_issue`.
431
432	Note that SCSI midlayer invokes hostt->queuecommand while holding
433	host_set lock, so all above occur while holding host_set lock.
434
435	How commands are processed
436	--------------------------
437
438	Depending on which protocol and which controller are used, commands are
439	processed differently. For the purpose of discussion, a controller which
440	uses taskfile interface and all standard callbacks is assumed.
441
442	Currently 6 ATA command protocols are used. They can be sorted into the
443	following four categories according to how they are processed.
444
445	ATA NO DATA or DMA
446	ATA_PROT_NODATA and ATA_PROT_DMA fall into this category. These
447	types of commands don't require any software intervention once
448	issued. Device will raise interrupt on completion.
449
450	ATA PIO
451	ATA_PROT_PIO is in this category. libata currently implements PIO
452	with polling. ATA_NIEN bit is set to turn off interrupt and
453	pio_task on ata_wq performs polling and IO.
454
455	ATAPI NODATA or DMA
456	ATA_PROT_ATAPI_NODATA and ATA_PROT_ATAPI_DMA are in this
457	category. packet_task is used to poll BSY bit after issuing PACKET
458	command. Once BSY is turned off by the device, packet_task
459	transfers CDB and hands off processing to interrupt handler.
460
461	ATAPI PIO
462	ATA_PROT_ATAPI is in this category. ATA_NIEN bit is set and, as
463	in ATAPI NODATA or DMA, packet_task submits cdb. However, after
464	submitting cdb, further processing (data transfer) is handed off to
465	pio_task.
466
467	How commands are completed
468	--------------------------
469
470	Once issued, all qc's are either completed with :c:func:`ata_qc_complete` or
471	time out. For commands which are handled by interrupts,
472	:c:func:`ata_host_intr` invokes :c:func:`ata_qc_complete`, and, for PIO tasks,
473	pio_task invokes :c:func:`ata_qc_complete`. In error cases, packet_task may
474	also complete commands.
475
476	:c:func:`ata_qc_complete` does the following.
477
478	1. DMA memory is unmapped.
479
480	2. ATA_QCFLAG_ACTIVE is cleared from qc->flags.
481
482	3. :c:expr:`qc->complete_fn` callback is invoked. If the return value of the
483	callback is not zero. Completion is short circuited and
484	:c:func:`ata_qc_complete` returns.
485
486	4. :c:func:`__ata_qc_complete` is called, which does
487
488	1. ``qc->flags`` is cleared to zero.
489
490	2. ``ap->active_tag`` and ``qc->tag`` are poisoned.
491
492	3. ``qc->waiting`` is cleared & completed (in that order).
493
494	4. qc is deallocated by clearing appropriate bit in ``ap->qactive``.
495
496	So, it basically notifies upper layer and deallocates qc. One exception
497	is short-circuit path in #3 which is used by :c:func:`atapi_qc_complete`.
498
499	For all non-ATAPI commands, whether it fails or not, almost the same
500	code path is taken and very little error handling takes place. A qc is
501	completed with success status if it succeeded, with failed status
502	otherwise.
503
504	However, failed ATAPI commands require more handling as REQUEST SENSE is
505	needed to acquire sense data. If an ATAPI command fails,
506	:c:func:`ata_qc_complete` is invoked with error status, which in turn invokes
507	:c:func:`atapi_qc_complete` via ``qc->complete_fn()`` callback.
508
509	This makes :c:func:`atapi_qc_complete` set ``scmd->result`` to
510	SAM_STAT_CHECK_CONDITION, complete the scmd and return 1. As the
511	sense data is empty but ``scmd->result`` is CHECK CONDITION, SCSI midlayer
512	will invoke EH for the scmd, and returning 1 makes :c:func:`ata_qc_complete`
513	to return without deallocating the qc. This leads us to
514	:c:func:`ata_scsi_error` with partially completed qc.
515
516	:c:func:`ata_scsi_error`
517	------------------------
518
519	:c:func:`ata_scsi_error` is the current ``transportt->eh_strategy_handler()``
520	for libata. As discussed above, this will be entered in two cases -
521	timeout and ATAPI error completion. This function will check if a qc is active
522	and has not failed yet. Such a qc will be marked with AC_ERR_TIMEOUT such that
523	EH will know to handle it later. Then it calls low level libata driver's
524	:c:func:`error_handler` callback.
525
526	When the :c:func:`error_handler` callback is invoked it stops BMDMA and
527	completes the qc. Note that as we're currently in EH, we cannot call
528	scsi_done. As described in SCSI EH doc, a recovered scmd should be
529	either retried with :c:func:`scsi_queue_insert` or finished with
530	:c:func:`scsi_finish_command`. Here, we override ``qc->scsidone`` with
531	:c:func:`scsi_finish_command` and calls :c:func:`ata_qc_complete`.
532
533	If EH is invoked due to a failed ATAPI qc, the qc here is completed but
534	not deallocated. The purpose of this half-completion is to use the qc as
535	place holder to make EH code reach this place. This is a bit hackish,
536	but it works.
537
538	Once control reaches here, the qc is deallocated by invoking
539	:c:func:`__ata_qc_complete` explicitly. Then, internal qc for REQUEST SENSE
540	is issued. Once sense data is acquired, scmd is finished by directly
541	invoking :c:func:`scsi_finish_command` on the scmd. Note that as we already
542	have completed and deallocated the qc which was associated with the
543	scmd, we don't need to/cannot call :c:func:`ata_qc_complete` again.
544
545	Problems with the current EH
546	----------------------------
547
548	- Error representation is too crude. Currently any and all error
549	conditions are represented with ATA STATUS and ERROR registers.
550	Errors which aren't ATA device errors are treated as ATA device
551	errors by setting ATA_ERR bit. Better error descriptor which can
552	properly represent ATA and other errors/exceptions is needed.
553
554	- When handling timeouts, no action is taken to make device forget
555	about the timed out command and ready for new commands.
556
557	- EH handling via :c:func:`ata_scsi_error` is not properly protected from
558	usual command processing. On EH entrance, the device is not in
559	quiescent state. Timed out commands may succeed or fail any time.
560	pio_task and atapi_task may still be running.
561
562	- Too weak error recovery. Devices / controllers causing HSM mismatch
563	errors and other errors quite often require reset to return to known
564	state. Also, advanced error handling is necessary to support features
565	like NCQ and hotplug.
566
567	- ATA errors are directly handled in the interrupt handler and PIO
568	errors in pio_task. This is problematic for advanced error handling
569	for the following reasons.
570
571	First, advanced error handling often requires context and internal qc
572	execution.
573
574	Second, even a simple failure (say, CRC error) needs information
575	gathering and could trigger complex error handling (say, resetting &
576	reconfiguring). Having multiple code paths to gather information,
577	enter EH and trigger actions makes life painful.
578
579	Third, scattered EH code makes implementing low level drivers
580	difficult. Low level drivers override libata callbacks. If EH is
581	scattered over several places, each affected callbacks should perform
582	its part of error handling. This can be error prone and painful.
583
584	libata Library
585	==============
586
587	.. kernel-doc:: drivers/ata/libata-core.c
588	:export:
589
590	libata Core Internals
591	=====================
592
593	.. kernel-doc:: drivers/ata/libata-core.c
594	:internal:
595
596	.. kernel-doc:: drivers/ata/libata-eh.c
597
598	libata SCSI translation/emulation
599	=================================
600
601	.. kernel-doc:: drivers/ata/libata-scsi.c
602	:export:
603
604	.. kernel-doc:: drivers/ata/libata-scsi.c
605	:internal:
606
607	ATA errors and exceptions
608	=========================
609
610	This chapter tries to identify what error/exception conditions exist for
611	ATA/ATAPI devices and describe how they should be handled in
612	implementation-neutral way.
613
614	The term 'error' is used to describe conditions where either an explicit
615	error condition is reported from device or a command has timed out.
616
617	The term 'exception' is either used to describe exceptional conditions
618	which are not errors (say, power or hotplug events), or to describe both
619	errors and non-error exceptional conditions. Where explicit distinction
620	between error and exception is necessary, the term 'non-error exception'
621	is used.
622
623	Exception categories
624	--------------------
625
626	Exceptions are described primarily with respect to legacy taskfile + bus
627	master IDE interface. If a controller provides other better mechanism
628	for error reporting, mapping those into categories described below
629	shouldn't be difficult.
630
631	In the following sections, two recovery actions - reset and
632	reconfiguring transport - are mentioned. These are described further in
633	`EH recovery actions <#exrec>`__.
634
635	HSM violation
636	~~~~~~~~~~~~~
637
638	This error is indicated when STATUS value doesn't match HSM requirement
639	during issuing or execution any ATA/ATAPI command.
640
641	- ATA_STATUS doesn't contain !BSY && DRDY && !DRQ while trying to
642	issue a command.
643
644	- !BSY && !DRQ during PIO data transfer.
645
646	- DRQ on command completion.
647
648	- !BSY && ERR after CDB transfer starts but before the last byte of CDB
649	is transferred. ATA/ATAPI standard states that "The device shall not
650	terminate the PACKET command with an error before the last byte of
651	the command packet has been written" in the error outputs description
652	of PACKET command and the state diagram doesn't include such
653	transitions.
654
655	In these cases, HSM is violated and not much information regarding the
656	error can be acquired from STATUS or ERROR register. IOW, this error can
657	be anything - driver bug, faulty device, controller and/or cable.
658
659	As HSM is violated, reset is necessary to restore known state.
660	Reconfiguring transport for lower speed might be helpful too as
661	transmission errors sometimes cause this kind of errors.
662
663	ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION)
664	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
665
666	These are errors detected and reported by ATA/ATAPI devices indicating
667	device problems. For this type of errors, STATUS and ERROR register
668	values are valid and describe error condition. Note that some of ATA bus
669	errors are detected by ATA/ATAPI devices and reported using the same
670	mechanism as device errors. Those cases are described later in this
671	section.
672
673	For ATA commands, this type of errors are indicated by !BSY && ERR
674	during command execution and on completion.
675
676	For ATAPI commands,
677
678	- !BSY && ERR && ABRT right after issuing PACKET indicates that PACKET
679	command is not supported and falls in this category.
680
681	- !BSY && ERR(==CHK) && !ABRT after the last byte of CDB is transferred
682	indicates CHECK CONDITION and doesn't fall in this category.
683
684	- !BSY && ERR(==CHK) && ABRT after the last byte of CDB is transferred
685	\probably\ indicates CHECK CONDITION and doesn't fall in this
686	category.
687
688	Of errors detected as above, the following are not ATA/ATAPI device
689	errors but ATA bus errors and should be handled according to
690	`ATA bus error <#excatATAbusErr>`__.
691
692	CRC error during data transfer
693	This is indicated by ICRC bit in the ERROR register and means that
694	corruption occurred during data transfer. Up to ATA/ATAPI-7, the
695	standard specifies that this bit is only applicable to UDMA
696	transfers but ATA/ATAPI-8 draft revision 1f says that the bit may be
697	applicable to multiword DMA and PIO.
698
699	ABRT error during data transfer or on completion
700	Up to ATA/ATAPI-7, the standard specifies that ABRT could be set on
701	ICRC errors and on cases where a device is not able to complete a
702	command. Combined with the fact that MWDMA and PIO transfer errors
703	aren't allowed to use ICRC bit up to ATA/ATAPI-7, it seems to imply
704	that ABRT bit alone could indicate transfer errors.
705
706	However, ATA/ATAPI-8 draft revision 1f removes the part that ICRC
707	errors can turn on ABRT. So, this is kind of gray area. Some
708	heuristics are needed here.
709
710	ATA/ATAPI device errors can be further categorized as follows.
711
712	Media errors
713	This is indicated by UNC bit in the ERROR register. ATA devices
714	reports UNC error only after certain number of retries cannot
715	recover the data, so there's nothing much else to do other than
716	notifying upper layer.
717
718	READ and WRITE commands report CHS or LBA of the first failed sector
719	but ATA/ATAPI standard specifies that the amount of transferred data
720	on error completion is indeterminate, so we cannot assume that
721	sectors preceding the failed sector have been transferred and thus
722	cannot complete those sectors successfully as SCSI does.
723
724	Media changed / media change requested error
725	<<TODO: fill here>>
726
727	Address error
728	This is indicated by IDNF bit in the ERROR register. Report to upper
729	layer.
730
731	Other errors
732	This can be invalid command or parameter indicated by ABRT ERROR bit
733	or some other error condition. Note that ABRT bit can indicate a lot
734	of things including ICRC and Address errors. Heuristics needed.
735
736	Depending on commands, not all STATUS/ERROR bits are applicable. These
737	non-applicable bits are marked with "na" in the output descriptions but
738	up to ATA/ATAPI-7 no definition of "na" can be found. However,
739	ATA/ATAPI-8 draft revision 1f describes "N/A" as follows.
740
741	3.2.3.3a N/A
742	A keyword the indicates a field has no defined value in this
743	standard and should not be checked by the host or device. N/A
744	fields should be cleared to zero.
745
746	So, it seems reasonable to assume that "na" bits are cleared to zero by
747	devices and thus need no explicit masking.
748
749	ATAPI device CHECK CONDITION
750	~~~~~~~~~~~~~~~~~~~~~~~~~~~~
751
752	ATAPI device CHECK CONDITION error is indicated by set CHK bit (ERR bit)
753	in the STATUS register after the last byte of CDB is transferred for a
754	PACKET command. For this kind of errors, sense data should be acquired
755	to gather information regarding the errors. REQUEST SENSE packet command
756	should be used to acquire sense data.
757
758	Once sense data is acquired, this type of errors can be handled
759	similarly to other SCSI errors. Note that sense data may indicate ATA
760	bus error (e.g. Sense Key 04h HARDWARE ERROR && ASC/ASCQ 47h/00h SCSI
761	PARITY ERROR). In such cases, the error should be considered as an ATA
762	bus error and handled according to `ATA bus error <#excatATAbusErr>`__.
763
764	ATA device error (NCQ)
765	~~~~~~~~~~~~~~~~~~~~~~
766
767	NCQ command error is indicated by cleared BSY and set ERR bit during NCQ
768	command phase (one or more NCQ commands outstanding). Although STATUS
769	and ERROR registers will contain valid values describing the error, READ
770	LOG EXT is required to clear the error condition, determine which
771	command has failed and acquire more information.
772
773	READ LOG EXT Log Page 10h reports which tag has failed and taskfile
774	register values describing the error. With this information the failed
775	command can be handled as a normal ATA command error as in
776	`ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION) <#excatDevErr>`__
777	and all other in-flight commands must be retried. Note that this retry
778	should not be counted - it's likely that commands retried this way would
779	have completed normally if it were not for the failed command.
780
781	Note that ATA bus errors can be reported as ATA device NCQ errors. This
782	should be handled as described in `ATA bus error <#excatATAbusErr>`__.
783
784	If READ LOG EXT Log Page 10h fails or reports NQ, we're thoroughly
785	screwed. This condition should be treated according to
786	`HSM violation <#excatHSMviolation>`__.
787
788	ATA bus error
789	~~~~~~~~~~~~~
790
791	ATA bus error means that data corruption occurred during transmission
792	over ATA bus (SATA or PATA). This type of errors can be indicated by
793
794	- ICRC or ABRT error as described in
795	`ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION) <#excatDevErr>`__.
796
797	- Controller-specific error completion with error information
798	indicating transmission error.
799
800	- On some controllers, command timeout. In this case, there may be a
801	mechanism to determine that the timeout is due to transmission error.
802
803	- Unknown/random errors, timeouts and all sorts of weirdities.
804
805	As described above, transmission errors can cause wide variety of
806	symptoms ranging from device ICRC error to random device lockup, and,
807	for many cases, there is no way to tell if an error condition is due to
808	transmission error or not; therefore, it's necessary to employ some kind
809	of heuristic when dealing with errors and timeouts. For example,
810	encountering repetitive ABRT errors for known supported command is
811	likely to indicate ATA bus error.
812
813	Once it's determined that ATA bus errors have possibly occurred,
814	lowering ATA bus transmission speed is one of actions which may
815	alleviate the problem. See `Reconfigure transport <#exrecReconf>`__ for
816	more information.
817
818	PCI bus error
819	~~~~~~~~~~~~~
820
821	Data corruption or other failures during transmission over PCI (or other
822	system bus). For standard BMDMA, this is indicated by Error bit in the
823	BMDMA Status register. This type of errors must be logged as it
824	indicates something is very wrong with the system. Resetting host
825	controller is recommended.
826
827	Late completion
828	~~~~~~~~~~~~~~~
829
830	This occurs when timeout occurs and the timeout handler finds out that
831	the timed out command has completed successfully or with error. This is
832	usually caused by lost interrupts. This type of errors must be logged.
833	Resetting host controller is recommended.
834
835	Unknown error (timeout)
836	~~~~~~~~~~~~~~~~~~~~~~~
837
838	This is when timeout occurs and the command is still processing or the
839	host and device are in unknown state. When this occurs, HSM could be in
840	any valid or invalid state. To bring the device to known state and make
841	it forget about the timed out command, resetting is necessary. The timed
842	out command may be retried.
843
844	Timeouts can also be caused by transmission errors. Refer to
845	`ATA bus error <#excatATAbusErr>`__ for more details.
846
847	Hotplug and power management exceptions
848	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
849
850	<<TODO: fill here>>
851
852	EH recovery actions
853	-------------------
854
855	This section discusses several important recovery actions.
856
857	Clearing error condition
858	~~~~~~~~~~~~~~~~~~~~~~~~
859
860	Many controllers require its error registers to be cleared by error
861	handler. Different controllers may have different requirements.
862
863	For SATA, it's strongly recommended to clear at least SError register
864	during error handling.
865
866	Reset
867	~~~~~
868
869	During EH, resetting is necessary in the following cases.
870
871	- HSM is in unknown or invalid state
872
873	- HBA is in unknown or invalid state
874
875	- EH needs to make HBA/device forget about in-flight commands
876
877	- HBA/device behaves weirdly
878
879	Resetting during EH might be a good idea regardless of error condition
880	to improve EH robustness. Whether to reset both or either one of HBA and
881	device depends on situation but the following scheme is recommended.
882
883	- When it's known that HBA is in ready state but ATA/ATAPI device is in
884	unknown state, reset only device.
885
886	- If HBA is in unknown state, reset both HBA and device.
887
888	HBA resetting is implementation specific. For a controller complying to
889	taskfile/BMDMA PCI IDE, stopping active DMA transaction may be
890	sufficient iff BMDMA state is the only HBA context. But even mostly
891	taskfile/BMDMA PCI IDE complying controllers may have implementation
892	specific requirements and mechanism to reset themselves. This must be
893	addressed by specific drivers.
894
895	OTOH, ATA/ATAPI standard describes in detail ways to reset ATA/ATAPI
896	devices.
897
898	PATA hardware reset
899	This is hardware initiated device reset signalled with asserted PATA
900	RESET- signal. There is no standard way to initiate hardware reset
901	from software although some hardware provides registers that allow
902	driver to directly tweak the RESET- signal.
903
904	Software reset
905	This is achieved by turning CONTROL SRST bit on for at least 5us.
906	Both PATA and SATA support it but, in case of SATA, this may require
907	controller-specific support as the second Register FIS to clear SRST
908	should be transmitted while BSY bit is still set. Note that on PATA,
909	this resets both master and slave devices on a channel.
910
911	EXECUTE DEVICE DIAGNOSTIC command
912	Although ATA/ATAPI standard doesn't describe exactly, EDD implies
913	some level of resetting, possibly similar level with software reset.
914	Host-side EDD protocol can be handled with normal command processing
915	and most SATA controllers should be able to handle EDD's just like
916	other commands. As in software reset, EDD affects both devices on a
917	PATA bus.
918
919	Although EDD does reset devices, this doesn't suit error handling as
920	EDD cannot be issued while BSY is set and it's unclear how it will
921	act when device is in unknown/weird state.
922
923	ATAPI DEVICE RESET command
924	This is very similar to software reset except that reset can be
925	restricted to the selected device without affecting the other device
926	sharing the cable.
927
928	SATA phy reset
929	This is the preferred way of resetting a SATA device. In effect,
930	it's identical to PATA hardware reset. Note that this can be done
931	with the standard SCR Control register. As such, it's usually easier
932	to implement than software reset.
933
934	One more thing to consider when resetting devices is that resetting
935	clears certain configuration parameters and they need to be set to their
936	previous or newly adjusted values after reset.
937
938	Parameters affected are.
939
940	- CHS set up with INITIALIZE DEVICE PARAMETERS (seldom used)
941
942	- Parameters set with SET FEATURES including transfer mode setting
943
944	- Block count set with SET MULTIPLE MODE
945
946	- Other parameters (SET MAX, MEDIA LOCK...)
947
948	ATA/ATAPI standard specifies that some parameters must be maintained
949	across hardware or software reset, but doesn't strictly specify all of
950	them. Always reconfiguring needed parameters after reset is required for
951	robustness. Note that this also applies when resuming from deep sleep
952	(power-off).
953
954	Also, ATA/ATAPI standard requires that IDENTIFY DEVICE / IDENTIFY PACKET
955	DEVICE is issued after any configuration parameter is updated or a
956	hardware reset and the result used for further operation. OS driver is
957	required to implement revalidation mechanism to support this.
958
959	Reconfigure transport
960	~~~~~~~~~~~~~~~~~~~~~
961
962	For both PATA and SATA, a lot of corners are cut for cheap connectors,
963	cables or controllers and it's quite common to see high transmission
964	error rate. This can be mitigated by lowering transmission speed.
965
966	The following is a possible scheme Jeff Garzik suggested.
967
968	If more than $N (3?) transmission errors happen in 15 minutes,
969
970	- if SATA, decrease SATA PHY speed. if speed cannot be decreased,
971
972	- decrease UDMA xfer speed. if at UDMA0, switch to PIO4,
973
974	- decrease PIO xfer speed. if at PIO3, complain, but continue
975
976	ata_piix Internals
977	===================
978
979	.. kernel-doc:: drivers/ata/ata_piix.c
980	:internal:
981
982	sata_sil Internals
983	===================
984
985	.. kernel-doc:: drivers/ata/sata_sil.c
986	:internal:
987
988	Thanks
989	======
990
991	The bulk of the ATA knowledge comes thanks to long conversations with
992	Andre Hedrick (www.linux-ide.org), and long hours pondering the ATA and
993	SCSI specifications.
994
995	Thanks to Alan Cox for pointing out similarities between SATA and SCSI,
996	and in general for motivation to hack on libata.
997
998	libata's device detection method, ata_pio_devchk, and in general all
999	the early probing was based on extensive study of Hale Landis's
1000	probe/reset code in his ATADRVR driver (www.ata-atapi.com).