| blob | 6c1303cff159e16daba2bf7c9a5fe8b72a3ebcad |
1 =======================
2 A Linux CD-ROM standard
3 =======================
5 :Author: David van Leeuwen <david@ElseWare.cistron.nl>
6 :Date: 12 March 1999
7 :Updated by: Erik Andersen (andersee@debian.org)
8 :Updated by: Jens Axboe (axboe@image.dk)
11 Introduction
12 ============
14 Linux is probably the Unix-like operating system that supports
15 the widest variety of hardware devices. The reasons for this are
16 presumably
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
21 driver for Linux.
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.
75 .. [#f1]
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
89 hardware will allow).
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.
114 .. _cdrom_api:
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
124 been discussed.
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 = {
149 NULL, /* lseek */
150 block _read , /* read--general block-dev read */
151 block _write, /* write--general block-dev write */
152 NULL, /* readdir */
153 NULL, /* select */
154 cdrom_ioctl, /* ioctl */
155 NULL, /* mmap */
156 cdrom_open, /* open */
157 cdrom_release, /* release */
158 NULL, /* fsync */
159 NULL, /* fasync */
160 NULL /* revalidate */
161 };
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
170 devices).
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:
179 cdrom_device_ops
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).
185 cdrom_device_info
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
199 low-level driver.
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 *,
216 unsigned int, int);
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 *, unsigned long);
221 int (*get_last_session) (struct cdrom_device_info *,
222 struct cdrom_multisession *);
223 int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
224 int (*reset)(struct cdrom_device_info *);
225 int (*audio_ioctl)(struct cdrom_device_info *,
226 unsigned int, void *);
227 const int capability; /* capability flags */
228 int (*generic_packet)(struct cdrom_device_info *,
229 struct packet_command *);
230 };
232 When a low-level device driver implements one of these capabilities,
233 it should add a function pointer to this *struct*. When a particular
234 function is not implemented, however, this *struct* should contain a
235 NULL instead. The *capability* flags specify the capabilities of the
236 CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
237 is registered with the Uniform CD-ROM Driver.
239 Note that most functions have fewer parameters than their
240 *blkdev_fops* counterparts. This is because very little of the
241 information in the structures *inode* and *file* is used. For most
242 drivers, the main parameter is the *struct* *cdrom_device_info*, from
243 which the major and minor number can be extracted. (Most low-level
244 CD-ROM drivers don't even look at the major and minor number though,
245 since many of them only support one device.) This will be available
246 through *dev* in *cdrom_device_info* described below.
248 The drive-specific, minor-like information that is registered with
249 `cdrom.c`, currently contains the following fields::
251 struct cdrom_device_info {
252 const struct cdrom_device_ops * ops; /* device operations for this major */
253 struct list_head list; /* linked list of all device_info */
254 struct gendisk * disk; /* matching block layer disk */
255 void * handle; /* driver-dependent data */
257 int mask; /* mask of capability: disables them */
258 int speed; /* maximum speed for reading data */
259 int capacity; /* number of discs in a jukebox */
261 unsigned int options:30; /* options flags */
262 unsigned mc_flags:2; /* media-change buffer flags */
263 unsigned int vfs_events; /* cached events for vfs path */
264 unsigned int ioctl_events; /* cached events for ioctl path */
265 int use_count; /* number of times device is opened */
266 char name[20]; /* name of the device type */
268 __u8 sanyo_slot : 2; /* Sanyo 3-CD changer support */
269 __u8 keeplocked : 1; /* CDROM_LOCKDOOR status */
270 __u8 reserved : 5; /* not used yet */
271 int cdda_method; /* see CDDA_* flags */
272 __u8 last_sense; /* saves last sense key */
273 __u8 media_written; /* dirty flag, DVD+RW bookkeeping */
274 unsigned short mmc3_profile; /* current MMC3 profile */
275 int for_data; /* unknown:TBD */
276 int (*exit)(struct cdrom_device_info *);/* unknown:TBD */
277 int mrw_mode_page; /* which MRW mode page is in use */
278 };
280 Using this *struct*, a linked list of the registered minor devices is
281 built, using the *next* field. The device number, the device operations
282 struct and specifications of properties of the drive are stored in this
283 structure.
285 The *mask* flags can be used to mask out some of the capabilities listed
286 in *ops->capability*, if a specific drive doesn't support a feature
287 of the driver. The value *speed* specifies the maximum head-rate of the
288 drive, measured in units of normal audio speed (176kB/sec raw data or
289 150kB/sec file system data). The parameters are declared *const*
290 because they describe properties of the drive, which don't change after
291 registration.
293 A few registers contain variables local to the CD-ROM drive. The
294 flags *options* are used to specify how the general CD-ROM routines
295 should behave. These various flags registers should provide enough
296 flexibility to adapt to the different users' wishes (and **not** the
297 `arbitrary` wishes of the author of the low-level device driver, as is
298 the case in the old scheme). The register *mc_flags* is used to buffer
299 the information from *media_changed()* to two separate queues. Other
300 data that is specific to a minor drive, can be accessed through *handle*,
301 which can point to a data structure specific to the low-level driver.
302 The fields *use_count*, *next*, *options* and *mc_flags* need not be
303 initialized.
305 The intermediate software layer that `cdrom.c` forms will perform some
306 additional bookkeeping. The use count of the device (the number of
307 processes that have the device opened) is registered in *use_count*. The
308 function *cdrom_ioctl()* will verify the appropriate user-memory regions
309 for read and write, and in case a location on the CD is transferred,
310 it will `sanitize` the format by making requests to the low-level
311 drivers in a standard format, and translating all formats between the
312 user-software and low level drivers. This relieves much of the drivers'
313 memory checking and format checking and translation. Also, the necessary
314 structures will be declared on the program stack.
316 The implementation of the functions should be as defined in the
317 following sections. Two functions **must** be implemented, namely
318 *open()* and *release()*. Other functions may be omitted, their
319 corresponding capability flags will be cleared upon registration.
320 Generally, a function returns zero on success and negative on error. A
321 function call should return only after the command has completed, but of
322 course waiting for the device should not use processor time.
324 ::
326 int open(struct cdrom_device_info *cdi, int purpose)
328 *Open()* should try to open the device for a specific *purpose*, which
329 can be either:
331 - Open for reading data, as done by `mount()` (2), or the
332 user commands `dd` or `cat`.
333 - Open for *ioctl* commands, as done by audio-CD playing programs.
335 Notice that any strategic code (closing tray upon *open()*, etc.) is
336 done by the calling routine in `cdrom.c`, so the low-level routine
337 should only be concerned with proper initialization, such as spinning
338 up the disc, etc.
340 ::
342 void release(struct cdrom_device_info *cdi)
344 Device-specific actions should be taken such as spinning down the device.
345 However, strategic actions such as ejection of the tray, or unlocking
346 the door, should be left over to the general routine *cdrom_release()*.
347 This is the only function returning type *void*.
349 .. _cdrom_drive_status:
351 ::
353 int drive_status(struct cdrom_device_info *cdi, int slot_nr)
355 The function *drive_status*, if implemented, should provide
356 information on the status of the drive (not the status of the disc,
357 which may or may not be in the drive). If the drive is not a changer,
358 *slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::
361 CDS_NO_INFO /* no information available */
362 CDS_NO_DISC /* no disc is inserted, tray is closed */
363 CDS_TRAY_OPEN /* tray is opened */
364 CDS_DRIVE_NOT_READY /* something is wrong, tray is moving? */
365 CDS_DISC_OK /* a disc is loaded and everything is fine */
367 ::
369 int tray_move(struct cdrom_device_info *cdi, int position)
371 This function, if implemented, should control the tray movement. (No
372 other function should control this.) The parameter *position* controls
373 the desired direction of movement:
375 - 0 Close tray
376 - 1 Open tray
378 This function returns 0 upon success, and a non-zero value upon
379 error. Note that if the tray is already in the desired position, no
380 action need be taken, and the return value should be 0.
382 ::
384 int lock_door(struct cdrom_device_info *cdi, int lock)
386 This function (and no other code) controls locking of the door, if the
387 drive allows this. The value of *lock* controls the desired locking
388 state:
390 - 0 Unlock door, manual opening is allowed
391 - 1 Lock door, tray cannot be ejected manually
393 This function returns 0 upon success, and a non-zero value upon
394 error. Note that if the door is already in the requested state, no
395 action need be taken, and the return value should be 0.
397 ::
399 int select_speed(struct cdrom_device_info *cdi, unsigned long speed)
401 Some CD-ROM drives are capable of changing their head-speed. There
402 are several reasons for changing the speed of a CD-ROM drive. Badly
403 pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
404 CD-ROM drives can obtain very high head rates (up to *24x* is
405 common). It has been reported that these drives can make reading
406 errors at these high speeds, reducing the speed can prevent data loss
407 in these circumstances. Finally, some of these drives can
408 make an annoyingly loud noise, which a lower speed may reduce.
410 This function specifies the speed at which data is read or audio is
411 played back. The value of *speed* specifies the head-speed of the
412 drive, measured in units of standard cdrom speed (176kB/sec raw data
413 or 150kB/sec file system data). So to request that a CD-ROM drive
414 operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
415 with *speed=2*. The special value `0` means `auto-selection`, i. e.,
416 maximum data-rate or real-time audio rate. If the drive doesn't have
417 this `auto-selection` capability, the decision should be made on the
418 current disc loaded and the return value should be positive. A negative
419 return value indicates an error.
421 ::
423 int get_last_session(struct cdrom_device_info *cdi,
424 struct cdrom_multisession *ms_info)
426 This function should implement the old corresponding *ioctl()*. For
427 device *cdi->dev*, the start of the last session of the current disc
428 should be returned in the pointer argument *ms_info*. Note that
429 routines in `cdrom.c` have sanitized this argument: its requested
430 format will **always** be of the type *CDROM_LBA* (linear block
431 addressing mode), whatever the calling software requested. But
432 sanitization goes even further: the low-level implementation may
433 return the requested information in *CDROM_MSF* format if it wishes so
434 (setting the *ms_info->addr_format* field appropriately, of
435 course) and the routines in `cdrom.c` will make the transformation if
436 necessary. The return value is 0 upon success.
438 ::
440 int get_mcn(struct cdrom_device_info *cdi,
441 struct cdrom_mcn *mcn)
443 Some discs carry a `Media Catalog Number` (MCN), also called
444 `Universal Product Code` (UPC). This number should reflect the number
445 that is generally found in the bar-code on the product. Unfortunately,
446 the few discs that carry such a number on the disc don't even use the
447 same format. The return argument to this function is a pointer to a
448 pre-declared memory region of type *struct cdrom_mcn*. The MCN is
449 expected as a 13-character string, terminated by a null-character.
451 ::
453 int reset(struct cdrom_device_info *cdi)
455 This call should perform a hard-reset on the drive (although in
456 circumstances that a hard-reset is necessary, a drive may very well not
457 listen to commands anymore). Preferably, control is returned to the
458 caller only after the drive has finished resetting. If the drive is no
459 longer listening, it may be wise for the underlying low-level cdrom
460 driver to time out.
462 ::
464 int audio_ioctl(struct cdrom_device_info *cdi,
465 unsigned int cmd, void *arg)
467 Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
468 implemented by the routines described above, and hence the function
469 *cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
470 audio-control. We have decided to leave these to be accessed through a
471 single function, repeating the arguments *cmd* and *arg*. Note that
472 the latter is of type *void*, rather than *unsigned long int*.
473 The routine *cdrom_ioctl()* does do some useful things,
474 though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
475 Seconds, Frames) for all audio calls. It also verifies the memory
476 location of *arg*, and reserves stack-memory for the argument. This
477 makes implementation of the *audio_ioctl()* much simpler than in the
478 old driver scheme. For example, you may look up the function
479 *cm206_audio_ioctl()* `cm206.c` that should be updated with
480 this documentation.
482 An unimplemented ioctl should return *-ENOSYS*, but a harmless request
483 (e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
484 errors should be according to the standards, whatever they are. When
485 an error is returned by the low-level driver, the Uniform CD-ROM Driver
486 tries whenever possible to return the error code to the calling program.
487 (We may decide to sanitize the return value in *cdrom_ioctl()* though, in
488 order to guarantee a uniform interface to the audio-player software.)
490 ::
492 int dev_ioctl(struct cdrom_device_info *cdi,
493 unsigned int cmd, unsigned long arg)
495 Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
496 they are introduced to service some capabilities of certain drives. In
497 fact, there are 6 different *ioctl()'s* for reading data, either in some
498 particular kind of format, or audio data. Not many drives support
499 reading audio tracks as data, I believe this is because of protection
500 of copyrights of artists. Moreover, I think that if audio-tracks are
501 supported, it should be done through the VFS and not via *ioctl()'s*. A
502 problem here could be the fact that audio-frames are 2352 bytes long,
503 so either the audio-file-system should ask for 75264 bytes at once
504 (the least common multiple of 512 and 2352), or the drivers should
505 bend their backs to cope with this incoherence (to which I would be
506 opposed). Furthermore, it is very difficult for the hardware to find
507 the exact frame boundaries, since there are no synchronization headers
508 in audio frames. Once these issues are resolved, this code should be
509 standardized in `cdrom.c`.
511 Because there are so many *ioctl()'s* that seem to be introduced to
512 satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
513 are routed through the call *dev_ioctl()*. In principle, `private`
514 *ioctl()*\ 's should be numbered after the device's major number, and not
515 the general CD-ROM *ioctl* number, `0x53`. Currently the
516 non-supported *ioctl()'s* are:
518 CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW,
519 CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL
521 .. [#f2]
523 Is there software around that actually uses these? I'd be interested!
525 .. _cdrom_capabilities:
527 CD-ROM capabilities
528 -------------------
530 Instead of just implementing some *ioctl* calls, the interface in
531 `cdrom.c` supplies the possibility to indicate the **capabilities**
532 of a CD-ROM drive. This can be done by ORing any number of
533 capability-constants that are defined in `cdrom.h` at the registration
534 phase. Currently, the capabilities are any of::
536 CDC_CLOSE_TRAY /* can close tray by software control */
537 CDC_OPEN_TRAY /* can open tray */
538 CDC_LOCK /* can lock and unlock the door */
539 CDC_SELECT_SPEED /* can select speed, in units of * sim*150 ,kB/s */
540 CDC_SELECT_DISC /* drive is juke-box */
541 CDC_MULTI_SESSION /* can read sessions *> rm1* */
542 CDC_MCN /* can read Media Catalog Number */
543 CDC_MEDIA_CHANGED /* can report if disc has changed */
544 CDC_PLAY_AUDIO /* can perform audio-functions (play, pause, etc) */
545 CDC_RESET /* hard reset device */
546 CDC_IOCTLS /* driver has non-standard ioctls */
547 CDC_DRIVE_STATUS /* driver implements drive status */
549 The capability flag is declared *const*, to prevent drivers from
550 accidentally tampering with the contents. The capability flags actually
551 inform `cdrom.c` of what the driver can do. If the drive found
552 by the driver does not have the capability, is can be masked out by
553 the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
554 driver has implemented the code for loading and ejecting CD-ROM's, and
555 hence its corresponding flags in *capability* will be set. But a SCSI
556 CD-ROM drive might be a caddy system, which can't load the tray, and
557 hence for this drive the *cdrom_device_info* struct will have set
558 the *CDC_CLOSE_TRAY* bit in *mask*.
560 In the file `cdrom.c` you will encounter many constructions of the type::
562 if (cdo->capability & ~cdi->mask & CDC _<capability>) ...
564 There is no *ioctl* to set the mask... The reason is that
565 I think it is better to control the **behavior** rather than the
566 **capabilities**.
568 Options
569 -------
571 A final flag register controls the **behavior** of the CD-ROM
572 drives, in order to satisfy different users' wishes, hopefully
573 independently of the ideas of the respective author who happened to
574 have made the drive's support available to the Linux community. The
575 current behavior options are::
577 CDO_AUTO_CLOSE /* try to close tray upon device open() */
578 CDO_AUTO_EJECT /* try to open tray on last device close() */
579 CDO_USE_FFLAGS /* use file_pointer->f_flags to indicate purpose for open() */
580 CDO_LOCK /* try to lock door if device is opened */
581 CDO_CHECK_TYPE /* ensure disc type is data if opened for data */
583 The initial value of this register is
584 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
585 interface and software standards. Before you protest, there are two
586 new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
587 behavior by software. These are::
589 CDROM_SET_OPTIONS /* set options specified in (int)arg */
590 CDROM_CLEAR_OPTIONS /* clear options specified in (int)arg */
592 One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
593 newsection we explain what the need for this option is.
595 A software package `setcd`, available from the Debian distribution
596 and `sunsite.unc.edu`, allows user level control of these flags.
599 The need to know the purpose of opening the CD-ROM device
600 =========================================================
602 Traditionally, Unix devices can be used in two different `modes`,
603 either by reading/writing to the device file, or by issuing
604 controlling commands to the device, by the device's *ioctl()*
605 call. The problem with CD-ROM drives, is that they can be used for
606 two entirely different purposes. One is to mount removable
607 file systems, CD-ROM's, the other is to play audio CD's. Audio commands
608 are implemented entirely through *ioctl()\'s*, presumably because the
609 first implementation (SUN?) has been such. In principle there is
610 nothing wrong with this, but a good control of the `CD player` demands
611 that the device can **always** be opened in order to give the
612 *ioctl* commands, regardless of the state the drive is in.
614 On the other hand, when used as a removable-media disc drive (what the
615 original purpose of CD-ROM s is) we would like to make sure that the
616 disc drive is ready for operation upon opening the device. In the old
617 scheme, some CD-ROM drivers don't do any integrity checking, resulting
618 in a number of i/o errors reported by the VFS to the kernel when an
619 attempt for mounting a CD-ROM on an empty drive occurs. This is not a
620 particularly elegant way to find out that there is no CD-ROM inserted;
621 it more-or-less looks like the old IBM-PC trying to read an empty floppy
622 drive for a couple of seconds, after which the system complains it
623 can't read from it. Nowadays we can **sense** the existence of a
624 removable medium in a drive, and we believe we should exploit that
625 fact. An integrity check on opening of the device, that verifies the
626 availability of a CD-ROM and its correct type (data), would be
627 desirable.
629 These two ways of using a CD-ROM drive, principally for data and
630 secondarily for playing audio discs, have different demands for the
631 behavior of the *open()* call. Audio use simply wants to open the
632 device in order to get a file handle which is needed for issuing
633 *ioctl* commands, while data use wants to open for correct and
634 reliable data transfer. The only way user programs can indicate what
635 their *purpose* of opening the device is, is through the *flags*
636 parameter (see `open(2)`). For CD-ROM devices, these flags aren't
637 implemented (some drivers implement checking for write-related flags,
638 but this is not strictly necessary if the device file has correct
639 permission flags). Most option flags simply don't make sense to
640 CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
641 *O_SYNC* have no meaning to a CD-ROM.
643 We therefore propose to use the flag *O_NONBLOCK* to indicate
644 that the device is opened just for issuing *ioctl*
645 commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
646 subsequent calls to the device don't cause the calling process to
647 wait. We could interpret this as don't wait until someone has
648 inserted some valid data-CD-ROM. Thus, our proposal of the
649 implementation for the *open()* call for CD-ROM s is:
651 - If no other flags are set than *O_RDONLY*, the device is opened
652 for data transfer, and the return value will be 0 only upon successful
653 initialization of the transfer. The call may even induce some actions
654 on the CD-ROM, such as closing the tray.
655 - If the option flag *O_NONBLOCK* is set, opening will always be
656 successful, unless the whole device doesn't exist. The drive will take
657 no actions whatsoever.
659 And what about standards?
660 -------------------------
662 You might hesitate to accept this proposal as it comes from the
663 Linux community, and not from some standardizing institute. What
664 about SUN, SGI, HP and all those other Unix and hardware vendors?
665 Well, these companies are in the lucky position that they generally
666 control both the hardware and software of their supported products,
667 and are large enough to set their own standard. They do not have to
668 deal with a dozen or more different, competing hardware
669 configurations\ [#f3]_.
671 .. [#f3]
673 Incidentally, I think that SUN's approach to mounting CD-ROM s is very
674 good in origin: under Solaris a volume-daemon automatically mounts a
675 newly inserted CD-ROM under `/cdrom/*<volume-name>*`.
677 In my opinion they should have pushed this
678 further and have **every** CD-ROM on the local area network be
679 mounted at the similar location, i. e., no matter in which particular
680 machine you insert a CD-ROM, it will always appear at the same
681 position in the directory tree, on every system. When I wanted to
682 implement such a user-program for Linux, I came across the
683 differences in behavior of the various drivers, and the need for an
684 *ioctl* informing about media changes.
686 We believe that using *O_NONBLOCK* to indicate that a device is being opened
687 for *ioctl* commands only can be easily introduced in the Linux
688 community. All the CD-player authors will have to be informed, we can
689 even send in our own patches to the programs. The use of *O_NONBLOCK*
690 has most likely no influence on the behavior of the CD-players on
691 other operating systems than Linux. Finally, a user can always revert
692 to old behavior by a call to
693 *ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.
695 The preferred strategy of *open()*
696 ----------------------------------
698 The routines in `cdrom.c` are designed in such a way that run-time
699 configuration of the behavior of CD-ROM devices (of **any** type)
700 can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
701 modes of operation can be set:
703 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`
704 This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
705 the future.) If the device is not yet opened by any other process, and if
706 the device is being opened for data (*O_NONBLOCK* is not set) and the
707 tray is found to be open, an attempt to close the tray is made. Then,
708 it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
709 set, that it contains tracks of type `data mode 1`. Only if all tests
710 are passed is the return value zero. The door is locked to prevent file
711 system corruption. If the drive is opened for audio (*O_NONBLOCK* is
712 set), no actions are taken and a value of 0 will be returned.
714 `CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK`
715 This mimics the behavior of the current sbpcd-driver. The option flags are
716 ignored, the tray is closed on the first open, if necessary. Similarly,
717 the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
718 it is automatically ejected, such that the user can replace it.
720 We hope that these option can convince everybody (both driver
721 maintainers and user program developers) to adopt the new CD-ROM
722 driver scheme and option flag interpretation.
724 Description of routines in `cdrom.c`
725 ====================================
727 Only a few routines in `cdrom.c` are exported to the drivers. In this
728 new section we will discuss these, as well as the functions that `take
729 over` the CD-ROM interface to the kernel. The header file belonging
730 to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
731 file were placed in the file `ucdrom.h`, but this file has now been
732 merged back into `cdrom.h`.
734 ::
736 struct file_operations cdrom_fops
738 The contents of this structure were described in cdrom_api_.
739 A pointer to this structure is assigned to the *fops* field
740 of the *struct gendisk*.
742 ::
744 int register_cdrom(struct cdrom_device_info *cdi)
746 This function is used in about the same way one registers *cdrom_fops*
747 with the kernel, the device operations and information structures,
748 as described in cdrom_api_, should be registered with the
749 Uniform CD-ROM Driver::
751 register_cdrom(&<device>_info);
754 This function returns zero upon success, and non-zero upon
755 failure. The structure *<device>_info* should have a pointer to the
756 driver's *<device>_dops*, as in::
758 struct cdrom_device_info <device>_info = {
759 <device>_dops;
760 ...
761 }
763 Note that a driver must have one static structure, *<device>_dops*, while
764 it may have as many structures *<device>_info* as there are minor devices
765 active. *Register_cdrom()* builds a linked list from these.
768 ::
770 void unregister_cdrom(struct cdrom_device_info *cdi)
772 Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
773 the minor device from the list. If it was the last registered minor for
774 the low-level driver, this disconnects the registered device-operation
775 routines from the CD-ROM interface. This function returns zero upon
776 success, and non-zero upon failure.
778 ::
780 int cdrom_open(struct inode * ip, struct file * fp)
782 This function is not called directly by the low-level drivers, it is
783 listed in the standard *cdrom_fops*. If the VFS opens a file, this
784 function becomes active. A strategy is implemented in this routine,
785 taking care of all capabilities and options that are set in the
786 *cdrom_device_ops* connected to the device. Then, the program flow is
787 transferred to the device_dependent *open()* call.
789 ::
791 void cdrom_release(struct inode *ip, struct file *fp)
793 This function implements the reverse-logic of *cdrom_open()*, and then
794 calls the device-dependent *release()* routine. When the use-count has
795 reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
796 and *invalidate_buffers(dev)*.
799 .. _cdrom_ioctl:
801 ::
803 int cdrom_ioctl(struct inode *ip, struct file *fp,
804 unsigned int cmd, unsigned long arg)
806 This function handles all the standard *ioctl* requests for CD-ROM
807 devices in a uniform way. The different calls fall into three
808 categories: *ioctl()'s* that can be directly implemented by device
809 operations, ones that are routed through the call *audio_ioctl()*, and
810 the remaining ones, that are presumable device-dependent. Generally, a
811 negative return value indicates an error.
813 Directly implemented *ioctl()'s*
814 --------------------------------
816 The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
817 calling device-operations in *cdrom_device_ops*, if implemented and
818 not masked:
820 `CDROMMULTISESSION`
821 Requests the last session on a CD-ROM.
822 `CDROMEJECT`
823 Open tray.
824 `CDROMCLOSETRAY`
825 Close tray.
826 `CDROMEJECT_SW`
827 If *arg\not=0*, set behavior to auto-close (close
828 tray on first open) and auto-eject (eject on last release), otherwise
829 set behavior to non-moving on *open()* and *release()* calls.
830 `CDROM_GET_MCN`
831 Get the Media Catalog Number from a CD.
833 *Ioctl*s routed through *audio_ioctl()*
834 ---------------------------------------
836 The following set of *ioctl()'s* are all implemented through a call to
837 the *cdrom_fops* function *audio_ioctl()*. Memory checks and
838 allocation are performed in *cdrom_ioctl()*, and also sanitization of
839 address format (*CDROM_LBA*/*CDROM_MSF*) is done.
841 `CDROMSUBCHNL`
842 Get sub-channel data in argument *arg* of type
843 `struct cdrom_subchnl *`.
844 `CDROMREADTOCHDR`
845 Read Table of Contents header, in *arg* of type
846 `struct cdrom_tochdr *`.
847 `CDROMREADTOCENTRY`
848 Read a Table of Contents entry in *arg* and specified by *arg*
849 of type `struct cdrom_tocentry *`.
850 `CDROMPLAYMSF`
851 Play audio fragment specified in Minute, Second, Frame format,
852 delimited by *arg* of type `struct cdrom_msf *`.
853 `CDROMPLAYTRKIND`
854 Play audio fragment in track-index format delimited by *arg*
855 of type `struct cdrom_ti *`.
856 `CDROMVOLCTRL`
857 Set volume specified by *arg* of type `struct cdrom_volctrl *`.
858 `CDROMVOLREAD`
859 Read volume into by *arg* of type `struct cdrom_volctrl *`.
860 `CDROMSTART`
861 Spin up disc.
862 `CDROMSTOP`
863 Stop playback of audio fragment.
864 `CDROMPAUSE`
865 Pause playback of audio fragment.
866 `CDROMRESUME`
867 Resume playing.
869 New *ioctl()'s* in `cdrom.c`
870 ----------------------------
872 The following *ioctl()'s* have been introduced to allow user programs to
873 control the behavior of individual CD-ROM devices. New *ioctl*
874 commands can be identified by the underscores in their names.
876 `CDROM_SET_OPTIONS`
877 Set options specified by *arg*. Returns the option flag register
878 after modification. Use *arg = \rm0* for reading the current flags.
879 `CDROM_CLEAR_OPTIONS`
880 Clear options specified by *arg*. Returns the option flag register
881 after modification.
882 `CDROM_SELECT_SPEED`
883 Select head-rate speed of disc specified as by *arg* in units
884 of standard cdrom speed (176\,kB/sec raw data or
885 150kB/sec file system data). The value 0 means `auto-select`,
886 i. e., play audio discs at real time and data discs at maximum speed.
887 The value *arg* is checked against the maximum head rate of the
888 drive found in the *cdrom_dops*.
889 `CDROM_SELECT_DISC`
890 Select disc numbered *arg* from a juke-box.
892 First disc is numbered 0. The number *arg* is checked against the
893 maximum number of discs in the juke-box found in the *cdrom_dops*.
894 `CDROM_MEDIA_CHANGED`
895 Returns 1 if a disc has been changed since the last call.
896 For juke-boxes, an extra argument *arg*
897 specifies the slot for which the information is given. The special
898 value *CDSL_CURRENT* requests that information about the currently
899 selected slot be returned.
900 `CDROM_TIMED_MEDIA_CHANGE`
901 Checks whether the disc has been changed since a user supplied time
902 and returns the time of the last disc change.
904 *arg* is a pointer to a *cdrom_timed_media_change_info* struct.
905 *arg->last_media_change* may be set by calling code to signal
906 the timestamp of the last known media change (by the caller).
907 Upon successful return, this ioctl call will set
908 *arg->last_media_change* to the latest media change timestamp (in ms)
909 known by the kernel/driver and set *arg->has_changed* to 1 if
910 that timestamp is more recent than the timestamp set by the caller.
911 `CDROM_DRIVE_STATUS`
912 Returns the status of the drive by a call to
913 *drive_status()*. Return values are defined in cdrom_drive_status_.
914 Note that this call doesn't return information on the
915 current playing activity of the drive; this can be polled through
916 an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
917 *arg* specifies the slot for which (possibly limited) information is
918 given. The special value *CDSL_CURRENT* requests that information
919 about the currently selected slot be returned.
920 `CDROM_DISC_STATUS`
921 Returns the type of the disc currently in the drive.
922 It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
923 This *ioctl* can provide *some* information about the current
924 disc that is inserted in the drive. This functionality used to be
925 implemented in the low level drivers, but is now carried out
926 entirely in Uniform CD-ROM Driver.
928 The history of development of the CD's use as a carrier medium for
929 various digital information has lead to many different disc types.
930 This *ioctl* is useful only in the case that CDs have \emph {only
931 one} type of data on them. While this is often the case, it is
932 also very common for CDs to have some tracks with data, and some
933 tracks with audio. Because this is an existing interface, rather
934 than fixing this interface by changing the assumptions it was made
935 under, thereby breaking all user applications that use this
936 function, the Uniform CD-ROM Driver implements this *ioctl* as
937 follows: If the CD in question has audio tracks on it, and it has
938 absolutely no CD-I, XA, or data tracks on it, it will be reported
939 as *CDS_AUDIO*. If it has both audio and data tracks, it will
940 return *CDS_MIXED*. If there are no audio tracks on the disc, and
941 if the CD in question has any CD-I tracks on it, it will be
942 reported as *CDS_XA_2_2*. Failing that, if the CD in question
943 has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
944 Finally, if the CD in question has any data tracks on it,
945 it will be reported as a data CD (*CDS_DATA_1*).
947 This *ioctl* can return::
949 CDS_NO_INFO /* no information available */
950 CDS_NO_DISC /* no disc is inserted, or tray is opened */
951 CDS_AUDIO /* Audio disc (2352 audio bytes/frame) */
952 CDS_DATA_1 /* data disc, mode 1 (2048 user bytes/frame) */
953 CDS_XA_2_1 /* mixed data (XA), mode 2, form 1 (2048 user bytes) */
954 CDS_XA_2_2 /* mixed data (XA), mode 2, form 1 (2324 user bytes) */
955 CDS_MIXED /* mixed audio/data disc */
957 For some information concerning frame layout of the various disc
958 types, see a recent version of `cdrom.h`.
960 `CDROM_CHANGER_NSLOTS`
961 Returns the number of slots in a juke-box.
962 `CDROMRESET`
963 Reset the drive.
964 `CDROM_GET_CAPABILITY`
965 Returns the *capability* flags for the drive. Refer to section
966 cdrom_capabilities_ for more information on these flags.
967 `CDROM_LOCKDOOR`
968 Locks the door of the drive. `arg == 0` unlocks the door,
969 any other value locks it.
970 `CDROM_DEBUG`
971 Turns on debugging info. Only root is allowed to do this.
972 Same semantics as CDROM_LOCKDOOR.
975 Device dependent *ioctl()'s*
976 ----------------------------
978 Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
979 if implemented. No memory allocation or verification is carried out.
981 How to update your driver
982 =========================
984 - Make a backup of your current driver.
985 - Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
986 the directory tree that came with this documentation.
987 - Make sure you include `cdrom.h`.
988 - Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
989 to `&cdrom_fops`.
990 - Just after that line, add the following to register with the Uniform
991 CD-ROM Driver::
993 register_cdrom(&<your-drive>_info);*
995 Similarly, add a call to *unregister_cdrom()* at the appropriate place.
996 - Copy an example of the device-operations *struct* to your
997 source, e. g., from `cm206.c` *cm206_dops*, and change all
998 entries to names corresponding to your driver, or names you just
999 happen to like. If your driver doesn't support a certain function,
1000 make the entry *NULL*. At the entry *capability* you should list all
1001 capabilities your driver currently supports. If your driver
1002 has a capability that is not listed, please send me a message.
1003 - Copy the *cdrom_device_info* declaration from the same example
1004 driver, and modify the entries according to your needs. If your
1005 driver dynamically determines the capabilities of the hardware, this
1006 structure should also be declared dynamically.
1007 - Implement all functions in your `<device>_dops` structure,
1008 according to prototypes listed in `cdrom.h`, and specifications given
1009 in cdrom_api_. Most likely you have already implemented
1010 the code in a large part, and you will almost certainly need to adapt the
1011 prototype and return values.
1012 - Rename your `<device>_ioctl()` function to *audio_ioctl* and
1013 change the prototype a little. Remove entries listed in the first
1014 part in cdrom_ioctl_, if your code was OK, these are
1015 just calls to the routines you adapted in the previous step.
1016 - You may remove all remaining memory checking code in the
1017 *audio_ioctl()* function that deals with audio commands (these are
1018 listed in the second part of cdrom_ioctl_. There is no
1019 need for memory allocation either, so most *case*s in the *switch*
1020 statement look similar to::
1022 case CDROMREADTOCENTRY:
1023 get_toc_entry\bigl((struct cdrom_tocentry *) arg);
1025 - All remaining *ioctl* cases must be moved to a separate
1026 function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
1027 memory checking and allocation must be kept in this code!
1028 - Change the prototypes of *<device>_open()* and
1029 *<device>_release()*, and remove any strategic code (i. e., tray
1030 movement, door locking, etc.).
1031 - Try to recompile the drivers. We advise you to use modules, both
1032 for `cdrom.o` and your driver, as debugging is much easier this
1033 way.
1035 Thanks
1036 ======
1038 Thanks to all the people involved. First, Erik Andersen, who has
1039 taken over the torch in maintaining `cdrom.c` and integrating much
1040 CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
1041 Gerd Knorr, who were the first to implement this interface for SCSI
1042 and IDE-CD drivers and added many ideas for extension of the data
1043 structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt,
1044 Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll,
1045 the Linux CD-ROM device driver developers who were kind
1046 enough to give suggestions and criticisms during the writing. Finally
1047 of course, I want to thank Linus Torvalds for making this possible in
1048 the first place.