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30 \maketitle
34 \linux\ is probably the Unix-like operating system that supports
35 the widest variety of hardware devices. The reasons for this are
36 presumably
38 \item
39 The large list of hardware devices available for the many platforms
41 \item
42 The open design of the operating system, such that anybody can write a
43 driver for \linux.
44 \item
45 There is plenty of source code around as examples of how to write a driver.
47 The openness of \linux, and the many different types of available
48 hardware has allowed \linux\ to support many different hardware devices.
49 Unfortunately, the very openness that has allowed \linux\ to support
50 all these different devices has also allowed the behavior of each
51 device driver to differ significantly from one device to another.
52 This divergence of behavior has been very significant for \cdrom\
53 devices; the way a particular drive reacts to a `standard' $ioctl()$
54 call varies greatly from one device driver to another. To avoid making
56 drivers generally created new device drivers by understanding, copying,
57 and then changing an existing one. Unfortunately, this practice did not
60 This document describes an effort to establish Uniform behavior across
62 defines the various $ioctl$s, and how the low-level \cdrom\ device
64 development kernels) several low-level \cdrom\ device drivers, including
65 both IDE/ATAPI and SCSI, now use this Uniform interface.
68 and the computer was not specified in the standards. As a result, many
69 different \cdrom\ interfaces were developed. Some of them had their
70 own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
71 manufacturers adopted an existing electrical interface and changed
72 the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
73 adapted their drives to one or more of the already existing electrical
74 interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
75 most of the `NoName' manufacturers). In cases where a new drive really
76 brought its own interface or used its own command set and flow control
77 scheme, either a separate driver had to be written, or an existing
78 driver had to be enhanced. History has delivered us \cdrom\ support for
79 many of these different interfaces. Nowadays, almost all new \cdrom\
80 drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
81 manufacturer will create a new interface. Even finding drives for the
82 old proprietary interfaces is getting difficult.
84 When (in the 1.3.70's) I looked at the existing software interface,
85 which was expressed through \cdromh, it appeared to be a rather wild
88 latest kernel that I was indirectly involved in.} It seemed that many
89 features of the software interface had been added to accommodate the
91 importantly, it appeared that the behavior of the `standard' commands
92 was different for most of the different drivers: \eg, some drivers
93 close the tray if an $open()$ call occurs when the tray is open, while
94 others do not. Some drivers lock the door upon opening the device, to
95 prevent an incoherent file system, but others don't, to allow software
96 ejection. Undoubtedly, the capabilities of the different drives vary,
97 but even when two drives have the same capability their drivers'
98 behavior was usually different.
101 drivers behave more uniformly. I began by contacting the developers of
105 driver is intended to be an additional software layer that sits on top
106 of the low-level device drivers for each \cdrom\ drive. By adding this
107 additional layer, it is possible to have all the different \cdrom\
109 hardware will allow).
112 have not yet taken steps to support this effort. The goal of \UCD\ is
113 simply to give people writing application programs for \cdrom\ drives
115 \cdrom\ devices. In addition, this also provides a consistent interface
116 between the low-level device driver code and the \linux\ kernel. Care
117 is taken that 100\,\% compatibility exists with the data structures and
118 programmer's interface defined in \cdromh. This guide was written to
120 defined in \cdromc.
122 Personally, I think that the most important hardware interfaces are
123 the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
124 of hardware drop continuously, it is also likely that people may have
125 more than one \cdrom\ drive, possibly of mixed types. It is important
126 that these drives behave in the same way. In December 1994, one of the
127 cheapest \cdrom\ drives was a Philips cm206, a double-speed proprietary
128 drive. In the months that I was busy writing a \linux\ driver for it,
129 proprietary drives became obsolete and IDE/ATAPI drives became the
130 standard. At the time of the last update to this document (November
137 At the time this document was conceived, all drivers directly
138 implemented the \cdrom\ $ioctl()$ calls through their own routines. This
139 led to the danger of different drivers forgetting to do important things
140 like checking that the user was giving the driver valid data. More
141 importantly, this led to the divergence of behavior, which has already
142 been discussed.
145 drive behavior, and to provide a common set of services to the various
147 software-level, that separates the $ioctl()$ and $open()$ implementation
148 from the actual hardware implementation. Note that this effort has
149 made few changes which will affect a user's application programs. The
150 greatest change involved moving the contents of the various low-level
151 \cdrom\ drivers' header files to the kernel's cdrom directory. This was
152 done to help ensure that the user is only presented with only one cdrom
153 interface, the interface defined in \cdromh.
156 block-devices such as floppy or hard disc drives), to define a set
158 These operations are different from the classical block-device file
159 operations, $<block-device>_fops$.
161 The routines for the \UCD\ interface level are implemented in the file
163 device by registering the following general $struct\ file_operations$:
164 $$
167 &NULL, & lseek \cr
168 &block_read, & read---general block-dev read \cr
169 &block_write, & write---general block-dev write \cr
170 &NULL, & readdir \cr
171 &NULL, & select \cr
172 &cdrom_ioctl, & ioctl \cr
173 &NULL, & mmap \cr
174 &cdrom_open, & open \cr
175 &cdrom_release, & release \cr
176 &NULL, & fsync \cr
177 &NULL, & fasync \cr
178 &cdrom_media_changed, & media change \cr
179 &NULL & revalidate \cr
181 }
182 $$
184 Every active \cdrom\ device shares this $struct$. The routines
185 declared above are all implemented in \cdromc, since this file is the
186 place where the behavior of all \cdrom-devices is defined and
187 standardized. The actual interface to the various types of \cdrom\
188 hardware is still performed by various low-level \cdrom-device
190 that are common to all \cdrom\ (and really, all removable-media
191 devices).
193 Registration of a low-level \cdrom\ device driver is now done through
194 the general routines in \cdromc, not through the Virtual File System
195 (VFS) any more. The interface implemented in \cdromc\ is carried out
196 through two general structures that contain information about the
197 capabilities of the driver, and the specific drives on which the
198 driver operates. The structures are:
201 This structure contains information about the low-level driver for a
202 \cdrom\ device. This structure is conceptually connected to the major
203 number of the device (although some drivers may have different
204 major numbers, as is the case for the IDE driver).
206 This structure contains information about a particular \cdrom\ drive,
207 such as its device name, speed, etc. This structure is conceptually
208 connected to the minor number of the device.
212 low-level device driver though a call to:
213 $$register_cdrom(struct\ cdrom_device_info * <device>_info)
214 $$
215 The device information structure, $<device>_info$, contains all the
216 information needed for the kernel to interface with the low-level
217 \cdrom\ device driver. One of the most important entries in this
218 structure is a pointer to the $cdrom_device_ops$ structure of the
219 low-level driver.
221 The device operations structure, $cdrom_device_ops$, contains a list
222 of pointers to the functions which are implemented in the low-level
224 through the functions in this structure. It is impossible to know all
225 the capabilities of future \cdrom\ drives, so it is expected that this
226 list may need to be expanded from time to time as new technologies are
227 developed. For example, CD-R and CD-R/W drives are beginning to become
228 popular, and support will soon need to be added for them. For now, the
229 current $struct$ is:
230 $$
234 &int& (* open)(struct\ cdrom_device_info *, int)\cr
236 &int& (* drive_status)(struct\ cdrom_device_info *, int);\cr
237 &int& (* media_changed)(struct\ cdrom_device_info *, int);\cr
238 &int& (* tray_move)(struct\ cdrom_device_info *, int);\cr
239 &int& (* lock_door)(struct\ cdrom_device_info *, int);\cr
240 &int& (* select_speed)(struct\ cdrom_device_info *, int);\cr
241 &int& (* select_disc)(struct\ cdrom_device_info *, int);\cr
242 &int& (* get_last_session) (struct\ cdrom_device_info *,
243 struct\ cdrom_multisession *{});\cr
244 &int& (* get_mcn)(struct\ cdrom_device_info *, struct\ cdrom_mcn *{});\cr
246 &int& (* audio_ioctl)(struct\ cdrom_device_info *, unsigned\ int,
247 void *{});\cr
248 &int& (* dev_ioctl)(struct\ cdrom_device_info *, unsigned\ int,
249 unsigned\ long);\cr
250 \noalign{\medskip}
251 &const\ int& capability;& capability flags \cr
252 &int& n_minors;& number of active minor devices \cr
253 \};\cr
254 }
255 $$
256 When a low-level device driver implements one of these capabilities,
257 it should add a function pointer to this $struct$. When a particular
258 function is not implemented, however, this $struct$ should contain a
259 NULL instead. The $capability$ flags specify the capabilities of the
260 \cdrom\ hardware and/or low-level \cdrom\ driver when a \cdrom\ drive
261 is registered with the \UCD. The value $n_minors$ should be a positive
262 value indicating the number of minor devices that are supported by
263 the low-level device driver, normally~1. Although these two variables
264 are `informative' rather than `operational,' they are included in
265 $cdrom_device_ops$ because they describe the capability of the {\em
266 driver\/} rather than the {\em drive}. Nomenclature has always been
267 difficult in computer programming.
269 Note that most functions have fewer parameters than their
270 $blkdev_fops$ counterparts. This is because very little of the
271 information in the structures $inode$ and $file$ is used. For most
272 drivers, the main parameter is the $struct$ $cdrom_device_info$, from
273 which the major and minor number can be extracted. (Most low-level
274 \cdrom\ drivers don't even look at the major and minor number though,
275 since many of them only support one device.) This will be available
276 through $dev$ in $cdrom_device_info$ described below.
278 The drive-specific, minor-like information that is registered with
279 \cdromc, currently contains the following fields:
280 $$
281 \halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}&
282 $/*$ \rm# $*/$\hfil\cr
283 struct& cdrom_device_info\ \{ \hidewidth\cr
284 & struct\ cdrom_device_ops *& ops;& device operations for this major\cr
285 & struct\ cdrom_device_info *& next;& next device_info for this major\cr
286 & void *& handle;& driver-dependent data\cr
287 \noalign{\medskip}
288 & kdev_t& dev;& device number (incorporates minor)\cr
289 & int& mask;& mask of capability: disables them \cr
290 & int& speed;& maximum speed for reading data \cr
291 & int& capacity;& number of discs in a jukebox \cr
292 \noalign{\medskip}
293 &int& options : 30;& options flags \cr
294 &unsigned& mc_flags : 2;& media-change buffer flags \cr
295 & int& use_count;& number of times device is opened\cr
296 & char& name[20];& name of the device type\cr
297 \}\cr
298 }$$
299 Using this $struct$, a linked list of the registered minor devices is
300 built, using the $next$ field. The device number, the device operations
301 struct and specifications of properties of the drive are stored in this
302 structure.
304 The $mask$ flags can be used to mask out some of the capabilities listed
305 in $ops\to capability$, if a specific drive doesn't support a feature
306 of the driver. The value $speed$ specifies the maximum head-rate of the
307 drive, measured in units of normal audio speed (176\,kB/sec raw data or
308 150\,kB/sec file system data). The value $n_discs$ should reflect the
309 number of discs the drive can hold simultaneously, if it is designed
310 as a juke-box, or otherwise~1. The parameters are declared $const$
311 because they describe properties of the drive, which don't change after
312 registration.
314 A few registers contain variables local to the \cdrom\ drive. The
315 flags $options$ are used to specify how the general \cdrom\ routines
316 should behave. These various flags registers should provide enough
317 flexibility to adapt to the different users' wishes (and {\em not\/} the
318 `arbitrary' wishes of the author of the low-level device driver, as is
319 the case in the old scheme). The register $mc_flags$ is used to buffer
320 the information from $media_changed()$ to two separate queues. Other
321 data that is specific to a minor drive, can be accessed through $handle$,
322 which can point to a data structure specific to the low-level driver.
323 The fields $use_count$, $next$, $options$ and $mc_flags$ need not be
324 initialized.
326 The intermediate software layer that \cdromc\ forms will perform some
327 additional bookkeeping. The use count of the device (the number of
328 processes that have the device opened) is registered in $use_count$. The
329 function $cdrom_ioctl()$ will verify the appropriate user-memory regions
330 for read and write, and in case a location on the CD is transferred,
331 it will `sanitize' the format by making requests to the low-level
332 drivers in a standard format, and translating all formats between the
333 user-software and low level drivers. This relieves much of the drivers'
334 memory checking and format checking and translation. Also, the necessary
335 structures will be declared on the program stack.
337 The implementation of the functions should be as defined in the
338 following sections. Two functions {\em must\/} be implemented, namely
339 $open()$ and $release()$. Other functions may be omitted, their
340 corresponding capability flags will be cleared upon registration.
341 Generally, a function returns zero on success and negative on error. A
342 function call should return only after the command has completed, but of
343 course waiting for the device should not use processor time.
345 \subsection{$Int\ open(struct\ cdrom_device_info * cdi, int\ purpose)$}
347 $Open()$ should try to open the device for a specific $purpose$, which
348 can be either:
349 \begin{itemize}
350 \item[0] Open for reading data, as done by {\tt {mount()}} (2), or the
351 user commands {\tt {dd}} or {\tt {cat}}.
352 \item[1] Open for $ioctl$ commands, as done by audio-CD playing
353 programs.
354 \end{itemize}
355 Notice that any strategic code (closing tray upon $open()$, etc.)\ is
356 done by the calling routine in \cdromc, so the low-level routine
357 should only be concerned with proper initialization, such as spinning
358 up the disc, etc. % and device-use count
361 \subsection{$Void\ release(struct\ cdrom_device_info * cdi)$}
364 Device-specific actions should be taken such as spinning down the device.
365 However, strategic actions such as ejection of the tray, or unlocking
366 the door, should be left over to the general routine $cdrom_release()$.
367 This is the only function returning type $void$.
369 \subsection{$Int\ drive_status(struct\ cdrom_device_info * cdi, int\ slot_nr)$}
370 \label{drive status}
372 The function $drive_status$, if implemented, should provide
373 information on the status of the drive (not the status of the disc,
374 which may or may not be in the drive). If the drive is not a changer,
375 $slot_nr$ should be ignored. In \cdromh\ the possibilities are listed:
376 $$
377 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
378 CDS_NO_INFO& no information available\cr
379 CDS_NO_DISC& no disc is inserted, tray is closed\cr
380 CDS_TRAY_OPEN& tray is opened\cr
381 CDS_DRIVE_NOT_READY& something is wrong, tray is moving?\cr
382 CDS_DISC_OK& a disc is loaded and everything is fine\cr
383 }
384 $$
386 \subsection{$Int\ media_changed(struct\ cdrom_device_info * cdi, int\ disc_nr)$}
388 This function is very similar to the original function in $struct\
389 file_operations$. It returns 1 if the medium of the device $cdi\to
390 dev$ has changed since the last call, and 0 otherwise. The parameter
391 $disc_nr$ identifies a specific slot in a juke-box, it should be
392 ignored for single-disc drives. Note that by `re-routing' this
393 function through $cdrom_media_changed()$, we can implement separate
394 queues for the VFS and a new $ioctl()$ function that can report device
395 changes to software (\eg, an auto-mounting daemon).
397 \subsection{$Int\ tray_move(struct\ cdrom_device_info * cdi, int\ position)$}
399 This function, if implemented, should control the tray movement. (No
400 other function should control this.) The parameter $position$ controls
401 the desired direction of movement:
402 \begin{itemize}
403 \item[0] Close tray
404 \item[1] Open tray
405 \end{itemize}
406 This function returns 0 upon success, and a non-zero value upon
407 error. Note that if the tray is already in the desired position, no
408 action need be taken, and the return value should be 0.
410 \subsection{$Int\ lock_door(struct\ cdrom_device_info * cdi, int\ lock)$}
412 This function (and no other code) controls locking of the door, if the
413 drive allows this. The value of $lock$ controls the desired locking
414 state:
415 \begin{itemize}
416 \item[0] Unlock door, manual opening is allowed
417 \item[1] Lock door, tray cannot be ejected manually
418 \end{itemize}
419 This function returns 0 upon success, and a non-zero value upon
420 error. Note that if the door is already in the requested state, no
421 action need be taken, and the return value should be 0.
423 \subsection{$Int\ select_speed(struct\ cdrom_device_info * cdi, int\ speed)$}
425 Some \cdrom\ drives are capable of changing their head-speed. There
426 are several reasons for changing the speed of a \cdrom\ drive. Badly
427 pressed \cdrom s may benefit from less-than-maximum head rate. Modern
428 \cdrom\ drives can obtain very high head rates (up to $24\times$ is
429 common). It has been reported that these drives can make reading
430 errors at these high speeds, reducing the speed can prevent data loss
431 in these circumstances. Finally, some of these drives can
432 make an annoyingly loud noise, which a lower speed may reduce. %Finally,
433 %although the audio-low-pass filters probably aren't designed for it,
434 %more than real-time playback of audio might be used for high-speed
435 %copying of audio tracks.
437 This function specifies the speed at which data is read or audio is
438 played back. The value of $speed$ specifies the head-speed of the
439 drive, measured in units of standard cdrom speed (176\,kB/sec raw data
440 or 150\,kB/sec file system data). So to request that a \cdrom\ drive
441 operate at 300\,kB/sec you would call the CDROM_SELECT_SPEED $ioctl$
442 with $speed=2$. The special value `0' means `auto-selection', \ie,
443 maximum data-rate or real-time audio rate. If the drive doesn't have
444 this `auto-selection' capability, the decision should be made on the
445 current disc loaded and the return value should be positive. A negative
446 return value indicates an error.
448 \subsection{$Int\ select_disc(struct\ cdrom_device_info * cdi, int\ number)$}
450 If the drive can store multiple discs (a juke-box) this function
451 will perform disc selection. It should return the number of the
452 selected disc on success, a negative value on error. Currently, only
453 the ide-cd driver supports this functionality.
455 \subsection{$Int\ get_last_session(struct\ cdrom_device_info * cdi, struct\
456 cdrom_multisession * ms_info)$}
458 This function should implement the old corresponding $ioctl()$. For
459 device $cdi\to dev$, the start of the last session of the current disc
460 should be returned in the pointer argument $ms_info$. Note that
461 routines in \cdromc\ have sanitized this argument: its requested
462 format will {\em always\/} be of the type $CDROM_LBA$ (linear block
463 addressing mode), whatever the calling software requested. But
464 sanitization goes even further: the low-level implementation may
465 return the requested information in $CDROM_MSF$ format if it wishes so
466 (setting the $ms_info\rightarrow addr_format$ field appropriately, of
467 course) and the routines in \cdromc\ will make the transformation if
468 necessary. The return value is 0 upon success.
470 \subsection{$Int\ get_mcn(struct\ cdrom_device_info * cdi, struct\
471 cdrom_mcn * mcn)$}
473 Some discs carry a `Media Catalog Number' (MCN), also called
474 `Universal Product Code' (UPC). This number should reflect the number
475 that is generally found in the bar-code on the product. Unfortunately,
476 the few discs that carry such a number on the disc don't even use the
477 same format. The return argument to this function is a pointer to a
478 pre-declared memory region of type $struct\ cdrom_mcn$. The MCN is
479 expected as a 13-character string, terminated by a null-character.
481 \subsection{$Int\ reset(struct\ cdrom_device_info * cdi)$}
483 This call should perform a hard-reset on the drive (although in
484 circumstances that a hard-reset is necessary, a drive may very well not
485 listen to commands anymore). Preferably, control is returned to the
486 caller only after the drive has finished resetting. If the drive is no
487 longer listening, it may be wise for the underlying low-level cdrom
488 driver to time out.
490 \subsection{$Int\ audio_ioctl(struct\ cdrom_device_info * cdi, unsigned\
491 int\ cmd, void * arg)$}
493 Some of the \cdrom-$ioctl$s defined in \cdromh\ can be
494 implemented by the routines described above, and hence the function
495 $cdrom_ioctl$ will use those. However, most $ioctl$s deal with
496 audio-control. We have decided to leave these to be accessed through a
497 single function, repeating the arguments $cmd$ and $arg$. Note that
498 the latter is of type $void*{}$, rather than $unsigned\ long\
499 int$. The routine $cdrom_ioctl()$ does do some useful things,
500 though. It sanitizes the address format type to $CDROM_MSF$ (Minutes,
501 Seconds, Frames) for all audio calls. It also verifies the memory
502 location of $arg$, and reserves stack-memory for the argument. This
503 makes implementation of the $audio_ioctl()$ much simpler than in the
504 old driver scheme. For example, you may look up the function
505 $cm206_audio_ioctl()$ in {\tt {cm206.c}} that should be updated with
506 this documentation.
508 An unimplemented ioctl should return $-ENOSYS$, but a harmless request
509 (\eg, $CDROMSTART$) may be ignored by returning 0 (success). Other
510 errors should be according to the standards, whatever they are. When
511 an error is returned by the low-level driver, the \UCD\ tries whenever
512 possible to return the error code to the calling program. (We may decide
513 to sanitize the return value in $cdrom_ioctl()$ though, in order to
514 guarantee a uniform interface to the audio-player software.)
516 \subsection{$Int\ dev_ioctl(struct\ cdrom_device_info * cdi, unsigned\ int\
517 cmd, unsigned\ long\ arg)$}
519 Some $ioctl$s seem to be specific to certain \cdrom\ drives. That is,
520 they are introduced to service some capabilities of certain drives. In
521 fact, there are 6 different $ioctl$s for reading data, either in some
522 particular kind of format, or audio data. Not many drives support
523 reading audio tracks as data, I believe this is because of protection
524 of copyrights of artists. Moreover, I think that if audio-tracks are
525 supported, it should be done through the VFS and not via $ioctl$s. A
526 problem here could be the fact that audio-frames are 2352 bytes long,
527 so either the audio-file-system should ask for 75264 bytes at once
528 (the least common multiple of 512 and 2352), or the drivers should
529 bend their backs to cope with this incoherence (to which I would be
530 opposed). Furthermore, it is very difficult for the hardware to find
531 the exact frame boundaries, since there are no synchronization headers
532 in audio frames. Once these issues are resolved, this code should be
533 standardized in \cdromc.
535 Because there are so many $ioctl$s that seem to be introduced to
536 satisfy certain drivers,\footnote{Is there software around that
537 actually uses these? I'd be interested!} any `non-standard' $ioctl$s
538 are routed through the call $dev_ioctl()$. In principle, `private'
539 $ioctl$s should be numbered after the device's major number, and not
540 the general \cdrom\ $ioctl$ number, {\tt {0x53}}. Currently the
541 non-supported $ioctl$s are: {\it CDROMREADMODE1, CDROMREADMODE2,
542 CDROMREADAUDIO, CDROMREADRAW, CDROMREADCOOKED, CDROMSEEK,
543 CDROMPLAY\-BLK and CDROM\-READALL}.
546 \subsection{\cdrom\ capabilities}
547 \label{capability}
549 Instead of just implementing some $ioctl$ calls, the interface in
550 \cdromc\ supplies the possibility to indicate the {\em capabilities\/}
551 of a \cdrom\ drive. This can be done by ORing any number of
552 capability-constants that are defined in \cdromh\ at the registration
553 phase. Currently, the capabilities are any of:
554 $$
555 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
556 CDC_CLOSE_TRAY& can close tray by software control\cr
557 CDC_OPEN_TRAY& can open tray\cr
558 CDC_LOCK& can lock and unlock the door\cr
559 CDC_SELECT_SPEED& can select speed, in units of $\sim$150\,kB/s\cr
560 CDC_SELECT_DISC& drive is juke-box\cr
561 CDC_MULTI_SESSION& can read sessions $>\rm1$\cr
562 CDC_MCN& can read Media Catalog Number\cr
563 CDC_MEDIA_CHANGED& can report if disc has changed\cr
564 CDC_PLAY_AUDIO& can perform audio-functions (play, pause, etc)\cr
565 CDC_RESET& hard reset device\cr
566 CDC_IOCTLS& driver has non-standard ioctls\cr
567 CDC_DRIVE_STATUS& driver implements drive status\cr
568 }
569 $$
570 The capability flag is declared $const$, to prevent drivers from
571 accidentally tampering with the contents. The capability fags actually
572 inform \cdromc\ of what the driver can do. If the drive found
573 by the driver does not have the capability, is can be masked out by
574 the $cdrom_device_info$ variable $mask$. For instance, the SCSI \cdrom\
575 driver has implemented the code for loading and ejecting \cdrom's, and
576 hence its corresponding flags in $capability$ will be set. But a SCSI
577 \cdrom\ drive might be a caddy system, which can't load the tray, and
578 hence for this drive the $cdrom_device_info$ struct will have set
579 the $CDC_CLOSE_TRAY$ bit in $mask$.
581 In the file \cdromc\ you will encounter many constructions of the type
582 $$\it
583 if\ (cdo\rightarrow capability \mathrel\& \mathord{\sim} cdi\rightarrow mask
584 \mathrel{\&} CDC_<capability>) \ldots
585 $$
586 There is no $ioctl$ to set the mask\dots The reason is that
587 I think it is better to control the {\em behavior\/} rather than the
588 {\em capabilities}.
590 \subsection{Options}
592 A final flag register controls the {\em behavior\/} of the \cdrom\
593 drives, in order to satisfy different users' wishes, hopefully
594 independently of the ideas of the respective author who happened to
595 have made the drive's support available to the \linux\ community. The
596 current behavior options are:
597 $$
598 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
599 CDO_AUTO_CLOSE& try to close tray upon device $open()$\cr
600 CDO_AUTO_EJECT& try to open tray on last device $close()$\cr
601 CDO_USE_FFLAGS& use $file_pointer\rightarrow f_flags$ to indicate
602 purpose for $open()$\cr
603 CDO_LOCK& try to lock door if device is opened\cr
604 CDO_CHECK_TYPE& ensure disc type is data if opened for data\cr
605 }
606 $$
608 The initial value of this register is $CDO_AUTO_CLOSE \mathrel|
609 CDO_USE_FFLAGS \mathrel| CDO_LOCK$, reflecting my own view on user
610 interface and software standards. Before you protest, there are two
611 new $ioctl$s implemented in \cdromc, that allow you to control the
612 behavior by software. These are:
613 $$
614 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
615 CDROM_SET_OPTIONS& set options specified in $(int)\ arg$\cr
616 CDROM_CLEAR_OPTIONS& clear options specified in $(int)\ arg$\cr
617 }
618 $$
619 One option needs some more explanation: $CDO_USE_FFLAGS$. In the next
620 newsection we explain what the need for this option is.
622 A software package {\tt setcd}, available from the Debian distribution
623 and {\tt sunsite.unc.edu}, allows user level control of these flags.
625 \newsection{The need to know the purpose of opening the \cdrom\ device}
627 Traditionally, Unix devices can be used in two different `modes',
628 either by reading/writing to the device file, or by issuing
629 controlling commands to the device, by the device's $ioctl()$
630 call. The problem with \cdrom\ drives, is that they can be used for
631 two entirely different purposes. One is to mount removable
632 file systems, \cdrom s, the other is to play audio CD's. Audio commands
633 are implemented entirely through $ioctl$s, presumably because the
634 first implementation (SUN?) has been such. In principle there is
635 nothing wrong with this, but a good control of the `CD player' demands
636 that the device can {\em always\/} be opened in order to give the
637 $ioctl$ commands, regardless of the state the drive is in.
639 On the other hand, when used as a removable-media disc drive (what the
640 original purpose of \cdrom s is) we would like to make sure that the
641 disc drive is ready for operation upon opening the device. In the old
642 scheme, some \cdrom\ drivers don't do any integrity checking, resulting
643 in a number of i/o errors reported by the VFS to the kernel when an
644 attempt for mounting a \cdrom\ on an empty drive occurs. This is not a
645 particularly elegant way to find out that there is no \cdrom\ inserted;
646 it more-or-less looks like the old IBM-PC trying to read an empty floppy
647 drive for a couple of seconds, after which the system complains it
648 can't read from it. Nowadays we can {\em sense\/} the existence of a
649 removable medium in a drive, and we believe we should exploit that
650 fact. An integrity check on opening of the device, that verifies the
651 availability of a \cdrom\ and its correct type (data), would be
652 desirable.
654 These two ways of using a \cdrom\ drive, principally for data and
655 secondarily for playing audio discs, have different demands for the
656 behavior of the $open()$ call. Audio use simply wants to open the
657 device in order to get a file handle which is needed for issuing
658 $ioctl$ commands, while data use wants to open for correct and
659 reliable data transfer. The only way user programs can indicate what
660 their {\em purpose\/} of opening the device is, is through the $flags$
661 parameter (see {\tt {open(2)}}). For \cdrom\ devices, these flags aren't
662 implemented (some drivers implement checking for write-related flags,
663 but this is not strictly necessary if the device file has correct
664 permission flags). Most option flags simply don't make sense to
665 \cdrom\ devices: $O_CREAT$, $O_NOCTTY$, $O_TRUNC$, $O_APPEND$, and
666 $O_SYNC$ have no meaning to a \cdrom.
668 We therefore propose to use the flag $O_NONBLOCK$ to indicate
669 that the device is opened just for issuing $ioctl$
670 commands. Strictly, the meaning of $O_NONBLOCK$ is that opening and
671 subsequent calls to the device don't cause the calling process to
672 wait. We could interpret this as ``don't wait until someone has
673 inserted some valid data-\cdrom.'' Thus, our proposal of the
674 implementation for the $open()$ call for \cdrom s is:
675 \begin{itemize}
676 \item If no other flags are set than $O_RDONLY$, the device is opened
677 for data transfer, and the return value will be 0 only upon successful
678 initialization of the transfer. The call may even induce some actions
679 on the \cdrom, such as closing the tray.
680 \item If the option flag $O_NONBLOCK$ is set, opening will always be
681 successful, unless the whole device doesn't exist. The drive will take
682 no actions whatsoever.
683 \end{itemize}
685 \subsection{And what about standards?}
687 You might hesitate to accept this proposal as it comes from the
688 \linux\ community, and not from some standardizing institute. What
689 about SUN, SGI, HP and all those other Unix and hardware vendors?
690 Well, these companies are in the lucky position that they generally
691 control both the hardware and software of their supported products,
692 and are large enough to set their own standard. They do not have to
693 deal with a dozen or more different, competing hardware
694 configurations.\footnote{Incidentally, I think that SUN's approach to
695 mounting \cdrom s is very good in origin: under Solaris a
696 volume-daemon automatically mounts a newly inserted \cdrom\ under {\tt
697 {/cdrom/$<volume-name>$/}}. In my opinion they should have pushed this
698 further and have {\em every\/} \cdrom\ on the local area network be
699 mounted at the similar location, \ie, no matter in which particular
700 machine you insert a \cdrom, it will always appear at the same
701 position in the directory tree, on every system. When I wanted to
702 implement such a user-program for \linux, I came across the
703 differences in behavior of the various drivers, and the need for an
704 $ioctl$ informing about media changes.}
706 We believe that using $O_NONBLOCK$ to indicate that a device is being opened
707 for $ioctl$ commands only can be easily introduced in the \linux\
708 community. All the CD-player authors will have to be informed, we can
709 even send in our own patches to the programs. The use of $O_NONBLOCK$
710 has most likely no influence on the behavior of the CD-players on
711 other operating systems than \linux. Finally, a user can always revert
712 to old behavior by a call to $ioctl(file_descriptor, CDROM_CLEAR_OPTIONS,
713 CDO_USE_FFLAGS)$.
715 \subsection{The preferred strategy of $open()$}
717 The routines in \cdromc\ are designed in such a way that run-time
718 configuration of the behavior of \cdrom\ devices (of {\em any\/} type)
719 can be carried out, by the $CDROM_SET/CLEAR_OPTIONS$ $ioctls$. Thus, various
720 modes of operation can be set:
721 \begin{description}
722 \item[$CDO_AUTO_CLOSE \mathrel| CDO_USE_FFLAGS \mathrel| CDO_LOCK$] This
723 is the default setting. (With $CDO_CHECK_TYPE$ it will be better, in the
724 future.) If the device is not yet opened by any other process, and if
725 the device is being opened for data ($O_NONBLOCK$ is not set) and the
726 tray is found to be open, an attempt to close the tray is made. Then,
727 it is verified that a disc is in the drive and, if $CDO_CHECK_TYPE$ is
728 set, that it contains tracks of type `data mode 1.' Only if all tests
729 are passed is the return value zero. The door is locked to prevent file
730 system corruption. If the drive is opened for audio ($O_NONBLOCK$ is
731 set), no actions are taken and a value of 0 will be returned.
732 \item[$CDO_AUTO_CLOSE \mathrel| CDO_AUTO_EJECT \mathrel| CDO_LOCK$] This
733 mimics the behavior of the current sbpcd-driver. The option flags are
734 ignored, the tray is closed on the first open, if necessary. Similarly,
735 the tray is opened on the last release, \ie, if a \cdrom\ is unmounted,
736 it is automatically ejected, such that the user can replace it.
737 \end{description}
738 We hope that these option can convince everybody (both driver
739 maintainers and user program developers) to adopt the new \cdrom\
740 driver scheme and option flag interpretation.
742 \newsection{Description of routines in \cdromc}
744 Only a few routines in \cdromc\ are exported to the drivers. In this
745 new section we will discuss these, as well as the functions that `take
746 over' the \cdrom\ interface to the kernel. The header file belonging
747 to \cdromc\ is called \cdromh. Formerly, some of the contents of this
748 file were placed in the file {\tt {ucdrom.h}}, but this file has now been
749 merged back into \cdromh.
751 \subsection{$Struct\ file_operations\ cdrom_fops$}
753 The contents of this structure were described in section~\ref{cdrom.c}.
754 A pointer to this structure is assigned to the $fops$ field
755 of the $struct gendisk$.
757 \subsection{$Int\ register_cdrom( struct\ cdrom_device_info\ * cdi)$}
759 This function is used in about the same way one registers $cdrom_fops$
760 with the kernel, the device operations and information structures,
761 as described in section~\ref{cdrom.c}, should be registered with the
762 \UCD:
763 $$
764 register_cdrom(\&<device>_info));
765 $$
766 This function returns zero upon success, and non-zero upon
767 failure. The structure $<device>_info$ should have a pointer to the
768 driver's $<device>_dops$, as in
769 $$
770 \vbox{\halign{&$#$\hfil\cr
771 struct\ &cdrom_device_info\ <device>_info = \{\cr
772 & <device>_dops;\cr
773 &\ldots\cr
774 \}\cr
775 }}$$
776 Note that a driver must have one static structure, $<device>_dops$, while
777 it may have as many structures $<device>_info$ as there are minor devices
778 active. $Register_cdrom()$ builds a linked list from these.
780 \subsection{$Int\ unregister_cdrom(struct\ cdrom_device_info * cdi)$}
782 Unregistering device $cdi$ with minor number $MINOR(cdi\to dev)$ 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 \cdrom\ interface. This function returns zero upon
786 success, and non-zero upon failure.
788 \subsection{$Int\ cdrom_open(struct\ inode * ip, struct\ file * fp)$}
790 This function is not called directly by the low-level drivers, it is
791 listed in the standard $cdrom_fops$. If the VFS opens a file, this
792 function becomes active. A strategy is implemented in this routine,
793 taking care of all capabilities and options that are set in the
794 $cdrom_device_ops$ connected to the device. Then, the program flow is
795 transferred to the device_dependent $open()$ call.
797 \subsection{$Void\ cdrom_release(struct\ inode *ip, struct\ file
798 *fp)$}
800 This function implements the reverse-logic of $cdrom_open()$, and then
801 calls the device-dependent $release()$ routine. When the use-count has
802 reached 0, the allocated buffers are flushed by calls to $sync_dev(dev)$
803 and $invalidate_buffers(dev)$.
806 \subsection{$Int\ cdrom_ioctl(struct\ inode *ip, struct\ file *fp,
807 unsigned\ int\ cmd, unsigned\ long\ arg)$}
808 \label{cdrom-ioctl}
810 This function handles all the standard $ioctl$ requests for \cdrom\
811 devices in a uniform way. The different calls fall into three
812 categories: $ioctl$s that can be directly implemented by device
813 operations, ones that are routed through the call $audio_ioctl()$, and
814 the remaining ones, that are presumable device-dependent. Generally, a
815 negative return value indicates an error.
817 \subsubsection{Directly implemented $ioctl$s}
818 \label{ioctl-direct}
820 The following `old' \cdrom-$ioctl$s are implemented by directly
821 calling device-operations in $cdrom_device_ops$, if implemented and
822 not masked:
823 \begin{description}
824 \item[CDROMMULTISESSION] Requests the last session on a \cdrom.
825 \item[CDROMEJECT] Open tray.
826 \item[CDROMCLOSETRAY] Close tray.
827 \item[CDROMEJECT_SW] 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 \item[CDROM_GET_MCN] Get the Media Catalog Number from a CD.
831 \end{description}
833 \subsubsection{$Ioctl$s routed through $audio_ioctl()$}
834 \label{ioctl-audio}
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.
840 \begin{description}
841 \item[CDROMSUBCHNL] Get sub-channel data in argument $arg$ of type $struct\
842 cdrom_subchnl *{}$.
843 \item[CDROMREADTOCHDR] Read Table of Contents header, in $arg$ of type
844 $struct\ cdrom_tochdr *{}$.
845 \item[CDROMREADTOCENTRY] Read a Table of Contents entry in $arg$ and
846 specified by $arg$ of type $struct\ cdrom_tocentry *{}$.
847 \item[CDROMPLAYMSF] Play audio fragment specified in Minute, Second,
848 Frame format, delimited by $arg$ of type $struct\ cdrom_msf *{}$.
849 \item[CDROMPLAYTRKIND] Play audio fragment in track-index format
850 delimited by $arg$ of type $struct\ \penalty-1000 cdrom_ti *{}$.
851 \item[CDROMVOLCTRL] Set volume specified by $arg$ of type $struct\
852 cdrom_volctrl *{}$.
853 \item[CDROMVOLREAD] Read volume into by $arg$ of type $struct\
854 cdrom_volctrl *{}$.
855 \item[CDROMSTART] Spin up disc.
856 \item[CDROMSTOP] Stop playback of audio fragment.
857 \item[CDROMPAUSE] Pause playback of audio fragment.
858 \item[CDROMRESUME] Resume playing.
859 \end{description}
861 \subsubsection{New $ioctl$s in \cdromc}
863 The following $ioctl$s have been introduced to allow user programs to
864 control the behavior of individual \cdrom\ devices. New $ioctl$
865 commands can be identified by the underscores in their names.
866 \begin{description}
867 \item[CDROM_SET_OPTIONS] Set options specified by $arg$. Returns the
868 option flag register after modification. Use $arg = \rm0$ for reading
869 the current flags.
870 \item[CDROM_CLEAR_OPTIONS] Clear options specified by $arg$. Returns
871 the option flag register after modification.
872 \item[CDROM_SELECT_SPEED] Select head-rate speed of disc specified as
873 by $arg$ in units of standard cdrom speed (176\,kB/sec raw data or
874 150\,kB/sec file system data). The value 0 means `auto-select', \ie,
875 play audio discs at real time and data discs at maximum speed. The value
876 $arg$ is checked against the maximum head rate of the drive found in the
877 $cdrom_dops$.
878 \item[CDROM_SELECT_DISC] Select disc numbered $arg$ from a juke-box.
879 First disc is numbered 0. The number $arg$ is checked against the
880 maximum number of discs in the juke-box found in the $cdrom_dops$.
881 \item[CDROM_MEDIA_CHANGED] Returns 1 if a disc has been changed since
882 the last call. Note that calls to $cdrom_media_changed$ by the VFS
883 are treated by an independent queue, so both mechanisms will detect
884 a media change once. For juke-boxes, an extra argument $arg$
885 specifies the slot for which the information is given. The special
886 value $CDSL_CURRENT$ requests that information about the currently
887 selected slot be returned.
888 \item[CDROM_DRIVE_STATUS] Returns the status of the drive by a call to
889 $drive_status()$. Return values are defined in section~\ref{drive
890 status}. Note that this call doesn't return information on the
891 current playing activity of the drive; this can be polled through an
892 $ioctl$ call to $CDROMSUBCHNL$. For juke-boxes, an extra argument
893 $arg$ specifies the slot for which (possibly limited) information is
894 given. The special value $CDSL_CURRENT$ requests that information
895 about the currently selected slot be returned.
896 \item[CDROM_DISC_STATUS] Returns the type of the disc currently in the
897 drive. It should be viewed as a complement to $CDROM_DRIVE_STATUS$.
898 This $ioctl$ can provide \emph {some} information about the current
899 disc that is inserted in the drive. This functionality used to be
900 implemented in the low level drivers, but is now carried out
901 entirely in \UCD.
903 The history of development of the CD's use as a carrier medium for
904 various digital information has lead to many different disc types.
905 This $ioctl$ is useful only in the case that CDs have \emph {only
906 one} type of data on them. While this is often the case, it is
907 also very common for CDs to have some tracks with data, and some
908 tracks with audio. Because this is an existing interface, rather
909 than fixing this interface by changing the assumptions it was made
910 under, thereby breaking all user applications that use this
911 function, the \UCD\ implements this $ioctl$ as follows: If the CD in
912 question has audio tracks on it, and it has absolutely no CD-I, XA,
913 or data tracks on it, it will be reported as $CDS_AUDIO$. If it has
914 both audio and data tracks, it will return $CDS_MIXED$. If there
915 are no audio tracks on the disc, and if the CD in question has any
916 CD-I tracks on it, it will be reported as $CDS_XA_2_2$. Failing
917 that, if the CD in question has any XA tracks on it, it will be
918 reported as $CDS_XA_2_1$. Finally, if the CD in question has any
919 data tracks on it, it will be reported as a data CD ($CDS_DATA_1$).
921 This $ioctl$ can return:
922 $$
923 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
924 CDS_NO_INFO& no information available\cr
925 CDS_NO_DISC& no disc is inserted, or tray is opened\cr
926 CDS_AUDIO& Audio disc (2352 audio bytes/frame)\cr
927 CDS_DATA_1& data disc, mode 1 (2048 user bytes/frame)\cr
928 CDS_XA_2_1& mixed data (XA), mode 2, form 1 (2048 user bytes)\cr
929 CDS_XA_2_2& mixed data (XA), mode 2, form 1 (2324 user bytes)\cr
930 CDS_MIXED& mixed audio/data disc\cr
931 }
932 $$
933 For some information concerning frame layout of the various disc
934 types, see a recent version of \cdromh.
936 \item[CDROM_CHANGER_NSLOTS] Returns the number of slots in a
937 juke-box.
938 \item[CDROMRESET] Reset the drive.
939 \item[CDROM_GET_CAPABILITY] Returns the $capability$ flags for the
940 drive. Refer to section \ref{capability} for more information on
941 these flags.
942 \item[CDROM_LOCKDOOR] Locks the door of the drive. $arg == \rm0$
943 unlocks the door, any other value locks it.
944 \item[CDROM_DEBUG] Turns on debugging info. Only root is allowed
945 to do this. Same semantics as CDROM_LOCKDOOR.
946 \end{description}
948 \subsubsection{Device dependent $ioctl$s}
950 Finally, all other $ioctl$s are passed to the function $dev_ioctl()$,
951 if implemented. No memory allocation or verification is carried out.
953 \newsection{How to update your driver}
955 \begin{enumerate}
956 \item Make a backup of your current driver.
957 \item Get hold of the files \cdromc\ and \cdromh, they should be in
958 the directory tree that came with this documentation.
959 \item Make sure you include \cdromh.
960 \item Change the 3rd argument of $register_blkdev$ from
961 $\&<your-drive>_fops$ to $\&cdrom_fops$.
962 \item Just after that line, add the following to register with the \UCD:
963 $$register_cdrom(\&<your-drive>_info);$$
964 Similarly, add a call to $unregister_cdrom()$ at the appropriate place.
965 \item Copy an example of the device-operations $struct$ to your
966 source, \eg, from {\tt {cm206.c}} $cm206_dops$, and change all
967 entries to names corresponding to your driver, or names you just
968 happen to like. If your driver doesn't support a certain function,
969 make the entry $NULL$. At the entry $capability$ you should list all
970 capabilities your driver currently supports. If your driver
971 has a capability that is not listed, please send me a message.
972 \item Copy the $cdrom_device_info$ declaration from the same example
973 driver, and modify the entries according to your needs. If your
974 driver dynamically determines the capabilities of the hardware, this
975 structure should also be declared dynamically.
976 \item Implement all functions in your $<device>_dops$ structure,
977 according to prototypes listed in \cdromh, and specifications given
978 in section~\ref{cdrom.c}. Most likely you have already implemented
979 the code in a large part, and you will almost certainly need to adapt the
980 prototype and return values.
981 \item Rename your $<device>_ioctl()$ function to $audio_ioctl$ and
982 change the prototype a little. Remove entries listed in the first
983 part in section~\ref{cdrom-ioctl}, if your code was OK, these are
984 just calls to the routines you adapted in the previous step.
985 \item You may remove all remaining memory checking code in the
986 $audio_ioctl()$ function that deals with audio commands (these are
987 listed in the second part of section~\ref{cdrom-ioctl}). There is no
988 need for memory allocation either, so most $case$s in the $switch$
989 statement look similar to:
990 $$
991 case\ CDROMREADTOCENTRY\colon get_toc_entry\bigl((struct\
992 cdrom_tocentry *{})\ arg\bigr);
993 $$
994 \item All remaining $ioctl$ cases must be moved to a separate
995 function, $<device>_ioctl$, the device-dependent $ioctl$s. Note that
996 memory checking and allocation must be kept in this code!
997 \item Change the prototypes of $<device>_open()$ and
998 $<device>_release()$, and remove any strategic code (\ie, tray
999 movement, door locking, etc.).
1000 \item Try to recompile the drivers. We advise you to use modules, both
1001 for {\tt {cdrom.o}} and your driver, as debugging is much easier this
1002 way.
1003 \end{enumerate}
1005 \newsection{Thanks}
1007 Thanks to all the people involved. First, Erik Andersen, who has
1008 taken over the torch in maintaining \cdromc\ and integrating much
1009 \cdrom-related code in the 2.1-kernel. Thanks to Scott Snyder and
1010 Gerd Knorr, who were the first to implement this interface for SCSI
1011 and IDE-CD drivers and added many ideas for extension of the data
1012 structures relative to kernel~2.0. Further thanks to Heiko Eissfeldt,
1013 Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard M\"onkeberg and Andrew
1014 Kroll, the \linux\ \cdrom\ device driver developers who were kind
1015 enough to give suggestions and criticisms during the writing. Finally
1016 of course, I want to thank Linus Torvalds for making this possible in
1017 the first place.
1019 \vfill
1020 $ \version\ $
1021 \eject
1022 \end{document}