1 =======================
2 A Linux CD-ROM standard
3 =======================
5 :Author: David van Leeuwen <david@ElseWare.cistron.nl>
7 :Updated by: Erik Andersen (andersee@debian.org)
8 :Updated by: Jens Axboe (axboe@image.dk)
14 Linux is probably the Unix-like operating system that supports
15 the widest variety of hardware devices. The reasons for this are
18 - The large list of hardware devices available for the many platforms
19 that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
20 - The open design of the operating system, such that anybody can write a
22 - There is plenty of source code around as examples of how to write a driver.
24 The openness of Linux, and the many different types of available
25 hardware has allowed Linux to support many different hardware devices.
26 Unfortunately, the very openness that has allowed Linux to support
27 all these different devices has also allowed the behavior of each
28 device driver to differ significantly from one device to another.
29 This divergence of behavior has been very significant for CD-ROM
30 devices; the way a particular drive reacts to a `standard` *ioctl()*
31 call varies greatly from one device driver to another. To avoid making
32 their drivers totally inconsistent, the writers of Linux CD-ROM
33 drivers generally created new device drivers by understanding, copying,
34 and then changing an existing one. Unfortunately, this practice did not
35 maintain uniform behavior across all the Linux CD-ROM drivers.
37 This document describes an effort to establish Uniform behavior across
38 all the different CD-ROM device drivers for Linux. This document also
39 defines the various *ioctl()'s*, and how the low-level CD-ROM device
40 drivers should implement them. Currently (as of the Linux 2.1.\ *x*
41 development kernels) several low-level CD-ROM device drivers, including
42 both IDE/ATAPI and SCSI, now use this Uniform interface.
44 When the CD-ROM was developed, the interface between the CD-ROM drive
45 and the computer was not specified in the standards. As a result, many
46 different CD-ROM interfaces were developed. Some of them had their
47 own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
48 manufacturers adopted an existing electrical interface and changed
49 the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
50 adapted their drives to one or more of the already existing electrical
51 interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
52 most of the `NoName` manufacturers). In cases where a new drive really
53 brought its own interface or used its own command set and flow control
54 scheme, either a separate driver had to be written, or an existing
55 driver had to be enhanced. History has delivered us CD-ROM support for
56 many of these different interfaces. Nowadays, almost all new CD-ROM
57 drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
58 manufacturer will create a new interface. Even finding drives for the
59 old proprietary interfaces is getting difficult.
61 When (in the 1.3.70's) I looked at the existing software interface,
62 which was expressed through `cdrom.h`, it appeared to be a rather wild
63 set of commands and data formats [#f1]_. It seemed that many
64 features of the software interface had been added to accommodate the
65 capabilities of a particular drive, in an *ad hoc* manner. More
66 importantly, it appeared that the behavior of the `standard` commands
67 was different for most of the different drivers: e. g., some drivers
68 close the tray if an *open()* call occurs when the tray is open, while
69 others do not. Some drivers lock the door upon opening the device, to
70 prevent an incoherent file system, but others don't, to allow software
71 ejection. Undoubtedly, the capabilities of the different drives vary,
72 but even when two drives have the same capability their drivers'
73 behavior was usually different.
76 I cannot recollect what kernel version I looked at, then,
77 presumably 1.2.13 and 1.3.34 --- the latest kernel that I was
78 indirectly involved in.
80 I decided to start a discussion on how to make all the Linux CD-ROM
81 drivers behave more uniformly. I began by contacting the developers of
82 the many CD-ROM drivers found in the Linux kernel. Their reactions
83 encouraged me to write the Uniform CD-ROM Driver which this document is
84 intended to describe. The implementation of the Uniform CD-ROM Driver is
85 in the file `cdrom.c`. This driver is intended to be an additional software
86 layer that sits on top of the low-level device drivers for each CD-ROM drive.
87 By adding this additional layer, it is possible to have all the different
88 CD-ROM devices behave **exactly** the same (insofar as the underlying
91 The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
92 whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
93 Driver is simply to give people writing application programs for CD-ROM drives
94 **one** Linux CD-ROM interface with consistent behavior for all
95 CD-ROM devices. In addition, this also provides a consistent interface
96 between the low-level device driver code and the Linux kernel. Care
97 is taken that 100% compatibility exists with the data structures and
98 programmer's interface defined in `cdrom.h`. This guide was written to
99 help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
100 Driver code defined in `cdrom.c`.
102 Personally, I think that the most important hardware interfaces are
103 the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
104 of hardware drop continuously, it is also likely that people may have
105 more than one CD-ROM drive, possibly of mixed types. It is important
106 that these drives behave in the same way. In December 1994, one of the
107 cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary
108 drive. In the months that I was busy writing a Linux driver for it,
109 proprietary drives became obsolete and IDE/ATAPI drives became the
110 standard. At the time of the last update to this document (November
111 1997) it is becoming difficult to even **find** anything less than a
112 16 speed CD-ROM drive, and 24 speed drives are common.
116 Standardizing through another software level
117 ============================================
119 At the time this document was conceived, all drivers directly
120 implemented the CD-ROM *ioctl()* calls through their own routines. This
121 led to the danger of different drivers forgetting to do important things
122 like checking that the user was giving the driver valid data. More
123 importantly, this led to the divergence of behavior, which has already
126 For this reason, the Uniform CD-ROM Driver was created to enforce consistent
127 CD-ROM drive behavior, and to provide a common set of services to the various
128 low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another
129 software-level, that separates the *ioctl()* and *open()* implementation
130 from the actual hardware implementation. Note that this effort has
131 made few changes which will affect a user's application programs. The
132 greatest change involved moving the contents of the various low-level
133 CD-ROM drivers\' header files to the kernel's cdrom directory. This was
134 done to help ensure that the user is only presented with only one cdrom
135 interface, the interface defined in `cdrom.h`.
137 CD-ROM drives are specific enough (i. e., different from other
138 block-devices such as floppy or hard disc drives), to define a set
139 of common **CD-ROM device operations**, *<cdrom-device>_dops*.
140 These operations are different from the classical block-device file
141 operations, *<block-device>_fops*.
143 The routines for the Uniform CD-ROM Driver interface level are implemented
144 in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
145 with the kernel as a block device by registering the following general
146 *struct file_operations*::
148 struct file_operations cdrom_fops = {
150 block _read , /* read--general block-dev read */
151 block _write, /* write--general block-dev write */
154 cdrom_ioctl, /* ioctl */
156 cdrom_open, /* open */
157 cdrom_release, /* release */
160 NULL /* revalidate */
163 Every active CD-ROM device shares this *struct*. The routines
164 declared above are all implemented in `cdrom.c`, since this file is the
165 place where the behavior of all CD-ROM-devices is defined and
166 standardized. The actual interface to the various types of CD-ROM
167 hardware is still performed by various low-level CD-ROM-device
168 drivers. These routines simply implement certain **capabilities**
169 that are common to all CD-ROM (and really, all removable-media
172 Registration of a low-level CD-ROM device driver is now done through
173 the general routines in `cdrom.c`, not through the Virtual File System
174 (VFS) any more. The interface implemented in `cdrom.c` is carried out
175 through two general structures that contain information about the
176 capabilities of the driver, and the specific drives on which the
177 driver operates. The structures are:
180 This structure contains information about the low-level driver for a
181 CD-ROM device. This structure is conceptually connected to the major
182 number of the device (although some drivers may have different
183 major numbers, as is the case for the IDE driver).
186 This structure contains information about a particular CD-ROM drive,
187 such as its device name, speed, etc. This structure is conceptually
188 connected to the minor number of the device.
190 Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
191 is done by the low-level device driver though a call to::
193 register_cdrom(struct cdrom_device_info * <device>_info)
195 The device information structure, *<device>_info*, contains all the
196 information needed for the kernel to interface with the low-level
197 CD-ROM device driver. One of the most important entries in this
198 structure is a pointer to the *cdrom_device_ops* structure of the
201 The device operations structure, *cdrom_device_ops*, contains a list
202 of pointers to the functions which are implemented in the low-level
203 device driver. When `cdrom.c` accesses a CD-ROM device, it does it
204 through the functions in this structure. It is impossible to know all
205 the capabilities of future CD-ROM drives, so it is expected that this
206 list may need to be expanded from time to time as new technologies are
207 developed. For example, CD-R and CD-R/W drives are beginning to become
208 popular, and support will soon need to be added for them. For now, the
209 current *struct* is::
211 struct cdrom_device_ops {
212 int (*open)(struct cdrom_device_info *, int)
213 void (*release)(struct cdrom_device_info *);
214 int (*drive_status)(struct cdrom_device_info *, int);
215 unsigned int (*check_events)(struct cdrom_device_info *,
217 int (*media_changed)(struct cdrom_device_info *, int);
218 int (*tray_move)(struct cdrom_device_info *, int);
219 int (*lock_door)(struct cdrom_device_info *, int);
220 int (*select_speed)(struct cdrom_device_info *, int);
221 int (*select_disc)(struct cdrom_device_info *, int);
222 int (*get_last_session) (struct cdrom_device_info *,
223 struct cdrom_multisession *);
224 int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
225 int (*reset)(struct cdrom_device_info *);
226 int (*audio_ioctl)(struct cdrom_device_info *,
227 unsigned int, void *);
228 const int capability; /* capability flags */
229 int (*generic_packet)(struct cdrom_device_info *,
230 struct packet_command *);
233 When a low-level device driver implements one of these capabilities,
234 it should add a function pointer to this *struct*. When a particular
235 function is not implemented, however, this *struct* should contain a
236 NULL instead. The *capability* flags specify the capabilities of the
237 CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
238 is registered with the Uniform CD-ROM Driver.
240 Note that most functions have fewer parameters than their
241 *blkdev_fops* counterparts. This is because very little of the
242 information in the structures *inode* and *file* is used. For most
243 drivers, the main parameter is the *struct* *cdrom_device_info*, from
244 which the major and minor number can be extracted. (Most low-level
245 CD-ROM drivers don't even look at the major and minor number though,
246 since many of them only support one device.) This will be available
247 through *dev* in *cdrom_device_info* described below.
249 The drive-specific, minor-like information that is registered with
250 `cdrom.c`, currently contains the following fields::
252 struct cdrom_device_info {
253 const struct cdrom_device_ops * ops; /* device operations for this major */
254 struct list_head list; /* linked list of all device_info */
255 struct gendisk * disk; /* matching block layer disk */
256 void * handle; /* driver-dependent data */
258 int mask; /* mask of capability: disables them */
259 int speed; /* maximum speed for reading data */
260 int capacity; /* number of discs in a jukebox */
262 unsigned int options:30; /* options flags */
263 unsigned mc_flags:2; /* media-change buffer flags */
264 unsigned int vfs_events; /* cached events for vfs path */
265 unsigned int ioctl_events; /* cached events for ioctl path */
266 int use_count; /* number of times device is opened */
267 char name[20]; /* name of the device type */
269 __u8 sanyo_slot : 2; /* Sanyo 3-CD changer support */
270 __u8 keeplocked : 1; /* CDROM_LOCKDOOR status */
271 __u8 reserved : 5; /* not used yet */
272 int cdda_method; /* see CDDA_* flags */
273 __u8 last_sense; /* saves last sense key */
274 __u8 media_written; /* dirty flag, DVD+RW bookkeeping */
275 unsigned short mmc3_profile; /* current MMC3 profile */
276 int for_data; /* unknown:TBD */
277 int (*exit)(struct cdrom_device_info *);/* unknown:TBD */
278 int mrw_mode_page; /* which MRW mode page is in use */
281 Using this *struct*, a linked list of the registered minor devices is
282 built, using the *next* field. The device number, the device operations
283 struct and specifications of properties of the drive are stored in this
286 The *mask* flags can be used to mask out some of the capabilities listed
287 in *ops->capability*, if a specific drive doesn't support a feature
288 of the driver. The value *speed* specifies the maximum head-rate of the
289 drive, measured in units of normal audio speed (176kB/sec raw data or
290 150kB/sec file system data). The parameters are declared *const*
291 because they describe properties of the drive, which don't change after
294 A few registers contain variables local to the CD-ROM drive. The
295 flags *options* are used to specify how the general CD-ROM routines
296 should behave. These various flags registers should provide enough
297 flexibility to adapt to the different users' wishes (and **not** the
298 `arbitrary` wishes of the author of the low-level device driver, as is
299 the case in the old scheme). The register *mc_flags* is used to buffer
300 the information from *media_changed()* to two separate queues. Other
301 data that is specific to a minor drive, can be accessed through *handle*,
302 which can point to a data structure specific to the low-level driver.
303 The fields *use_count*, *next*, *options* and *mc_flags* need not be
306 The intermediate software layer that `cdrom.c` forms will perform some
307 additional bookkeeping. The use count of the device (the number of
308 processes that have the device opened) is registered in *use_count*. The
309 function *cdrom_ioctl()* will verify the appropriate user-memory regions
310 for read and write, and in case a location on the CD is transferred,
311 it will `sanitize` the format by making requests to the low-level
312 drivers in a standard format, and translating all formats between the
313 user-software and low level drivers. This relieves much of the drivers'
314 memory checking and format checking and translation. Also, the necessary
315 structures will be declared on the program stack.
317 The implementation of the functions should be as defined in the
318 following sections. Two functions **must** be implemented, namely
319 *open()* and *release()*. Other functions may be omitted, their
320 corresponding capability flags will be cleared upon registration.
321 Generally, a function returns zero on success and negative on error. A
322 function call should return only after the command has completed, but of
323 course waiting for the device should not use processor time.
327 int open(struct cdrom_device_info *cdi, int purpose)
329 *Open()* should try to open the device for a specific *purpose*, which
332 - Open for reading data, as done by `mount()` (2), or the
333 user commands `dd` or `cat`.
334 - Open for *ioctl* commands, as done by audio-CD playing programs.
336 Notice that any strategic code (closing tray upon *open()*, etc.) is
337 done by the calling routine in `cdrom.c`, so the low-level routine
338 should only be concerned with proper initialization, such as spinning
343 void release(struct cdrom_device_info *cdi)
345 Device-specific actions should be taken such as spinning down the device.
346 However, strategic actions such as ejection of the tray, or unlocking
347 the door, should be left over to the general routine *cdrom_release()*.
348 This is the only function returning type *void*.
350 .. _cdrom_drive_status:
354 int drive_status(struct cdrom_device_info *cdi, int slot_nr)
356 The function *drive_status*, if implemented, should provide
357 information on the status of the drive (not the status of the disc,
358 which may or may not be in the drive). If the drive is not a changer,
359 *slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::
362 CDS_NO_INFO /* no information available */
363 CDS_NO_DISC /* no disc is inserted, tray is closed */
364 CDS_TRAY_OPEN /* tray is opened */
365 CDS_DRIVE_NOT_READY /* something is wrong, tray is moving? */
366 CDS_DISC_OK /* a disc is loaded and everything is fine */
370 int tray_move(struct cdrom_device_info *cdi, int position)
372 This function, if implemented, should control the tray movement. (No
373 other function should control this.) The parameter *position* controls
374 the desired direction of movement:
379 This function returns 0 upon success, and a non-zero value upon
380 error. Note that if the tray is already in the desired position, no
381 action need be taken, and the return value should be 0.
385 int lock_door(struct cdrom_device_info *cdi, int lock)
387 This function (and no other code) controls locking of the door, if the
388 drive allows this. The value of *lock* controls the desired locking
391 - 0 Unlock door, manual opening is allowed
392 - 1 Lock door, tray cannot be ejected manually
394 This function returns 0 upon success, and a non-zero value upon
395 error. Note that if the door is already in the requested state, no
396 action need be taken, and the return value should be 0.
400 int select_speed(struct cdrom_device_info *cdi, int speed)
402 Some CD-ROM drives are capable of changing their head-speed. There
403 are several reasons for changing the speed of a CD-ROM drive. Badly
404 pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
405 CD-ROM drives can obtain very high head rates (up to *24x* is
406 common). It has been reported that these drives can make reading
407 errors at these high speeds, reducing the speed can prevent data loss
408 in these circumstances. Finally, some of these drives can
409 make an annoyingly loud noise, which a lower speed may reduce.
411 This function specifies the speed at which data is read or audio is
412 played back. The value of *speed* specifies the head-speed of the
413 drive, measured in units of standard cdrom speed (176kB/sec raw data
414 or 150kB/sec file system data). So to request that a CD-ROM drive
415 operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
416 with *speed=2*. The special value `0` means `auto-selection`, i. e.,
417 maximum data-rate or real-time audio rate. If the drive doesn't have
418 this `auto-selection` capability, the decision should be made on the
419 current disc loaded and the return value should be positive. A negative
420 return value indicates an error.
424 int select_disc(struct cdrom_device_info *cdi, int number)
426 If the drive can store multiple discs (a juke-box) this function
427 will perform disc selection. It should return the number of the
428 selected disc on success, a negative value on error. Currently, only
429 the ide-cd driver supports this functionality.
433 int get_last_session(struct cdrom_device_info *cdi,
434 struct cdrom_multisession *ms_info)
436 This function should implement the old corresponding *ioctl()*. For
437 device *cdi->dev*, the start of the last session of the current disc
438 should be returned in the pointer argument *ms_info*. Note that
439 routines in `cdrom.c` have sanitized this argument: its requested
440 format will **always** be of the type *CDROM_LBA* (linear block
441 addressing mode), whatever the calling software requested. But
442 sanitization goes even further: the low-level implementation may
443 return the requested information in *CDROM_MSF* format if it wishes so
444 (setting the *ms_info->addr_format* field appropriately, of
445 course) and the routines in `cdrom.c` will make the transformation if
446 necessary. The return value is 0 upon success.
450 int get_mcn(struct cdrom_device_info *cdi,
451 struct cdrom_mcn *mcn)
453 Some discs carry a `Media Catalog Number` (MCN), also called
454 `Universal Product Code` (UPC). This number should reflect the number
455 that is generally found in the bar-code on the product. Unfortunately,
456 the few discs that carry such a number on the disc don't even use the
457 same format. The return argument to this function is a pointer to a
458 pre-declared memory region of type *struct cdrom_mcn*. The MCN is
459 expected as a 13-character string, terminated by a null-character.
463 int reset(struct cdrom_device_info *cdi)
465 This call should perform a hard-reset on the drive (although in
466 circumstances that a hard-reset is necessary, a drive may very well not
467 listen to commands anymore). Preferably, control is returned to the
468 caller only after the drive has finished resetting. If the drive is no
469 longer listening, it may be wise for the underlying low-level cdrom
474 int audio_ioctl(struct cdrom_device_info *cdi,
475 unsigned int cmd, void *arg)
477 Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
478 implemented by the routines described above, and hence the function
479 *cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
480 audio-control. We have decided to leave these to be accessed through a
481 single function, repeating the arguments *cmd* and *arg*. Note that
482 the latter is of type *void*, rather than *unsigned long int*.
483 The routine *cdrom_ioctl()* does do some useful things,
484 though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
485 Seconds, Frames) for all audio calls. It also verifies the memory
486 location of *arg*, and reserves stack-memory for the argument. This
487 makes implementation of the *audio_ioctl()* much simpler than in the
488 old driver scheme. For example, you may look up the function
489 *cm206_audio_ioctl()* `cm206.c` that should be updated with
492 An unimplemented ioctl should return *-ENOSYS*, but a harmless request
493 (e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
494 errors should be according to the standards, whatever they are. When
495 an error is returned by the low-level driver, the Uniform CD-ROM Driver
496 tries whenever possible to return the error code to the calling program.
497 (We may decide to sanitize the return value in *cdrom_ioctl()* though, in
498 order to guarantee a uniform interface to the audio-player software.)
502 int dev_ioctl(struct cdrom_device_info *cdi,
503 unsigned int cmd, unsigned long arg)
505 Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
506 they are introduced to service some capabilities of certain drives. In
507 fact, there are 6 different *ioctl()'s* for reading data, either in some
508 particular kind of format, or audio data. Not many drives support
509 reading audio tracks as data, I believe this is because of protection
510 of copyrights of artists. Moreover, I think that if audio-tracks are
511 supported, it should be done through the VFS and not via *ioctl()'s*. A
512 problem here could be the fact that audio-frames are 2352 bytes long,
513 so either the audio-file-system should ask for 75264 bytes at once
514 (the least common multiple of 512 and 2352), or the drivers should
515 bend their backs to cope with this incoherence (to which I would be
516 opposed). Furthermore, it is very difficult for the hardware to find
517 the exact frame boundaries, since there are no synchronization headers
518 in audio frames. Once these issues are resolved, this code should be
519 standardized in `cdrom.c`.
521 Because there are so many *ioctl()'s* that seem to be introduced to
522 satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
523 are routed through the call *dev_ioctl()*. In principle, `private`
524 *ioctl()*\ 's should be numbered after the device's major number, and not
525 the general CD-ROM *ioctl* number, `0x53`. Currently the
526 non-supported *ioctl()'s* are:
528 CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW,
529 CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL
533 Is there software around that actually uses these? I'd be interested!
535 .. _cdrom_capabilities:
540 Instead of just implementing some *ioctl* calls, the interface in
541 `cdrom.c` supplies the possibility to indicate the **capabilities**
542 of a CD-ROM drive. This can be done by ORing any number of
543 capability-constants that are defined in `cdrom.h` at the registration
544 phase. Currently, the capabilities are any of::
546 CDC_CLOSE_TRAY /* can close tray by software control */
547 CDC_OPEN_TRAY /* can open tray */
548 CDC_LOCK /* can lock and unlock the door */
549 CDC_SELECT_SPEED /* can select speed, in units of * sim*150 ,kB/s */
550 CDC_SELECT_DISC /* drive is juke-box */
551 CDC_MULTI_SESSION /* can read sessions *> rm1* */
552 CDC_MCN /* can read Media Catalog Number */
553 CDC_MEDIA_CHANGED /* can report if disc has changed */
554 CDC_PLAY_AUDIO /* can perform audio-functions (play, pause, etc) */
555 CDC_RESET /* hard reset device */
556 CDC_IOCTLS /* driver has non-standard ioctls */
557 CDC_DRIVE_STATUS /* driver implements drive status */
559 The capability flag is declared *const*, to prevent drivers from
560 accidentally tampering with the contents. The capability flags actually
561 inform `cdrom.c` of what the driver can do. If the drive found
562 by the driver does not have the capability, is can be masked out by
563 the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
564 driver has implemented the code for loading and ejecting CD-ROM's, and
565 hence its corresponding flags in *capability* will be set. But a SCSI
566 CD-ROM drive might be a caddy system, which can't load the tray, and
567 hence for this drive the *cdrom_device_info* struct will have set
568 the *CDC_CLOSE_TRAY* bit in *mask*.
570 In the file `cdrom.c` you will encounter many constructions of the type::
572 if (cdo->capability & ~cdi->mask & CDC _<capability>) ...
574 There is no *ioctl* to set the mask... The reason is that
575 I think it is better to control the **behavior** rather than the
581 A final flag register controls the **behavior** of the CD-ROM
582 drives, in order to satisfy different users' wishes, hopefully
583 independently of the ideas of the respective author who happened to
584 have made the drive's support available to the Linux community. The
585 current behavior options are::
587 CDO_AUTO_CLOSE /* try to close tray upon device open() */
588 CDO_AUTO_EJECT /* try to open tray on last device close() */
589 CDO_USE_FFLAGS /* use file_pointer->f_flags to indicate purpose for open() */
590 CDO_LOCK /* try to lock door if device is opened */
591 CDO_CHECK_TYPE /* ensure disc type is data if opened for data */
593 The initial value of this register is
594 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
595 interface and software standards. Before you protest, there are two
596 new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
597 behavior by software. These are::
599 CDROM_SET_OPTIONS /* set options specified in (int)arg */
600 CDROM_CLEAR_OPTIONS /* clear options specified in (int)arg */
602 One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
603 newsection we explain what the need for this option is.
605 A software package `setcd`, available from the Debian distribution
606 and `sunsite.unc.edu`, allows user level control of these flags.
609 The need to know the purpose of opening the CD-ROM device
610 =========================================================
612 Traditionally, Unix devices can be used in two different `modes`,
613 either by reading/writing to the device file, or by issuing
614 controlling commands to the device, by the device's *ioctl()*
615 call. The problem with CD-ROM drives, is that they can be used for
616 two entirely different purposes. One is to mount removable
617 file systems, CD-ROM's, the other is to play audio CD's. Audio commands
618 are implemented entirely through *ioctl()\'s*, presumably because the
619 first implementation (SUN?) has been such. In principle there is
620 nothing wrong with this, but a good control of the `CD player` demands
621 that the device can **always** be opened in order to give the
622 *ioctl* commands, regardless of the state the drive is in.
624 On the other hand, when used as a removable-media disc drive (what the
625 original purpose of CD-ROM s is) we would like to make sure that the
626 disc drive is ready for operation upon opening the device. In the old
627 scheme, some CD-ROM drivers don't do any integrity checking, resulting
628 in a number of i/o errors reported by the VFS to the kernel when an
629 attempt for mounting a CD-ROM on an empty drive occurs. This is not a
630 particularly elegant way to find out that there is no CD-ROM inserted;
631 it more-or-less looks like the old IBM-PC trying to read an empty floppy
632 drive for a couple of seconds, after which the system complains it
633 can't read from it. Nowadays we can **sense** the existence of a
634 removable medium in a drive, and we believe we should exploit that
635 fact. An integrity check on opening of the device, that verifies the
636 availability of a CD-ROM and its correct type (data), would be
639 These two ways of using a CD-ROM drive, principally for data and
640 secondarily for playing audio discs, have different demands for the
641 behavior of the *open()* call. Audio use simply wants to open the
642 device in order to get a file handle which is needed for issuing
643 *ioctl* commands, while data use wants to open for correct and
644 reliable data transfer. The only way user programs can indicate what
645 their *purpose* of opening the device is, is through the *flags*
646 parameter (see `open(2)`). For CD-ROM devices, these flags aren't
647 implemented (some drivers implement checking for write-related flags,
648 but this is not strictly necessary if the device file has correct
649 permission flags). Most option flags simply don't make sense to
650 CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
651 *O_SYNC* have no meaning to a CD-ROM.
653 We therefore propose to use the flag *O_NONBLOCK* to indicate
654 that the device is opened just for issuing *ioctl*
655 commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
656 subsequent calls to the device don't cause the calling process to
657 wait. We could interpret this as don't wait until someone has
658 inserted some valid data-CD-ROM. Thus, our proposal of the
659 implementation for the *open()* call for CD-ROM s is:
661 - If no other flags are set than *O_RDONLY*, the device is opened
662 for data transfer, and the return value will be 0 only upon successful
663 initialization of the transfer. The call may even induce some actions
664 on the CD-ROM, such as closing the tray.
665 - If the option flag *O_NONBLOCK* is set, opening will always be
666 successful, unless the whole device doesn't exist. The drive will take
667 no actions whatsoever.
669 And what about standards?
670 -------------------------
672 You might hesitate to accept this proposal as it comes from the
673 Linux community, and not from some standardizing institute. What
674 about SUN, SGI, HP and all those other Unix and hardware vendors?
675 Well, these companies are in the lucky position that they generally
676 control both the hardware and software of their supported products,
677 and are large enough to set their own standard. They do not have to
678 deal with a dozen or more different, competing hardware
679 configurations\ [#f3]_.
683 Incidentally, I think that SUN's approach to mounting CD-ROM s is very
684 good in origin: under Solaris a volume-daemon automatically mounts a
685 newly inserted CD-ROM under `/cdrom/*<volume-name>*`.
687 In my opinion they should have pushed this
688 further and have **every** CD-ROM on the local area network be
689 mounted at the similar location, i. e., no matter in which particular
690 machine you insert a CD-ROM, it will always appear at the same
691 position in the directory tree, on every system. When I wanted to
692 implement such a user-program for Linux, I came across the
693 differences in behavior of the various drivers, and the need for an
694 *ioctl* informing about media changes.
696 We believe that using *O_NONBLOCK* to indicate that a device is being opened
697 for *ioctl* commands only can be easily introduced in the Linux
698 community. All the CD-player authors will have to be informed, we can
699 even send in our own patches to the programs. The use of *O_NONBLOCK*
700 has most likely no influence on the behavior of the CD-players on
701 other operating systems than Linux. Finally, a user can always revert
702 to old behavior by a call to
703 *ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.
705 The preferred strategy of *open()*
706 ----------------------------------
708 The routines in `cdrom.c` are designed in such a way that run-time
709 configuration of the behavior of CD-ROM devices (of **any** type)
710 can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
711 modes of operation can be set:
713 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`
714 This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
715 the future.) If the device is not yet opened by any other process, and if
716 the device is being opened for data (*O_NONBLOCK* is not set) and the
717 tray is found to be open, an attempt to close the tray is made. Then,
718 it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
719 set, that it contains tracks of type `data mode 1`. Only if all tests
720 are passed is the return value zero. The door is locked to prevent file
721 system corruption. If the drive is opened for audio (*O_NONBLOCK* is
722 set), no actions are taken and a value of 0 will be returned.
724 `CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK`
725 This mimics the behavior of the current sbpcd-driver. The option flags are
726 ignored, the tray is closed on the first open, if necessary. Similarly,
727 the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
728 it is automatically ejected, such that the user can replace it.
730 We hope that these option can convince everybody (both driver
731 maintainers and user program developers) to adopt the new CD-ROM
732 driver scheme and option flag interpretation.
734 Description of routines in `cdrom.c`
735 ====================================
737 Only a few routines in `cdrom.c` are exported to the drivers. In this
738 new section we will discuss these, as well as the functions that `take
739 over` the CD-ROM interface to the kernel. The header file belonging
740 to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
741 file were placed in the file `ucdrom.h`, but this file has now been
742 merged back into `cdrom.h`.
746 struct file_operations cdrom_fops
748 The contents of this structure were described in cdrom_api_.
749 A pointer to this structure is assigned to the *fops* field
750 of the *struct gendisk*.
754 int register_cdrom(struct cdrom_device_info *cdi)
756 This function is used in about the same way one registers *cdrom_fops*
757 with the kernel, the device operations and information structures,
758 as described in cdrom_api_, should be registered with the
759 Uniform CD-ROM Driver::
761 register_cdrom(&<device>_info);
764 This function returns zero upon success, and non-zero upon
765 failure. The structure *<device>_info* should have a pointer to the
766 driver's *<device>_dops*, as in::
768 struct cdrom_device_info <device>_info = {
773 Note that a driver must have one static structure, *<device>_dops*, while
774 it may have as many structures *<device>_info* as there are minor devices
775 active. *Register_cdrom()* builds a linked list from these.
780 void unregister_cdrom(struct cdrom_device_info *cdi)
782 Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
783 the minor device from the list. If it was the last registered minor for
784 the low-level driver, this disconnects the registered device-operation
785 routines from the CD-ROM interface. This function returns zero upon
786 success, and non-zero upon failure.
790 int cdrom_open(struct inode * ip, struct file * fp)
792 This function is not called directly by the low-level drivers, it is
793 listed in the standard *cdrom_fops*. If the VFS opens a file, this
794 function becomes active. A strategy is implemented in this routine,
795 taking care of all capabilities and options that are set in the
796 *cdrom_device_ops* connected to the device. Then, the program flow is
797 transferred to the device_dependent *open()* call.
801 void cdrom_release(struct inode *ip, struct file *fp)
803 This function implements the reverse-logic of *cdrom_open()*, and then
804 calls the device-dependent *release()* routine. When the use-count has
805 reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
806 and *invalidate_buffers(dev)*.
813 int cdrom_ioctl(struct inode *ip, struct file *fp,
814 unsigned int cmd, unsigned long arg)
816 This function handles all the standard *ioctl* requests for CD-ROM
817 devices in a uniform way. The different calls fall into three
818 categories: *ioctl()'s* that can be directly implemented by device
819 operations, ones that are routed through the call *audio_ioctl()*, and
820 the remaining ones, that are presumable device-dependent. Generally, a
821 negative return value indicates an error.
823 Directly implemented *ioctl()'s*
824 --------------------------------
826 The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
827 calling device-operations in *cdrom_device_ops*, if implemented and
831 Requests the last session on a CD-ROM.
837 If *arg\not=0*, set behavior to auto-close (close
838 tray on first open) and auto-eject (eject on last release), otherwise
839 set behavior to non-moving on *open()* and *release()* calls.
841 Get the Media Catalog Number from a CD.
843 *Ioctl*s routed through *audio_ioctl()*
844 ---------------------------------------
846 The following set of *ioctl()'s* are all implemented through a call to
847 the *cdrom_fops* function *audio_ioctl()*. Memory checks and
848 allocation are performed in *cdrom_ioctl()*, and also sanitization of
849 address format (*CDROM_LBA*/*CDROM_MSF*) is done.
852 Get sub-channel data in argument *arg* of type
853 `struct cdrom_subchnl *`.
855 Read Table of Contents header, in *arg* of type
856 `struct cdrom_tochdr *`.
858 Read a Table of Contents entry in *arg* and specified by *arg*
859 of type `struct cdrom_tocentry *`.
861 Play audio fragment specified in Minute, Second, Frame format,
862 delimited by *arg* of type `struct cdrom_msf *`.
864 Play audio fragment in track-index format delimited by *arg*
865 of type `struct cdrom_ti *`.
867 Set volume specified by *arg* of type `struct cdrom_volctrl *`.
869 Read volume into by *arg* of type `struct cdrom_volctrl *`.
873 Stop playback of audio fragment.
875 Pause playback of audio fragment.
879 New *ioctl()'s* in `cdrom.c`
880 ----------------------------
882 The following *ioctl()'s* have been introduced to allow user programs to
883 control the behavior of individual CD-ROM devices. New *ioctl*
884 commands can be identified by the underscores in their names.
887 Set options specified by *arg*. Returns the option flag register
888 after modification. Use *arg = \rm0* for reading the current flags.
889 `CDROM_CLEAR_OPTIONS`
890 Clear options specified by *arg*. Returns the option flag register
893 Select head-rate speed of disc specified as by *arg* in units
894 of standard cdrom speed (176\,kB/sec raw data or
895 150kB/sec file system data). The value 0 means `auto-select`,
896 i. e., play audio discs at real time and data discs at maximum speed.
897 The value *arg* is checked against the maximum head rate of the
898 drive found in the *cdrom_dops*.
900 Select disc numbered *arg* from a juke-box.
902 First disc is numbered 0. The number *arg* is checked against the
903 maximum number of discs in the juke-box found in the *cdrom_dops*.
904 `CDROM_MEDIA_CHANGED`
905 Returns 1 if a disc has been changed since the last call.
906 For juke-boxes, an extra argument *arg*
907 specifies the slot for which the information is given. The special
908 value *CDSL_CURRENT* requests that information about the currently
909 selected slot be returned.
911 Returns the status of the drive by a call to
912 *drive_status()*. Return values are defined in cdrom_drive_status_.
913 Note that this call doesn't return information on the
914 current playing activity of the drive; this can be polled through
915 an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
916 *arg* specifies the slot for which (possibly limited) information is
917 given. The special value *CDSL_CURRENT* requests that information
918 about the currently selected slot be returned.
920 Returns the type of the disc currently in the drive.
921 It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
922 This *ioctl* can provide *some* information about the current
923 disc that is inserted in the drive. This functionality used to be
924 implemented in the low level drivers, but is now carried out
925 entirely in Uniform CD-ROM Driver.
927 The history of development of the CD's use as a carrier medium for
928 various digital information has lead to many different disc types.
929 This *ioctl* is useful only in the case that CDs have \emph {only
930 one} type of data on them. While this is often the case, it is
931 also very common for CDs to have some tracks with data, and some
932 tracks with audio. Because this is an existing interface, rather
933 than fixing this interface by changing the assumptions it was made
934 under, thereby breaking all user applications that use this
935 function, the Uniform CD-ROM Driver implements this *ioctl* as
936 follows: If the CD in question has audio tracks on it, and it has
937 absolutely no CD-I, XA, or data tracks on it, it will be reported
938 as *CDS_AUDIO*. If it has both audio and data tracks, it will
939 return *CDS_MIXED*. If there are no audio tracks on the disc, and
940 if the CD in question has any CD-I tracks on it, it will be
941 reported as *CDS_XA_2_2*. Failing that, if the CD in question
942 has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
943 Finally, if the CD in question has any data tracks on it,
944 it will be reported as a data CD (*CDS_DATA_1*).
946 This *ioctl* can return::
948 CDS_NO_INFO /* no information available */
949 CDS_NO_DISC /* no disc is inserted, or tray is opened */
950 CDS_AUDIO /* Audio disc (2352 audio bytes/frame) */
951 CDS_DATA_1 /* data disc, mode 1 (2048 user bytes/frame) */
952 CDS_XA_2_1 /* mixed data (XA), mode 2, form 1 (2048 user bytes) */
953 CDS_XA_2_2 /* mixed data (XA), mode 2, form 1 (2324 user bytes) */
954 CDS_MIXED /* mixed audio/data disc */
956 For some information concerning frame layout of the various disc
957 types, see a recent version of `cdrom.h`.
959 `CDROM_CHANGER_NSLOTS`
960 Returns the number of slots in a juke-box.
963 `CDROM_GET_CAPABILITY`
964 Returns the *capability* flags for the drive. Refer to section
965 cdrom_capabilities_ for more information on these flags.
967 Locks the door of the drive. `arg == 0` unlocks the door,
968 any other value locks it.
970 Turns on debugging info. Only root is allowed to do this.
971 Same semantics as CDROM_LOCKDOOR.
974 Device dependent *ioctl()'s*
975 ----------------------------
977 Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
978 if implemented. No memory allocation or verification is carried out.
980 How to update your driver
981 =========================
983 - Make a backup of your current driver.
984 - Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
985 the directory tree that came with this documentation.
986 - Make sure you include `cdrom.h`.
987 - Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
989 - Just after that line, add the following to register with the Uniform
992 register_cdrom(&<your-drive>_info);*
994 Similarly, add a call to *unregister_cdrom()* at the appropriate place.
995 - Copy an example of the device-operations *struct* to your
996 source, e. g., from `cm206.c` *cm206_dops*, and change all
997 entries to names corresponding to your driver, or names you just
998 happen to like. If your driver doesn't support a certain function,
999 make the entry *NULL*. At the entry *capability* you should list all
1000 capabilities your driver currently supports. If your driver
1001 has a capability that is not listed, please send me a message.
1002 - Copy the *cdrom_device_info* declaration from the same example
1003 driver, and modify the entries according to your needs. If your
1004 driver dynamically determines the capabilities of the hardware, this
1005 structure should also be declared dynamically.
1006 - Implement all functions in your `<device>_dops` structure,
1007 according to prototypes listed in `cdrom.h`, and specifications given
1008 in cdrom_api_. Most likely you have already implemented
1009 the code in a large part, and you will almost certainly need to adapt the
1010 prototype and return values.
1011 - Rename your `<device>_ioctl()` function to *audio_ioctl* and
1012 change the prototype a little. Remove entries listed in the first
1013 part in cdrom_ioctl_, if your code was OK, these are
1014 just calls to the routines you adapted in the previous step.
1015 - You may remove all remaining memory checking code in the
1016 *audio_ioctl()* function that deals with audio commands (these are
1017 listed in the second part of cdrom_ioctl_. There is no
1018 need for memory allocation either, so most *case*s in the *switch*
1019 statement look similar to::
1021 case CDROMREADTOCENTRY:
1022 get_toc_entry\bigl((struct cdrom_tocentry *) arg);
1024 - All remaining *ioctl* cases must be moved to a separate
1025 function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
1026 memory checking and allocation must be kept in this code!
1027 - Change the prototypes of *<device>_open()* and
1028 *<device>_release()*, and remove any strategic code (i. e., tray
1029 movement, door locking, etc.).
1030 - Try to recompile the drivers. We advise you to use modules, both
1031 for `cdrom.o` and your driver, as debugging is much easier this
1037 Thanks to all the people involved. First, Erik Andersen, who has
1038 taken over the torch in maintaining `cdrom.c` and integrating much
1039 CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
1040 Gerd Knorr, who were the first to implement this interface for SCSI
1041 and IDE-CD drivers and added many ideas for extension of the data
1042 structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt,
1043 Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll,
1044 the Linux CD-ROM device driver developers who were kind
1045 enough to give suggestions and criticisms during the writing. Finally
1046 of course, I want to thank Linus Torvalds for making this possible in
projects / platform / kernel / linux-rpi.git / blob