summaryrefslogtreecommitdiffstats
path: root/Documentation/cdrom/cdrom-standard.rst
blob: 6c1303cff159e16daba2bf7c9a5fe8b72a3ebcad (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
=======================
A Linux CD-ROM standard
=======================

:Author: David van Leeuwen <david@ElseWare.cistron.nl>
:Date: 12 March 1999
:Updated by: Erik Andersen (andersee@debian.org)
:Updated by: Jens Axboe (axboe@image.dk)


Introduction
============

Linux is probably the Unix-like operating system that supports
the widest variety of hardware devices. The reasons for this are
presumably

- The large list of hardware devices available for the many platforms
  that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
- The open design of the operating system, such that anybody can write a
  driver for Linux.
- There is plenty of source code around as examples of how to write a driver.

The openness of Linux, and the many different types of available
hardware has allowed Linux to support many different hardware devices.
Unfortunately, the very openness that has allowed Linux to support
all these different devices has also allowed the behavior of each
device driver to differ significantly from one device to another.
This divergence of behavior has been very significant for CD-ROM
devices; the way a particular drive reacts to a `standard` *ioctl()*
call varies greatly from one device driver to another. To avoid making
their drivers totally inconsistent, the writers of Linux CD-ROM
drivers generally created new device drivers by understanding, copying,
and then changing an existing one. Unfortunately, this practice did not
maintain uniform behavior across all the Linux CD-ROM drivers.

This document describes an effort to establish Uniform behavior across
all the different CD-ROM device drivers for Linux. This document also
defines the various *ioctl()'s*, and how the low-level CD-ROM device
drivers should implement them. Currently (as of the Linux 2.1.\ *x*
development kernels) several low-level CD-ROM device drivers, including
both IDE/ATAPI and SCSI, now use this Uniform interface.

When the CD-ROM was developed, the interface between the CD-ROM drive
and the computer was not specified in the standards. As a result, many
different CD-ROM interfaces were developed. Some of them had their
own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
manufacturers adopted an existing electrical interface and changed
the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
adapted their drives to one or more of the already existing electrical
interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
most of the `NoName` manufacturers). In cases where a new drive really
brought its own interface or used its own command set and flow control
scheme, either a separate driver had to be written, or an existing
driver had to be enhanced. History has delivered us CD-ROM support for
many of these different interfaces. Nowadays, almost all new CD-ROM
drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
manufacturer will create a new interface. Even finding drives for the
old proprietary interfaces is getting difficult.

When (in the 1.3.70's) I looked at the existing software interface,
which was expressed through `cdrom.h`, it appeared to be a rather wild
set of commands and data formats [#f1]_. It seemed that many
features of the software interface had been added to accommodate the
capabilities of a particular drive, in an *ad hoc* manner. More
importantly, it appeared that the behavior of the `standard` commands
was different for most of the different drivers: e. g., some drivers
close the tray if an *open()* call occurs when the tray is open, while
others do not. Some drivers lock the door upon opening the device, to
prevent an incoherent file system, but others don't, to allow software
ejection. Undoubtedly, the capabilities of the different drives vary,
but even when two drives have the same capability their drivers'
behavior was usually different.

.. [#f1]
   I cannot recollect what kernel version I looked at, then,
   presumably 1.2.13 and 1.3.34 --- the latest kernel that I was
   indirectly involved in.

I decided to start a discussion on how to make all the Linux CD-ROM
drivers behave more uniformly. I began by contacting the developers of
the many CD-ROM drivers found in the Linux kernel. Their reactions
encouraged me to write the Uniform CD-ROM Driver which this document is
intended to describe. The implementation of the Uniform CD-ROM Driver is
in the file `cdrom.c`. This driver is intended to be an additional software
layer that sits on top of the low-level device drivers for each CD-ROM drive.
By adding this additional layer, it is possible to have all the different
CD-ROM devices behave **exactly** the same (insofar as the underlying
hardware will allow).

The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
Driver is simply to give people writing application programs for CD-ROM drives
**one** Linux CD-ROM interface with consistent behavior for all
CD-ROM devices. In addition, this also provides a consistent interface
between the low-level device driver code and the Linux kernel. Care
is taken that 100% compatibility exists with the data structures and
programmer's interface defined in `cdrom.h`. This guide was written to
help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
Driver code defined in `cdrom.c`.

Personally, I think that the most important hardware interfaces are
the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
of hardware drop continuously, it is also likely that people may have
more than one CD-ROM drive, possibly of mixed types. It is important
that these drives behave in the same way. In December 1994, one of the
cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary
drive. In the months that I was busy writing a Linux driver for it,
proprietary drives became obsolete and IDE/ATAPI drives became the
standard. At the time of the last update to this document (November
1997) it is becoming difficult to even **find** anything less than a
16 speed CD-ROM drive, and 24 speed drives are common.

.. _cdrom_api:

Standardizing through another software level
============================================

At the time this document was conceived, all drivers directly
implemented the CD-ROM *ioctl()* calls through their own routines. This
led to the danger of different drivers forgetting to do important things
like checking that the user was giving the driver valid data. More
importantly, this led to the divergence of behavior, which has already
been discussed.

For this reason, the Uniform CD-ROM Driver was created to enforce consistent
CD-ROM drive behavior, and to provide a common set of services to the various
low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another
software-level, that separates the *ioctl()* and *open()* implementation
from the actual hardware implementation. Note that this effort has
made few changes which will affect a user's application programs. The
greatest change involved moving the contents of the various low-level
CD-ROM drivers\' header files to the kernel's cdrom directory. This was
done to help ensure that the user is only presented with only one cdrom
interface, the interface defined in `cdrom.h`.

CD-ROM drives are specific enough (i. e., different from other
block-devices such as floppy or hard disc drives), to define a set
of common **CD-ROM device operations**, *<cdrom-device>_dops*.
These operations are different from the classical block-device file
operations, *<block-device>_fops*.

The routines for the Uniform CD-ROM Driver interface level are implemented
in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
with the kernel as a block device by registering the following general
*struct file_operations*::

	struct file_operations cdrom_fops = {
		NULL,			/* lseek */
		block _read ,		/* read--general block-dev read */
		block _write,		/* write--general block-dev write */
		NULL,			/* readdir */
		NULL,			/* select */
		cdrom_ioctl,		/* ioctl */
		NULL,			/* mmap */
		cdrom_open,		/* open */
		cdrom_release,		/* release */
		NULL,			/* fsync */
		NULL,			/* fasync */
		NULL			/* revalidate */
	};

Every active CD-ROM device shares this *struct*. The routines
declared above are all implemented in `cdrom.c`, since this file is the
place where the behavior of all CD-ROM-devices is defined and
standardized. The actual interface to the various types of CD-ROM
hardware is still performed by various low-level CD-ROM-device
drivers. These routines simply implement certain **capabilities**
that are common to all CD-ROM (and really, all removable-media
devices).

Registration of a low-level CD-ROM device driver is now done through
the general routines in `cdrom.c`, not through the Virtual File System
(VFS) any more. The interface implemented in `cdrom.c` is carried out
through two general structures that contain information about the
capabilities of the driver, and the specific drives on which the
driver operates. The structures are:

cdrom_device_ops
  This structure contains information about the low-level driver for a
  CD-ROM device. This structure is conceptually connected to the major
  number of the device (although some drivers may have different
  major numbers, as is the case for the IDE driver).

cdrom_device_info
  This structure contains information about a particular CD-ROM drive,
  such as its device name, speed, etc. This structure is conceptually
  connected to the minor number of the device.

Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
is done by the low-level device driver though a call to::

	register_cdrom(struct cdrom_device_info * <device>_info)

The device information structure, *<device>_info*, contains all the
information needed for the kernel to interface with the low-level
CD-ROM device driver. One of the most important entries in this
structure is a pointer to the *cdrom_device_ops* structure of the
low-level driver.

The device operations structure, *cdrom_device_ops*, contains a list
of pointers to the functions which are implemented in the low-level
device driver. When `cdrom.c` accesses a CD-ROM device, it does it
through the functions in this structure. It is impossible to know all
the capabilities of future CD-ROM drives, so it is expected that this
list may need to be expanded from time to time as new technologies are
developed. For example, CD-R and CD-R/W drives are beginning to become
popular, and support will soon need to be added for them. For now, the
current *struct* is::

	struct cdrom_device_ops {
		int (*open)(struct cdrom_device_info *, int)
		void (*release)(struct cdrom_device_info *);
		int (*drive_status)(struct cdrom_device_info *, int);
		unsigned int (*check_events)(struct cdrom_device_info *,
					     unsigned int, int);
		int (*media_changed)(struct cdrom_device_info *, int);
		int (*tray_move)(struct cdrom_device_info *, int);
		int (*lock_door)(struct cdrom_device_info *, int);
		int (*select_speed)(struct cdrom_device_info *, unsigned long);
		int (*get_last_session) (struct cdrom_device_info *,
					 struct cdrom_multisession *);
		int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
		int (*reset)(struct cdrom_device_info *);
		int (*audio_ioctl)(struct cdrom_device_info *,
				   unsigned int, void *);
		const int capability;		/* capability flags */
		int (*generic_packet)(struct cdrom_device_info *,
				      struct packet_command *);
	};

When a low-level device driver implements one of these capabilities,
it should add a function pointer to this *struct*. When a particular
function is not implemented, however, this *struct* should contain a
NULL instead. The *capability* flags specify the capabilities of the
CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
is registered with the Uniform CD-ROM Driver.

Note that most functions have fewer parameters than their
*blkdev_fops* counterparts. This is because very little of the
information in the structures *inode* and *file* is used. For most
drivers, the main parameter is the *struct* *cdrom_device_info*, from
which the major and minor number can be extracted. (Most low-level
CD-ROM drivers don't even look at the major and minor number though,
since many of them only support one device.) This will be available
through *dev* in *cdrom_device_info* described below.

The drive-specific, minor-like information that is registered with
`cdrom.c`, currently contains the following fields::

  struct cdrom_device_info {
	const struct cdrom_device_ops * ops;	/* device operations for this major */
	struct list_head list;			/* linked list of all device_info */
	struct gendisk * disk;			/* matching block layer disk */
	void *  handle;				/* driver-dependent data */

	int mask;				/* mask of capability: disables them */
	int speed;				/* maximum speed for reading data */
	int capacity;				/* number of discs in a jukebox */

	unsigned int options:30;		/* options flags */
	unsigned mc_flags:2;			/*  media-change buffer flags */
	unsigned int vfs_events;		/*  cached events for vfs path */
	unsigned int ioctl_events;		/*  cached events for ioctl path */
	int use_count;				/*  number of times device is opened */
	char name[20];				/*  name of the device type */

	__u8 sanyo_slot : 2;			/*  Sanyo 3-CD changer support */
	__u8 keeplocked : 1;			/*  CDROM_LOCKDOOR status */
	__u8 reserved : 5;			/*  not used yet */
	int cdda_method;			/*  see CDDA_* flags */
	__u8 last_sense;			/*  saves last sense key */
	__u8 media_written;			/*  dirty flag, DVD+RW bookkeeping */
	unsigned short mmc3_profile;		/*  current MMC3 profile */
	int for_data;				/*  unknown:TBD */
	int (*exit)(struct cdrom_device_info *);/*  unknown:TBD */
	int mrw_mode_page;			/*  which MRW mode page is in use */
  };

Using this *struct*, a linked list of the registered minor devices is
built, using the *next* field. The device number, the device operations
struct and specifications of properties of the drive are stored in this
structure.

The *mask* flags can be used to mask out some of the capabilities listed
in *ops->capability*, if a specific drive doesn't support a feature
of the driver. The value *speed* specifies the maximum head-rate of the
drive, measured in units of normal audio speed (176kB/sec raw data or
150kB/sec file system data). The parameters are declared *const*
because they describe properties of the drive, which don't change after
registration.

A few registers contain variables local to the CD-ROM drive. The
flags *options* are used to specify how the general CD-ROM routines
should behave. These various flags registers should provide enough
flexibility to adapt to the different users' wishes (and **not** the
`arbitrary` wishes of the author of the low-level device driver, as is
the case in the old scheme). The register *mc_flags* is used to buffer
the information from *media_changed()* to two separate queues. Other
data that is specific to a minor drive, can be accessed through *handle*,
which can point to a data structure specific to the low-level driver.
The fields *use_count*, *next*, *options* and *mc_flags* need not be
initialized.

The intermediate software layer that `cdrom.c` forms will perform some
additional bookkeeping. The use count of the device (the number of
processes that have the device opened) is registered in *use_count*. The
function *cdrom_ioctl()* will verify the appropriate user-memory regions
for read and write, and in case a location on the CD is transferred,
it will `sanitize` the format by making requests to the low-level
drivers in a standard format, and translating all formats between the
user-software and low level drivers. This relieves much of the drivers'
memory checking and format checking and translation. Also, the necessary
structures will be declared on the program stack.

The implementation of the functions should be as defined in the
following sections. Two functions **must** be implemented, namely
*open()* and *release()*. Other functions may be omitted, their
corresponding capability flags will be cleared upon registration.
Generally, a function returns zero on success and negative on error. A
function call should return only after the command has completed, but of
course waiting for the device should not use processor time.

::

	int open(struct cdrom_device_info *cdi, int purpose)

*Open()* should try to open the device for a specific *purpose*, which
can be either:

- Open for reading data, as done by `mount()` (2), or the
  user commands `dd` or `cat`.
- Open for *ioctl* commands, as done by audio-CD playing programs.

Notice that any strategic code (closing tray upon *open()*, etc.) is
done by the calling routine in `cdrom.c`, so the low-level routine
should only be concerned with proper initialization, such as spinning
up the disc, etc.

::

	void release(struct cdrom_device_info *cdi)

Device-specific actions should be taken such as spinning down the device.
However, strategic actions such as ejection of the tray, or unlocking
the door, should be left over to the general routine *cdrom_release()*.
This is the only function returning type *void*.

.. _cdrom_drive_status:

::

	int drive_status(struct cdrom_device_info *cdi, int slot_nr)

The function *drive_status*, if implemented, should provide
information on the status of the drive (not the status of the disc,
which may or may not be in the drive). If the drive is not a changer,
*slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::


	CDS_NO_INFO		/* no information available */
	CDS_NO_DISC		/* no disc is inserted, tray is closed */
	CDS_TRAY_OPEN		/* tray is opened */
	CDS_DRIVE_NOT_READY	/* something is wrong, tray is moving? */
	CDS_DISC_OK		/* a disc is loaded and everything is fine */

::

	int tray_move(struct cdrom_device_info *cdi, int position)

This function, if implemented, should control the tray movement. (No
other function should control this.) The parameter *position* controls
the desired direction of movement:

- 0 Close tray
- 1 Open tray

This function returns 0 upon success, and a non-zero value upon
error. Note that if the tray is already in the desired position, no
action need be taken, and the return value should be 0.

::

	int lock_door(struct cdrom_device_info *cdi, int lock)

This function (and no other code) controls locking of the door, if the
drive allows this. The value of *lock* controls the desired locking
state:

- 0 Unlock door, manual opening is allowed
- 1 Lock door, tray cannot be ejected manually

This function returns 0 upon success, and a non-zero value upon
error. Note that if the door is already in the requested state, no
action need be taken, and the return value should be 0.

::

	int select_speed(struct cdrom_device_info *cdi, unsigned long speed)

Some CD-ROM drives are capable of changing their head-speed. There
are several reasons for changing the speed of a CD-ROM drive. Badly
pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
CD-ROM drives can obtain very high head rates (up to *24x* is
common). It has been reported that these drives can make reading
errors at these high speeds, reducing the speed can prevent data loss
in these circumstances. Finally, some of these drives can
make an annoyingly loud noise, which a lower speed may reduce.

This function specifies the speed at which data is read or audio is
played back. The value of *speed* specifies the head-speed of the
drive, measured in units of standard cdrom speed (176kB/sec raw data
or 150kB/sec file system data). So to request that a CD-ROM drive
operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
with *speed=2*. The special value `0` means `auto-selection`, i. e.,
maximum data-rate or real-time audio rate. If the drive doesn't have
this `auto-selection` capability, the decision should be made on the
current disc loaded and the return value should be positive. A negative
return value indicates an error.

::

	int get_last_session(struct cdrom_device_info *cdi,
			     struct cdrom_multisession *ms_info)

This function should implement the old corresponding *ioctl()*. For
device *cdi->dev*, the start of the last session of the current disc
should be returned in the pointer argument *ms_info*. Note that
routines in `cdrom.c` have sanitized this argument: its requested
format will **always** be of the type *CDROM_LBA* (linear block
addressing mode), whatever the calling software requested. But
sanitization goes even further: the low-level implementation may
return the requested information in *CDROM_MSF* format if it wishes so
(setting the *ms_info->addr_format* field appropriately, of
course) and the routines in `cdrom.c` will make the transformation if
necessary. The return value is 0 upon success.

::

	int get_mcn(struct cdrom_device_info *cdi,
		    struct cdrom_mcn *mcn)

Some discs carry a `Media Catalog Number` (MCN), also called
`Universal Product Code` (UPC). This number should reflect the number
that is generally found in the bar-code on the product. Unfortunately,
the few discs that carry such a number on the disc don't even use the
same format. The return argument to this function is a pointer to a
pre-declared memory region of type *struct cdrom_mcn*. The MCN is
expected as a 13-character string, terminated by a null-character.

::

	int reset(struct cdrom_device_info *cdi)

This call should perform a hard-reset on the drive (although in
circumstances that a hard-reset is necessary, a drive may very well not
listen to commands anymore). Preferably, control is returned to the
caller only after the drive has finished resetting. If the drive is no
longer listening, it may be wise for the underlying low-level cdrom
driver to time out.

::

	int audio_ioctl(struct cdrom_device_info *cdi,
			unsigned int cmd, void *arg)

Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
implemented by the routines described above, and hence the function
*cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
audio-control. We have decided to leave these to be accessed through a
single function, repeating the arguments *cmd* and *arg*. Note that
the latter is of type *void*, rather than *unsigned long int*.
The routine *cdrom_ioctl()* does do some useful things,
though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
Seconds, Frames) for all audio calls. It also verifies the memory
location of *arg*, and reserves stack-memory for the argument. This
makes implementation of the *audio_ioctl()* much simpler than in the
old driver scheme. For example, you may look up the function
*cm206_audio_ioctl()* `cm206.c` that should be updated with
this documentation.

An unimplemented ioctl should return *-ENOSYS*, but a harmless request
(e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
errors should be according to the standards, whatever they are. When
an error is returned by the low-level driver, the Uniform CD-ROM Driver
tries whenever possible to return the error code to the calling program.
(We may decide to sanitize the return value in *cdrom_ioctl()* though, in
order to guarantee a uniform interface to the audio-player software.)

::

	int dev_ioctl(struct cdrom_device_info *cdi,
		      unsigned int cmd, unsigned long arg)

Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
they are introduced to service some capabilities of certain drives. In
fact, there are 6 different *ioctl()'s* for reading data, either in some
particular kind of format, or audio data. Not many drives support
reading audio tracks as data, I believe this is because of protection
of copyrights of artists. Moreover, I think that if audio-tracks are
supported, it should be done through the VFS and not via *ioctl()'s*. A
problem here could be the fact that audio-frames are 2352 bytes long,
so either the audio-file-system should ask for 75264 bytes at once
(the least common multiple of 512 and 2352), or the drivers should
bend their backs to cope with this incoherence (to which I would be
opposed). Furthermore, it is very difficult for the hardware to find
the exact frame boundaries, since there are no synchronization headers
in audio frames. Once these issues are resolved, this code should be
standardized in `cdrom.c`.

Because there are so many *ioctl()'s* that seem to be introduced to
satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
are routed through the call *dev_ioctl()*. In principle, `private`
*ioctl()*\ 's should be numbered after the device's major number, and not
the general CD-ROM *ioctl* number, `0x53`. Currently the
non-supported *ioctl()'s* are:

	CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW,
	CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL

.. [#f2]

   Is there software around that actually uses these? I'd be interested!

.. _cdrom_capabilities:

CD-ROM capabilities
-------------------

Instead of just implementing some *ioctl* calls, the interface in
`cdrom.c` supplies the possibility to indicate the **capabilities**
of a CD-ROM drive. This can be done by ORing any number of
capability-constants that are defined in `cdrom.h` at the registration
phase. Currently, the capabilities are any of::

	CDC_CLOSE_TRAY		/* can close tray by software control */
	CDC_OPEN_TRAY		/* can open tray */
	CDC_LOCK		/* can lock and unlock the door */
	CDC_SELECT_SPEED	/* can select speed, in units of * sim*150 ,kB/s */
	CDC_SELECT_DISC		/* drive is juke-box */
	CDC_MULTI_SESSION	/* can read sessions *> rm1* */
	CDC_MCN			/* can read Media Catalog Number */
	CDC_MEDIA_CHANGED	/* can report if disc has changed */
	CDC_PLAY_AUDIO		/* can perform audio-functions (play, pause, etc) */
	CDC_RESET		/* hard reset device */
	CDC_IOCTLS		/* driver has non-standard ioctls */
	CDC_DRIVE_STATUS	/* driver implements drive status */

The capability flag is declared *const*, to prevent drivers from
accidentally tampering with the contents. The capability flags actually
inform `cdrom.c` of what the driver can do. If the drive found
by the driver does not have the capability, is can be masked out by
the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
driver has implemented the code for loading and ejecting CD-ROM's, and
hence its corresponding flags in *capability* will be set. But a SCSI
CD-ROM drive might be a caddy system, which can't load the tray, and
hence for this drive the *cdrom_device_info* struct will have set
the *CDC_CLOSE_TRAY* bit in *mask*.

In the file `cdrom.c` you will encounter many constructions of the type::

	if (cdo->capability & ~cdi->mask & CDC _<capability>) ...

There is no *ioctl* to set the mask... The reason is that
I think it is better to control the **behavior** rather than the
**capabilities**.

Options
-------

A final flag register controls the **behavior** of the CD-ROM
drives, in order to satisfy different users' wishes, hopefully
independently of the ideas of the respective author who happened to
have made the drive's support available to the Linux community. The
current behavior options are::

	CDO_AUTO_CLOSE	/* try to close tray upon device open() */
	CDO_AUTO_EJECT	/* try to open tray on last device close() */
	CDO_USE_FFLAGS	/* use file_pointer->f_flags to indicate purpose for open() */
	CDO_LOCK	/* try to lock door if device is opened */
	CDO_CHECK_TYPE	/* ensure disc type is data if opened for data */

The initial value of this register is
`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
interface and software standards. Before you protest, there are two
new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
behavior by software. These are::

	CDROM_SET_OPTIONS	/* set options specified in (int)arg */
	CDROM_CLEAR_OPTIONS	/* clear options specified in (int)arg */

One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
newsection we explain what the need for this option is.

A software package `setcd`, available from the Debian distribution
and `sunsite.unc.edu`, allows user level control of these flags.


The need to know the purpose of opening the CD-ROM device
=========================================================

Traditionally, Unix devices can be used in two different `modes`,
either by reading/writing to the device file, or by issuing
controlling commands to the device, by the device's *ioctl()*
call. The problem with CD-ROM drives, is that they can be used for
two entirely different purposes. One is to mount removable
file systems, CD-ROM's, the other is to play audio CD's. Audio commands
are implemented entirely through *ioctl()\'s*, presumably because the
first implementation (SUN?) has been such. In principle there is
nothing wrong with this, but a good control of the `CD player` demands
that the device can **always** be opened in order to give the
*ioctl* commands, regardless of the state the drive is in.

On the other hand, when used as a removable-media disc drive (what the
original purpose of CD-ROM s is) we would like to make sure that the
disc drive is ready for operation upon opening the device. In the old
scheme, some CD-ROM drivers don't do any integrity checking, resulting
in a number of i/o errors reported by the VFS to the kernel when an
attempt for mounting a CD-ROM on an empty drive occurs. This is not a
particularly elegant way to find out that there is no CD-ROM inserted;
it more-or-less looks like the old IBM-PC trying to read an empty floppy
drive for a couple of seconds, after which the system complains it
can't read from it. Nowadays we can **sense** the existence of a
removable medium in a drive, and we believe we should exploit that
fact. An integrity check on opening of the device, that verifies the
availability of a CD-ROM and its correct type (data), would be
desirable.

These two ways of using a CD-ROM drive, principally for data and
secondarily for playing audio discs, have different demands for the
behavior of the *open()* call. Audio use simply wants to open the
device in order to get a file handle which is needed for issuing
*ioctl* commands, while data use wants to open for correct and
reliable data transfer. The only way user programs can indicate what
their *purpose* of opening the device is, is through the *flags*
parameter (see `open(2)`). For CD-ROM devices, these flags aren't
implemented (some drivers implement checking for write-related flags,
but this is not strictly necessary if the device file has correct
permission flags). Most option flags simply don't make sense to
CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
*O_SYNC* have no meaning to a CD-ROM.

We therefore propose to use the flag *O_NONBLOCK* to indicate
that the device is opened just for issuing *ioctl*
commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
subsequent calls to the device don't cause the calling process to
wait. We could interpret this as don't wait until someone has
inserted some valid data-CD-ROM. Thus, our proposal of the
implementation for the *open()* call for CD-ROM s is:

- If no other flags are set than *O_RDONLY*, the device is opened
  for data transfer, and the return value will be 0 only upon successful
  initialization of the transfer. The call may even induce some actions
  on the CD-ROM, such as closing the tray.
- If the option flag *O_NONBLOCK* is set, opening will always be
  successful, unless the whole device doesn't exist. The drive will take
  no actions whatsoever.

And what about standards?
-------------------------

You might hesitate to accept this proposal as it comes from the
Linux community, and not from some standardizing institute. What
about SUN, SGI, HP and all those other Unix and hardware vendors?
Well, these companies are in the lucky position that they generally
control both the hardware and software of their supported products,
and are large enough to set their own standard. They do not have to
deal with a dozen or more different, competing hardware
configurations\ [#f3]_.

.. [#f3]

   Incidentally, I think that SUN's approach to mounting CD-ROM s is very
   good in origin: under Solaris a volume-daemon automatically mounts a
   newly inserted CD-ROM under `/cdrom/*<volume-name>*`.

   In my opinion they should have pushed this
   further and have **every** CD-ROM on the local area network be
   mounted at the similar location, i. e., no matter in which particular
   machine you insert a CD-ROM, it will always appear at the same
   position in the directory tree, on every system. When I wanted to
   implement such a user-program for Linux, I came across the
   differences in behavior of the various drivers, and the need for an
   *ioctl* informing about media changes.

We believe that using *O_NONBLOCK* to indicate that a device is being opened
for *ioctl* commands only can be easily introduced in the Linux
community. All the CD-player authors will have to be informed, we can
even send in our own patches to the programs. The use of *O_NONBLOCK*
has most likely no influence on the behavior of the CD-players on
other operating systems than Linux. Finally, a user can always revert
to old behavior by a call to
*ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.

The preferred strategy of *open()*
----------------------------------

The routines in `cdrom.c` are designed in such a way that run-time
configuration of the behavior of CD-ROM devices (of **any** type)
can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
modes of operation can be set:

`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`
   This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
   the future.) If the device is not yet opened by any other process, and if
   the device is being opened for data (*O_NONBLOCK* is not set) and the
   tray is found to be open, an attempt to close the tray is made. Then,
   it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
   set, that it contains tracks of type `data mode 1`. Only if all tests
   are passed is the return value zero. The door is locked to prevent file
   system corruption. If the drive is opened for audio (*O_NONBLOCK* is
   set), no actions are taken and a value of 0 will be returned.

`CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK`
   This mimics the behavior of the current sbpcd-driver. The option flags are
   ignored, the tray is closed on the first open, if necessary. Similarly,
   the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
   it is automatically ejected, such that the user can replace it.

We hope that these option can convince everybody (both driver
maintainers and user program developers) to adopt the new CD-ROM
driver scheme and option flag interpretation.

Description of routines in `cdrom.c`
====================================

Only a few routines in `cdrom.c` are exported to the drivers. In this
new section we will discuss these, as well as the functions that `take
over` the CD-ROM interface to the kernel. The header file belonging
to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
file were placed in the file `ucdrom.h`, but this file has now been
merged back into `cdrom.h`.

::

	struct file_operations cdrom_fops

The contents of this structure were described in cdrom_api_.
A pointer to this structure is assigned to the *fops* field
of the *struct gendisk*.

::

	int register_cdrom(struct cdrom_device_info *cdi)

This function is used in about the same way one registers *cdrom_fops*
with the kernel, the device operations and information structures,
as described in cdrom_api_, should be registered with the
Uniform CD-ROM Driver::

	register_cdrom(&<device>_info);


This function returns zero upon success, and non-zero upon
failure. The structure *<device>_info* should have a pointer to the
driver's *<device>_dops*, as in::

	struct cdrom_device_info <device>_info = {
		<device>_dops;
		...
	}

Note that a driver must have one static structure, *<device>_dops*, while
it may have as many structures *<device>_info* as there are minor devices
active. *Register_cdrom()* builds a linked list from these.


::

	void unregister_cdrom(struct cdrom_device_info *cdi)

Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
the minor device from the list. If it was the last registered minor for
the low-level driver, this disconnects the registered device-operation
routines from the CD-ROM interface. This function returns zero upon
success, and non-zero upon failure.

::

	int cdrom_open(struct inode * ip, struct file * fp)

This function is not called directly by the low-level drivers, it is
listed in the standard *cdrom_fops*. If the VFS opens a file, this
function becomes active. A strategy is implemented in this routine,
taking care of all capabilities and options that are set in the
*cdrom_device_ops* connected to the device. Then, the program flow is
transferred to the device_dependent *open()* call.

::

	void cdrom_release(struct inode *ip, struct file *fp)

This function implements the reverse-logic of *cdrom_open()*, and then
calls the device-dependent *release()* routine. When the use-count has
reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
and *invalidate_buffers(dev)*.


.. _cdrom_ioctl:

::

	int cdrom_ioctl(struct inode *ip, struct file *fp,
			unsigned int cmd, unsigned long arg)

This function handles all the standard *ioctl* requests for CD-ROM
devices in a uniform way. The different calls fall into three
categories: *ioctl()'s* that can be directly implemented by device
operations, ones that are routed through the call *audio_ioctl()*, and
the remaining ones, that are presumable device-dependent. Generally, a
negative return value indicates an error.

Directly implemented *ioctl()'s*
--------------------------------

The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
calling device-operations in *cdrom_device_ops*, if implemented and
not masked:

`CDROMMULTISESSION`
	Requests the last session on a CD-ROM.
`CDROMEJECT`
	Open tray.
`CDROMCLOSETRAY`
	Close tray.
`CDROMEJECT_SW`
	If *arg\not=0*, set behavior to auto-close (close
	tray on first open) and auto-eject (eject on last release), otherwise
	set behavior to non-moving on *open()* and *release()* calls.
`CDROM_GET_MCN`
	Get the Media Catalog Number from a CD.

*Ioctl*s routed through *audio_ioctl()*
---------------------------------------

The following set of *ioctl()'s* are all implemented through a call to
the *cdrom_fops* function *audio_ioctl()*. Memory checks and
allocation are performed in *cdrom_ioctl()*, and also sanitization of
address format (*CDROM_LBA*/*CDROM_MSF*) is done.

`CDROMSUBCHNL`
	Get sub-channel data in argument *arg* of type
	`struct cdrom_subchnl *`.
`CDROMREADTOCHDR`
	Read Table of Contents header, in *arg* of type
	`struct cdrom_tochdr *`.
`CDROMREADTOCENTRY`
	Read a Table of Contents entry in *arg* and specified by *arg*
	of type `struct cdrom_tocentry *`.
`CDROMPLAYMSF`
	Play audio fragment specified in Minute, Second, Frame format,
	delimited by *arg* of type `struct cdrom_msf *`.
`CDROMPLAYTRKIND`
	Play audio fragment in track-index format delimited by *arg*
	of type `struct cdrom_ti *`.
`CDROMVOLCTRL`
	Set volume specified by *arg* of type `struct cdrom_volctrl *`.
`CDROMVOLREAD`
	Read volume into by *arg* of type `struct cdrom_volctrl *`.
`CDROMSTART`
	Spin up disc.
`CDROMSTOP`
	Stop playback of audio fragment.
`CDROMPAUSE`
	Pause playback of audio fragment.
`CDROMRESUME`
	Resume playing.

New *ioctl()'s* in `cdrom.c`
----------------------------

The following *ioctl()'s* have been introduced to allow user programs to
control the behavior of individual CD-ROM devices. New *ioctl*
commands can be identified by the underscores in their names.

`CDROM_SET_OPTIONS`
	Set options specified by *arg*. Returns the option flag register
	after modification. Use *arg = \rm0* for reading the current flags.
`CDROM_CLEAR_OPTIONS`
	Clear options specified by *arg*. Returns the option flag register
	after modification.
`CDROM_SELECT_SPEED`
	Select head-rate speed of disc specified as by *arg* in units
	of standard cdrom speed (176\,kB/sec raw data or
	150kB/sec file system data). The value 0 means `auto-select`,
	i. e., play audio discs at real time and data discs at maximum speed.
	The value *arg* is checked against the maximum head rate of the
	drive found in the *cdrom_dops*.
`CDROM_SELECT_DISC`
	Select disc numbered *arg* from a juke-box.

	First disc is numbered 0. The number *arg* is checked against the
	maximum number of discs in the juke-box found in the *cdrom_dops*.
`CDROM_MEDIA_CHANGED`
	Returns 1 if a disc has been changed since the last call.
	For juke-boxes, an extra argument *arg*
	specifies the slot for which the information is given. The special
	value *CDSL_CURRENT* requests that information about the currently
	selected slot be returned.
`CDROM_TIMED_MEDIA_CHANGE`
	Checks whether the disc has been changed since a user supplied time
	and returns the time of the last disc change.

	*arg* is a pointer to a *cdrom_timed_media_change_info* struct.
	*arg->last_media_change* may be set by calling code to signal
	the timestamp of the last known media change (by the caller).
	Upon successful return, this ioctl call will set
	*arg->last_media_change* to the latest media change timestamp (in ms)
	known by the kernel/driver and set *arg->has_changed* to 1 if
	that timestamp is more recent than the timestamp set by the caller.
`CDROM_DRIVE_STATUS`
	Returns the status of the drive by a call to
	*drive_status()*. Return values are defined in cdrom_drive_status_.
	Note that this call doesn't return information on the
	current playing activity of the drive; this can be polled through
	an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
	*arg* specifies the slot for which (possibly limited) information is
	given. The special value *CDSL_CURRENT* requests that information
	about the currently selected slot be returned.
`CDROM_DISC_STATUS`
	Returns the type of the disc currently in the drive.
	It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
	This *ioctl* can provide *some* information about the current
	disc that is inserted in the drive. This functionality used to be
	implemented in the low level drivers, but is now carried out
	entirely in Uniform CD-ROM Driver.

	The history of development of the CD's use as a carrier medium for
	various digital information has lead to many different disc types.
	This *ioctl* is useful only in the case that CDs have \emph {only
	one} type of data on them. While this is often the case, it is
	also very common for CDs to have some tracks with data, and some
	tracks with audio. Because this is an existing interface, rather
	than fixing this interface by changing the assumptions it was made
	under, thereby breaking all user applications that use this
	function, the Uniform CD-ROM Driver implements this *ioctl* as
	follows: If the CD in question has audio tracks on it, and it has
	absolutely no CD-I, XA, or data tracks on it, it will be reported
	as *CDS_AUDIO*. If it has both audio and data tracks, it will
	return *CDS_MIXED*. If there are no audio tracks on the disc, and
	if the CD in question has any CD-I tracks on it, it will be
	reported as *CDS_XA_2_2*. Failing that, if the CD in question
	has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
	Finally, if the CD in question has any data tracks on it,
	it will be reported as a data CD (*CDS_DATA_1*).

	This *ioctl* can return::

		CDS_NO_INFO	/* no information available */
		CDS_NO_DISC	/* no disc is inserted, or tray is opened */
		CDS_AUDIO	/* Audio disc (2352 audio bytes/frame) */
		CDS_DATA_1	/* data disc, mode 1 (2048 user bytes/frame) */
		CDS_XA_2_1	/* mixed data (XA), mode 2, form 1 (2048 user bytes) */
		CDS_XA_2_2	/* mixed data (XA), mode 2, form 1 (2324 user bytes) */
		CDS_MIXED	/* mixed audio/data disc */

	For some information concerning frame layout of the various disc
	types, see a recent version of `cdrom.h`.

`CDROM_CHANGER_NSLOTS`
	Returns the number of slots in a juke-box.
`CDROMRESET`
	Reset the drive.
`CDROM_GET_CAPABILITY`
	Returns the *capability* flags for the drive. Refer to section
	cdrom_capabilities_ for more information on these flags.
`CDROM_LOCKDOOR`
	 Locks the door of the drive. `arg == 0` unlocks the door,
	 any other value locks it.
`CDROM_DEBUG`
	 Turns on debugging info. Only root is allowed to do this.
	 Same semantics as CDROM_LOCKDOOR.


Device dependent *ioctl()'s*
----------------------------

Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
if implemented. No memory allocation or verification is carried out.

How to update your driver
=========================

- Make a backup of your current driver.
- Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
  the directory tree that came with this documentation.
- Make sure you include `cdrom.h`.
- Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
  to `&cdrom_fops`.
- Just after that line, add the following to register with the Uniform
  CD-ROM Driver::

	register_cdrom(&<your-drive>_info);*

  Similarly, add a call to *unregister_cdrom()* at the appropriate place.
- Copy an example of the device-operations *struct* to your
  source, e. g., from `cm206.c` *cm206_dops*, and change all
  entries to names corresponding to your driver, or names you just
  happen to like. If your driver doesn't support a certain function,
  make the entry *NULL*. At the entry *capability* you should list all
  capabilities your driver currently supports. If your driver
  has a capability that is not listed, please send me a message.
- Copy the *cdrom_device_info* declaration from the same example
  driver, and modify the entries according to your needs. If your
  driver dynamically determines the capabilities of the hardware, this
  structure should also be declared dynamically.
- Implement all functions in your `<device>_dops` structure,
  according to prototypes listed in  `cdrom.h`, and specifications given
  in cdrom_api_. Most likely you have already implemented
  the code in a large part, and you will almost certainly need to adapt the
  prototype and return values.
- Rename your `<device>_ioctl()` function to *audio_ioctl* and
  change the prototype a little. Remove entries listed in the first
  part in cdrom_ioctl_, if your code was OK, these are
  just calls to the routines you adapted in the previous step.
- You may remove all remaining memory checking code in the
  *audio_ioctl()* function that deals with audio commands (these are
  listed in the second part of cdrom_ioctl_. There is no
  need for memory allocation either, so most *case*s in the *switch*
  statement look similar to::

	case CDROMREADTOCENTRY:
		get_toc_entry\bigl((struct cdrom_tocentry *) arg);

- All remaining *ioctl* cases must be moved to a separate
  function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
  memory checking and allocation must be kept in this code!
- Change the prototypes of *<device>_open()* and
  *<device>_release()*, and remove any strategic code (i. e., tray
  movement, door locking, etc.).
- Try to recompile the drivers. We advise you to use modules, both
  for `cdrom.o` and your driver, as debugging is much easier this
  way.

Thanks
======

Thanks to all the people involved. First, Erik Andersen, who has
taken over the torch in maintaining `cdrom.c` and integrating much
CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
Gerd Knorr, who were the first to implement this interface for SCSI
and IDE-CD drivers and added many ideas for extension of the data
structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt,
Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll,
the Linux CD-ROM device driver developers who were kind
enough to give suggestions and criticisms during the writing. Finally
of course, I want to thank Linus Torvalds for making this possible in
the first place.