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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 In case the driver supports modules, the call $MOD_INC_USE_COUNT$
356 should be performed exactly once, if the $open()$ was successful. The
357 return value is negative on error, and zero on success. The
358 open-for-ioctl call can only fail if there is no hardware.
360 Notice that any strategic code (closing tray upon $open()$, etc.)\ is
361 done by the calling routine in \cdromc, so the low-level routine
362 should only be concerned with proper initialization, such as spinning
363 up the disc, etc. % and device-use count
366 \subsection{$Void\ release(struct\ cdrom_device_info * cdi)$}
368 In case of module support, a single call $MOD_DEC_USE_COUNT$ should be
369 coded here. Possibly other device-specific actions should be taken
370 such as spinning down the device. However, strategic actions such as
371 ejection of the tray, or unlocking the door, should be left over to
372 the general routine $cdrom_release()$. Also, the invalidation of the
373 allocated buffers in the VFS is taken care of by the routine in
374 \cdromc. This is the only function returning type $void$.
376 \subsection{$Int\ drive_status(struct\ cdrom_device_info * cdi, int\ slot_nr)$}
377 \label{drive status}
379 The function $drive_status$, if implemented, should provide
380 information on the status of the drive (not the status of the disc,
381 which may or may not be in the drive). If the drive is not a changer,
382 $slot_nr$ should be ignored. In \cdromh\ the possibilities are listed:
383 $$
384 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
385 CDS_NO_INFO& no information available\cr
386 CDS_NO_DISC& no disc is inserted, tray is closed\cr
387 CDS_TRAY_OPEN& tray is opened\cr
388 CDS_DRIVE_NOT_READY& something is wrong, tray is moving?\cr
389 CDS_DISC_OK& a disc is loaded and everything is fine\cr
390 }
391 $$
393 \subsection{$Int\ media_changed(struct\ cdrom_device_info * cdi, int\ disc_nr)$}
395 This function is very similar to the original function in $struct\
396 file_operations$. It returns 1 if the medium of the device $cdi\to
397 dev$ has changed since the last call, and 0 otherwise. The parameter
398 $disc_nr$ identifies a specific slot in a juke-box, it should be
399 ignored for single-disc drives. Note that by `re-routing' this
400 function through $cdrom_media_changed()$, we can implement separate
401 queues for the VFS and a new $ioctl()$ function that can report device
402 changes to software (\eg, an auto-mounting daemon).
404 \subsection{$Int\ tray_move(struct\ cdrom_device_info * cdi, int\ position)$}
406 This function, if implemented, should control the tray movement. (No
407 other function should control this.) The parameter $position$ controls
408 the desired direction of movement:
409 \begin{itemize}
410 \item[0] Close tray
411 \item[1] Open tray
412 \end{itemize}
413 This function returns 0 upon success, and a non-zero value upon
414 error. Note that if the tray is already in the desired position, no
415 action need be taken, and the return value should be 0.
417 \subsection{$Int\ lock_door(struct\ cdrom_device_info * cdi, int\ lock)$}
419 This function (and no other code) controls locking of the door, if the
420 drive allows this. The value of $lock$ controls the desired locking
421 state:
422 \begin{itemize}
423 \item[0] Unlock door, manual opening is allowed
424 \item[1] Lock door, tray cannot be ejected manually
425 \end{itemize}
426 This function returns 0 upon success, and a non-zero value upon
427 error. Note that if the door is already in the requested state, no
428 action need be taken, and the return value should be 0.
430 \subsection{$Int\ select_speed(struct\ cdrom_device_info * cdi, int\ speed)$}
432 Some \cdrom\ drives are capable of changing their head-speed. There
433 are several reasons for changing the speed of a \cdrom\ drive. Badly
434 pressed \cdrom s may benefit from less-than-maximum head rate. Modern
435 \cdrom\ drives can obtain very high head rates (up to $24\times$ is
436 common). It has been reported that these drives can make reading
437 errors at these high speeds, reducing the speed can prevent data loss
438 in these circumstances. Finally, some of these drives can
439 make an annoyingly loud noise, which a lower speed may reduce. %Finally,
440 %although the audio-low-pass filters probably aren't designed for it,
441 %more than real-time playback of audio might be used for high-speed
442 %copying of audio tracks.
444 This function specifies the speed at which data is read or audio is
445 played back. The value of $speed$ specifies the head-speed of the
446 drive, measured in units of standard cdrom speed (176\,kB/sec raw data
447 or 150\,kB/sec file system data). So to request that a \cdrom\ drive
448 operate at 300\,kB/sec you would call the CDROM_SELECT_SPEED $ioctl$
449 with $speed=2$. The special value `0' means `auto-selection', \ie,
450 maximum data-rate or real-time audio rate. If the drive doesn't have
451 this `auto-selection' capability, the decision should be made on the
452 current disc loaded and the return value should be positive. A negative
453 return value indicates an error.
455 \subsection{$Int\ select_disc(struct\ cdrom_device_info * cdi, int\ number)$}
457 If the drive can store multiple discs (a juke-box) this function
458 will perform disc selection. It should return the number of the
459 selected disc on success, a negative value on error. Currently, only
460 the ide-cd driver supports this functionality.
462 \subsection{$Int\ get_last_session(struct\ cdrom_device_info * cdi, struct\
463 cdrom_multisession * ms_info)$}
465 This function should implement the old corresponding $ioctl()$. For
466 device $cdi\to dev$, the start of the last session of the current disc
467 should be returned in the pointer argument $ms_info$. Note that
468 routines in \cdromc\ have sanitized this argument: its requested
469 format will {\em always\/} be of the type $CDROM_LBA$ (linear block
470 addressing mode), whatever the calling software requested. But
471 sanitization goes even further: the low-level implementation may
472 return the requested information in $CDROM_MSF$ format if it wishes so
473 (setting the $ms_info\rightarrow addr_format$ field appropriately, of
474 course) and the routines in \cdromc\ will make the transformation if
475 necessary. The return value is 0 upon success.
477 \subsection{$Int\ get_mcn(struct\ cdrom_device_info * cdi, struct\
478 cdrom_mcn * mcn)$}
480 Some discs carry a `Media Catalog Number' (MCN), also called
481 `Universal Product Code' (UPC). This number should reflect the number
482 that is generally found in the bar-code on the product. Unfortunately,
483 the few discs that carry such a number on the disc don't even use the
484 same format. The return argument to this function is a pointer to a
485 pre-declared memory region of type $struct\ cdrom_mcn$. The MCN is
486 expected as a 13-character string, terminated by a null-character.
488 \subsection{$Int\ reset(struct\ cdrom_device_info * cdi)$}
490 This call should perform a hard-reset on the drive (although in
491 circumstances that a hard-reset is necessary, a drive may very well not
492 listen to commands anymore). Preferably, control is returned to the
493 caller only after the drive has finished resetting. If the drive is no
494 longer listening, it may be wise for the underlying low-level cdrom
495 driver to time out.
497 \subsection{$Int\ audio_ioctl(struct\ cdrom_device_info * cdi, unsigned\
498 int\ cmd, void * arg)$}
500 Some of the \cdrom-$ioctl$s defined in \cdromh\ can be
501 implemented by the routines described above, and hence the function
502 $cdrom_ioctl$ will use those. However, most $ioctl$s deal with
503 audio-control. We have decided to leave these to be accessed through a
504 single function, repeating the arguments $cmd$ and $arg$. Note that
505 the latter is of type $void*{}$, rather than $unsigned\ long\
506 int$. The routine $cdrom_ioctl()$ does do some useful things,
507 though. It sanitizes the address format type to $CDROM_MSF$ (Minutes,
508 Seconds, Frames) for all audio calls. It also verifies the memory
509 location of $arg$, and reserves stack-memory for the argument. This
510 makes implementation of the $audio_ioctl()$ much simpler than in the
511 old driver scheme. For example, you may look up the function
512 $cm206_audio_ioctl()$ in {\tt {cm206.c}} that should be updated with
513 this documentation.
515 An unimplemented ioctl should return $-ENOSYS$, but a harmless request
516 (\eg, $CDROMSTART$) may be ignored by returning 0 (success). Other
517 errors should be according to the standards, whatever they are. When
518 an error is returned by the low-level driver, the \UCD\ tries whenever
519 possible to return the error code to the calling program. (We may decide
520 to sanitize the return value in $cdrom_ioctl()$ though, in order to
521 guarantee a uniform interface to the audio-player software.)
523 \subsection{$Int\ dev_ioctl(struct\ cdrom_device_info * cdi, unsigned\ int\
524 cmd, unsigned\ long\ arg)$}
526 Some $ioctl$s seem to be specific to certain \cdrom\ drives. That is,
527 they are introduced to service some capabilities of certain drives. In
528 fact, there are 6 different $ioctl$s for reading data, either in some
529 particular kind of format, or audio data. Not many drives support
530 reading audio tracks as data, I believe this is because of protection
531 of copyrights of artists. Moreover, I think that if audio-tracks are
532 supported, it should be done through the VFS and not via $ioctl$s. A
533 problem here could be the fact that audio-frames are 2352 bytes long,
534 so either the audio-file-system should ask for 75264 bytes at once
535 (the least common multiple of 512 and 2352), or the drivers should
536 bend their backs to cope with this incoherence (to which I would be
537 opposed). Furthermore, it is very difficult for the hardware to find
538 the exact frame boundaries, since there are no synchronization headers
539 in audio frames. Once these issues are resolved, this code should be
540 standardized in \cdromc.
542 Because there are so many $ioctl$s that seem to be introduced to
543 satisfy certain drivers,\footnote{Is there software around that
544 actually uses these? I'd be interested!} any `non-standard' $ioctl$s
545 are routed through the call $dev_ioctl()$. In principle, `private'
546 $ioctl$s should be numbered after the device's major number, and not
547 the general \cdrom\ $ioctl$ number, {\tt {0x53}}. Currently the
548 non-supported $ioctl$s are: {\it CDROMREADMODE1, CDROMREADMODE2,
549 CDROMREADAUDIO, CDROMREADRAW, CDROMREADCOOKED, CDROMSEEK,
550 CDROMPLAY\-BLK and CDROM\-READALL}.
553 \subsection{\cdrom\ capabilities}
554 \label{capability}
556 Instead of just implementing some $ioctl$ calls, the interface in
557 \cdromc\ supplies the possibility to indicate the {\em capabilities\/}
558 of a \cdrom\ drive. This can be done by ORing any number of
559 capability-constants that are defined in \cdromh\ at the registration
560 phase. Currently, the capabilities are any of:
561 $$
562 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
563 CDC_CLOSE_TRAY& can close tray by software control\cr
564 CDC_OPEN_TRAY& can open tray\cr
565 CDC_LOCK& can lock and unlock the door\cr
566 CDC_SELECT_SPEED& can select speed, in units of $\sim$150\,kB/s\cr
567 CDC_SELECT_DISC& drive is juke-box\cr
568 CDC_MULTI_SESSION& can read sessions $>\rm1$\cr
569 CDC_MCN& can read Media Catalog Number\cr
570 CDC_MEDIA_CHANGED& can report if disc has changed\cr
571 CDC_PLAY_AUDIO& can perform audio-functions (play, pause, etc)\cr
572 CDC_RESET& hard reset device\cr
573 CDC_IOCTLS& driver has non-standard ioctls\cr
574 CDC_DRIVE_STATUS& driver implements drive status\cr
575 }
576 $$
577 The capability flag is declared $const$, to prevent drivers from
578 accidentally tampering with the contents. The capability fags actually
579 inform \cdromc\ of what the driver can do. If the drive found
580 by the driver does not have the capability, is can be masked out by
581 the $cdrom_device_info$ variable $mask$. For instance, the SCSI \cdrom\
582 driver has implemented the code for loading and ejecting \cdrom's, and
583 hence its corresponding flags in $capability$ will be set. But a SCSI
584 \cdrom\ drive might be a caddy system, which can't load the tray, and
585 hence for this drive the $cdrom_device_info$ struct will have set
586 the $CDC_CLOSE_TRAY$ bit in $mask$.
588 In the file \cdromc\ you will encounter many constructions of the type
589 $$\it
590 if\ (cdo\rightarrow capability \mathrel\& \mathord{\sim} cdi\rightarrow mask
591 \mathrel{\&} CDC_<capability>) \ldots
592 $$
593 There is no $ioctl$ to set the mask\dots The reason is that
594 I think it is better to control the {\em behavior\/} rather than the
595 {\em capabilities}.
597 \subsection{Options}
599 A final flag register controls the {\em behavior\/} of the \cdrom\
600 drives, in order to satisfy different users' wishes, hopefully
601 independently of the ideas of the respective author who happened to
602 have made the drive's support available to the \linux\ community. The
603 current behavior options are:
604 $$
605 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
606 CDO_AUTO_CLOSE& try to close tray upon device $open()$\cr
607 CDO_AUTO_EJECT& try to open tray on last device $close()$\cr
608 CDO_USE_FFLAGS& use $file_pointer\rightarrow f_flags$ to indicate
609 purpose for $open()$\cr
610 CDO_LOCK& try to lock door if device is opened\cr
611 CDO_CHECK_TYPE& ensure disc type is data if opened for data\cr
612 }
613 $$
615 The initial value of this register is $CDO_AUTO_CLOSE \mathrel|
616 CDO_USE_FFLAGS \mathrel| CDO_LOCK$, reflecting my own view on user
617 interface and software standards. Before you protest, there are two
618 new $ioctl$s implemented in \cdromc, that allow you to control the
619 behavior by software. These are:
620 $$
621 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
622 CDROM_SET_OPTIONS& set options specified in $(int)\ arg$\cr
623 CDROM_CLEAR_OPTIONS& clear options specified in $(int)\ arg$\cr
624 }
625 $$
626 One option needs some more explanation: $CDO_USE_FFLAGS$. In the next
627 newsection we explain what the need for this option is.
629 A software package {\tt setcd}, available from the Debian distribution
630 and {\tt sunsite.unc.edu}, allows user level control of these flags.
632 \newsection{The need to know the purpose of opening the \cdrom\ device}
634 Traditionally, Unix devices can be used in two different `modes',
635 either by reading/writing to the device file, or by issuing
636 controlling commands to the device, by the device's $ioctl()$
637 call. The problem with \cdrom\ drives, is that they can be used for
638 two entirely different purposes. One is to mount removable
639 file systems, \cdrom s, the other is to play audio CD's. Audio commands
640 are implemented entirely through $ioctl$s, presumably because the
641 first implementation (SUN?) has been such. In principle there is
642 nothing wrong with this, but a good control of the `CD player' demands
643 that the device can {\em always\/} be opened in order to give the
644 $ioctl$ commands, regardless of the state the drive is in.
646 On the other hand, when used as a removable-media disc drive (what the
647 original purpose of \cdrom s is) we would like to make sure that the
648 disc drive is ready for operation upon opening the device. In the old
649 scheme, some \cdrom\ drivers don't do any integrity checking, resulting
650 in a number of i/o errors reported by the VFS to the kernel when an
651 attempt for mounting a \cdrom\ on an empty drive occurs. This is not a
652 particularly elegant way to find out that there is no \cdrom\ inserted;
653 it more-or-less looks like the old IBM-PC trying to read an empty floppy
654 drive for a couple of seconds, after which the system complains it
655 can't read from it. Nowadays we can {\em sense\/} the existence of a
656 removable medium in a drive, and we believe we should exploit that
657 fact. An integrity check on opening of the device, that verifies the
658 availability of a \cdrom\ and its correct type (data), would be
659 desirable.
661 These two ways of using a \cdrom\ drive, principally for data and
662 secondarily for playing audio discs, have different demands for the
663 behavior of the $open()$ call. Audio use simply wants to open the
664 device in order to get a file handle which is needed for issuing
665 $ioctl$ commands, while data use wants to open for correct and
666 reliable data transfer. The only way user programs can indicate what
667 their {\em purpose\/} of opening the device is, is through the $flags$
668 parameter (see {\tt {open(2)}}). For \cdrom\ devices, these flags aren't
669 implemented (some drivers implement checking for write-related flags,
670 but this is not strictly necessary if the device file has correct
671 permission flags). Most option flags simply don't make sense to
672 \cdrom\ devices: $O_CREAT$, $O_NOCTTY$, $O_TRUNC$, $O_APPEND$, and
673 $O_SYNC$ have no meaning to a \cdrom.
675 We therefore propose to use the flag $O_NONBLOCK$ to indicate
676 that the device is opened just for issuing $ioctl$
677 commands. Strictly, the meaning of $O_NONBLOCK$ is that opening and
678 subsequent calls to the device don't cause the calling process to
679 wait. We could interpret this as ``don't wait until someone has
680 inserted some valid data-\cdrom.'' Thus, our proposal of the
681 implementation for the $open()$ call for \cdrom s is:
682 \begin{itemize}
683 \item If no other flags are set than $O_RDONLY$, the device is opened
684 for data transfer, and the return value will be 0 only upon successful
685 initialization of the transfer. The call may even induce some actions
686 on the \cdrom, such as closing the tray.
687 \item If the option flag $O_NONBLOCK$ is set, opening will always be
688 successful, unless the whole device doesn't exist. The drive will take
689 no actions whatsoever.
690 \end{itemize}
692 \subsection{And what about standards?}
694 You might hesitate to accept this proposal as it comes from the
695 \linux\ community, and not from some standardizing institute. What
696 about SUN, SGI, HP and all those other Unix and hardware vendors?
697 Well, these companies are in the lucky position that they generally
698 control both the hardware and software of their supported products,
699 and are large enough to set their own standard. They do not have to
700 deal with a dozen or more different, competing hardware
701 configurations.\footnote{Incidentally, I think that SUN's approach to
702 mounting \cdrom s is very good in origin: under Solaris a
703 volume-daemon automatically mounts a newly inserted \cdrom\ under {\tt
704 {/cdrom/$<volume-name>$/}}. In my opinion they should have pushed this
705 further and have {\em every\/} \cdrom\ on the local area network be
706 mounted at the similar location, \ie, no matter in which particular
707 machine you insert a \cdrom, it will always appear at the same
708 position in the directory tree, on every system. When I wanted to
709 implement such a user-program for \linux, I came across the
710 differences in behavior of the various drivers, and the need for an
711 $ioctl$ informing about media changes.}
713 We believe that using $O_NONBLOCK$ to indicate that a device is being opened
714 for $ioctl$ commands only can be easily introduced in the \linux\
715 community. All the CD-player authors will have to be informed, we can
716 even send in our own patches to the programs. The use of $O_NONBLOCK$
717 has most likely no influence on the behavior of the CD-players on
718 other operating systems than \linux. Finally, a user can always revert
719 to old behavior by a call to $ioctl(file_descriptor, CDROM_CLEAR_OPTIONS,
720 CDO_USE_FFLAGS)$.
722 \subsection{The preferred strategy of $open()$}
724 The routines in \cdromc\ are designed in such a way that run-time
725 configuration of the behavior of \cdrom\ devices (of {\em any\/} type)
726 can be carried out, by the $CDROM_SET/CLEAR_OPTIONS$ $ioctls$. Thus, various
727 modes of operation can be set:
728 \begin{description}
729 \item[$CDO_AUTO_CLOSE \mathrel| CDO_USE_FFLAGS \mathrel| CDO_LOCK$] This
730 is the default setting. (With $CDO_CHECK_TYPE$ it will be better, in the
731 future.) If the device is not yet opened by any other process, and if
732 the device is being opened for data ($O_NONBLOCK$ is not set) and the
733 tray is found to be open, an attempt to close the tray is made. Then,
734 it is verified that a disc is in the drive and, if $CDO_CHECK_TYPE$ is
735 set, that it contains tracks of type `data mode 1.' Only if all tests
736 are passed is the return value zero. The door is locked to prevent file
737 system corruption. If the drive is opened for audio ($O_NONBLOCK$ is
738 set), no actions are taken and a value of 0 will be returned.
739 \item[$CDO_AUTO_CLOSE \mathrel| CDO_AUTO_EJECT \mathrel| CDO_LOCK$] This
740 mimics the behavior of the current sbpcd-driver. The option flags are
741 ignored, the tray is closed on the first open, if necessary. Similarly,
742 the tray is opened on the last release, \ie, if a \cdrom\ is unmounted,
743 it is automatically ejected, such that the user can replace it.
744 \end{description}
745 We hope that these option can convince everybody (both driver
746 maintainers and user program developers) to adopt the new \cdrom\
747 driver scheme and option flag interpretation.
749 \newsection{Description of routines in \cdromc}
751 Only a few routines in \cdromc\ are exported to the drivers. In this
752 new section we will discuss these, as well as the functions that `take
753 over' the \cdrom\ interface to the kernel. The header file belonging
754 to \cdromc\ is called \cdromh. Formerly, some of the contents of this
755 file were placed in the file {\tt {ucdrom.h}}, but this file has now been
756 merged back into \cdromh.
758 \subsection{$Struct\ file_operations\ cdrom_fops$}
760 The contents of this structure were described in section~\ref{cdrom.c}.
761 As already stated, this structure should be used to register block
762 devices with the kernel:
763 $$
764 register_blkdev(major, <name>, \&cdrom_fops);
765 $$
767 \subsection{$Int\ register_cdrom( struct\ cdrom_device_info\ * cdi)$}
769 This function is used in about the same way one registers $cdrom_fops$
770 with the kernel, the device operations and information structures,
771 as described in section~\ref{cdrom.c}, should be registered with the
772 \UCD:
773 $$
774 register_cdrom(\&<device>_info));
775 $$
776 This function returns zero upon success, and non-zero upon
777 failure. The structure $<device>_info$ should have a pointer to the
778 driver's $<device>_dops$, as in
779 $$
780 \vbox{\halign{&$#$\hfil\cr
781 struct\ &cdrom_device_info\ <device>_info = \{\cr
782 & <device>_dops;\cr
783 &\ldots\cr
784 \}\cr
785 }}$$
786 Note that a driver must have one static structure, $<device>_dops$, while
787 it may have as many structures $<device>_info$ as there are minor devices
788 active. $Register_cdrom()$ builds a linked list from these.
790 \subsection{$Int\ unregister_cdrom(struct\ cdrom_device_info * cdi)$}
792 Unregistering device $cdi$ with minor number $MINOR(cdi\to dev)$ removes
793 the minor device from the list. If it was the last registered minor for
794 the low-level driver, this disconnects the registered device-operation
795 routines from the \cdrom\ interface. This function returns zero upon
796 success, and non-zero upon failure.
798 \subsection{$Int\ cdrom_open(struct\ inode * ip, struct\ file * fp)$}
800 This function is not called directly by the low-level drivers, it is
801 listed in the standard $cdrom_fops$. If the VFS opens a file, this
802 function becomes active. A strategy is implemented in this routine,
803 taking care of all capabilities and options that are set in the
804 $cdrom_device_ops$ connected to the device. Then, the program flow is
805 transferred to the device_dependent $open()$ call.
807 \subsection{$Void\ cdrom_release(struct\ inode *ip, struct\ file
808 *fp)$}
810 This function implements the reverse-logic of $cdrom_open()$, and then
811 calls the device-dependent $release()$ routine. When the use-count has
812 reached 0, the allocated buffers are flushed by calls to $sync_dev(dev)$
813 and $invalidate_buffers(dev)$.
816 \subsection{$Int\ cdrom_ioctl(struct\ inode *ip, struct\ file *fp,
817 unsigned\ int\ cmd, unsigned\ long\ arg)$}
818 \label{cdrom-ioctl}
820 This function handles all the standard $ioctl$ requests for \cdrom\
821 devices in a uniform way. The different calls fall into three
822 categories: $ioctl$s that can be directly implemented by device
823 operations, ones that are routed through the call $audio_ioctl()$, and
824 the remaining ones, that are presumable device-dependent. Generally, a
825 negative return value indicates an error.
827 \subsubsection{Directly implemented $ioctl$s}
828 \label{ioctl-direct}
830 The following `old' \cdrom-$ioctl$s are implemented by directly
831 calling device-operations in $cdrom_device_ops$, if implemented and
832 not masked:
833 \begin{description}
834 \item[CDROMMULTISESSION] Requests the last session on a \cdrom.
835 \item[CDROMEJECT] Open tray.
836 \item[CDROMCLOSETRAY] Close tray.
837 \item[CDROMEJECT_SW] If $arg\not=0$, set behavior to auto-close (close
838 tray on first open) and auto-eject (eject on last release), otherwise
839 set behavior to non-moving on $open()$ and $release()$ calls.
840 \item[CDROM_GET_MCN] Get the Media Catalog Number from a CD.
841 \end{description}
843 \subsubsection{$Ioctl$s routed through $audio_ioctl()$}
844 \label{ioctl-audio}
846 The following set of $ioctl$s are all implemented through a call to
847 the $cdrom_fops$ function $audio_ioctl()$. Memory checks and
848 allocation are performed in $cdrom_ioctl()$, and also sanitization of
849 address format ($CDROM_LBA$/$CDROM_MSF$) is done.
850 \begin{description}
851 \item[CDROMSUBCHNL] Get sub-channel data in argument $arg$ of type $struct\
852 cdrom_subchnl *{}$.
853 \item[CDROMREADTOCHDR] Read Table of Contents header, in $arg$ of type
854 $struct\ cdrom_tochdr *{}$.
855 \item[CDROMREADTOCENTRY] Read a Table of Contents entry in $arg$ and
856 specified by $arg$ of type $struct\ cdrom_tocentry *{}$.
857 \item[CDROMPLAYMSF] Play audio fragment specified in Minute, Second,
858 Frame format, delimited by $arg$ of type $struct\ cdrom_msf *{}$.
859 \item[CDROMPLAYTRKIND] Play audio fragment in track-index format
860 delimited by $arg$ of type $struct\ \penalty-1000 cdrom_ti *{}$.
861 \item[CDROMVOLCTRL] Set volume specified by $arg$ of type $struct\
862 cdrom_volctrl *{}$.
863 \item[CDROMVOLREAD] Read volume into by $arg$ of type $struct\
864 cdrom_volctrl *{}$.
865 \item[CDROMSTART] Spin up disc.
866 \item[CDROMSTOP] Stop playback of audio fragment.
867 \item[CDROMPAUSE] Pause playback of audio fragment.
868 \item[CDROMRESUME] Resume playing.
869 \end{description}
871 \subsubsection{New $ioctl$s in \cdromc}
873 The following $ioctl$s have been introduced to allow user programs to
874 control the behavior of individual \cdrom\ devices. New $ioctl$
875 commands can be identified by the underscores in their names.
876 \begin{description}
877 \item[CDROM_SET_OPTIONS] Set options specified by $arg$. Returns the
878 option flag register after modification. Use $arg = \rm0$ for reading
879 the current flags.
880 \item[CDROM_CLEAR_OPTIONS] Clear options specified by $arg$. Returns
881 the option flag register after modification.
882 \item[CDROM_SELECT_SPEED] Select head-rate speed of disc specified as
883 by $arg$ in units of standard cdrom speed (176\,kB/sec raw data or
884 150\,kB/sec file system data). The value 0 means `auto-select', \ie,
885 play audio discs at real time and data discs at maximum speed. The value
886 $arg$ is checked against the maximum head rate of the drive found in the
887 $cdrom_dops$.
888 \item[CDROM_SELECT_DISC] Select disc numbered $arg$ from a juke-box.
889 First disc is numbered 0. The number $arg$ is checked against the
890 maximum number of discs in the juke-box found in the $cdrom_dops$.
891 \item[CDROM_MEDIA_CHANGED] Returns 1 if a disc has been changed since
892 the last call. Note that calls to $cdrom_media_changed$ by the VFS
893 are treated by an independent queue, so both mechanisms will detect
894 a media change once. For juke-boxes, an extra argument $arg$
895 specifies the slot for which the information is given. The special
896 value $CDSL_CURRENT$ requests that information about the currently
897 selected slot be returned.
898 \item[CDROM_DRIVE_STATUS] Returns the status of the drive by a call to
899 $drive_status()$. Return values are defined in section~\ref{drive
900 status}. Note that this call doesn't return information on the
901 current playing activity of the drive; this can be polled through an
902 $ioctl$ call to $CDROMSUBCHNL$. For juke-boxes, an extra argument
903 $arg$ specifies the slot for which (possibly limited) information is
904 given. The special value $CDSL_CURRENT$ requests that information
905 about the currently selected slot be returned.
906 \item[CDROM_DISC_STATUS] Returns the type of the disc currently in the
907 drive. It should be viewed as a complement to $CDROM_DRIVE_STATUS$.
908 This $ioctl$ can provide \emph {some} information about the current
909 disc that is inserted in the drive. This functionality used to be
910 implemented in the low level drivers, but is now carried out
911 entirely in \UCD.
913 The history of development of the CD's use as a carrier medium for
914 various digital information has lead to many different disc types.
915 This $ioctl$ is useful only in the case that CDs have \emph {only
916 one} type of data on them. While this is often the case, it is
917 also very common for CDs to have some tracks with data, and some
918 tracks with audio. Because this is an existing interface, rather
919 than fixing this interface by changing the assumptions it was made
920 under, thereby breaking all user applications that use this
921 function, the \UCD\ implements this $ioctl$ as follows: If the CD in
922 question has audio tracks on it, and it has absolutly no CD-I, XA,
923 or data tracks on it, it will be reported as $CDS_AUDIO$. If it has
924 both audio and data tracks, it will return $CDS_MIXED$. If there
925 are no audio tracks on the disc, and if the CD in question has any
926 CD-I tracks on it, it will be reported as $CDS_XA_2_2$. Failing
927 that, if the CD in question has any XA tracks on it, it will be
928 reported as $CDS_XA_2_1$. Finally, if the CD in question has any
929 data tracks on it, it will be reported as a data CD ($CDS_DATA_1$).
931 This $ioctl$ can return:
932 $$
933 \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
934 CDS_NO_INFO& no information available\cr
935 CDS_NO_DISC& no disc is inserted, or tray is opened\cr
936 CDS_AUDIO& Audio disc (2352 audio bytes/frame)\cr
937 CDS_DATA_1& data disc, mode 1 (2048 user bytes/frame)\cr
938 CDS_XA_2_1& mixed data (XA), mode 2, form 1 (2048 user bytes)\cr
939 CDS_XA_2_2& mixed data (XA), mode 2, form 1 (2324 user bytes)\cr
940 CDS_MIXED& mixed audio/data disc\cr
941 }
942 $$
943 For some information concerning frame layout of the various disc
944 types, see a recent version of \cdromh.
946 \item[CDROM_CHANGER_NSLOTS] Returns the number of slots in a
947 juke-box.
948 \item[CDROMRESET] Reset the drive.
949 \item[CDROM_GET_CAPABILITY] Returns the $capability$ flags for the
950 drive. Refer to section \ref{capability} for more information on
951 these flags.
952 \item[CDROM_LOCKDOOR] Locks the door of the drive. $arg == \rm0$
953 unlocks the door, any other value locks it.
954 \item[CDROM_DEBUG] Turns on debugging info. Only root is allowed
955 to do this. Same semantics as CDROM_LOCKDOOR.
956 \end{description}
958 \subsubsection{Device dependent $ioctl$s}
960 Finally, all other $ioctl$s are passed to the function $dev_ioctl()$,
961 if implemented. No memory allocation or verification is carried out.
963 \newsection{How to update your driver}
965 \begin{enumerate}
966 \item Make a backup of your current driver.
967 \item Get hold of the files \cdromc\ and \cdromh, they should be in
968 the directory tree that came with this documentation.
969 \item Make sure you include \cdromh.
970 \item Change the 3rd argument of $register_blkdev$ from
971 $\&<your-drive>_fops$ to $\&cdrom_fops$.
972 \item Just after that line, add the following to register with the \UCD:
973 $$register_cdrom(\&<your-drive>_info);$$
974 Similarly, add a call to $unregister_cdrom()$ at the appropriate place.
975 \item Copy an example of the device-operations $struct$ to your
976 source, \eg, from {\tt {cm206.c}} $cm206_dops$, and change all
977 entries to names corresponding to your driver, or names you just
978 happen to like. If your driver doesn't support a certain function,
979 make the entry $NULL$. At the entry $capability$ you should list all
980 capabilities your driver currently supports. If your driver
981 has a capability that is not listed, please send me a message.
982 \item Copy the $cdrom_device_info$ declaration from the same example
983 driver, and modify the entries according to your needs. If your
984 driver dynamically determines the capabilities of the hardware, this
985 structure should also be declared dynamically.
986 \item Implement all functions in your $<device>_dops$ structure,
987 according to prototypes listed in \cdromh, and specifications given
988 in section~\ref{cdrom.c}. Most likely you have already implemented
989 the code in a large part, and you will almost certainly need to adapt the
990 prototype and return values.
991 \item Rename your $<device>_ioctl()$ function to $audio_ioctl$ and
992 change the prototype a little. Remove entries listed in the first
993 part in section~\ref{cdrom-ioctl}, if your code was OK, these are
994 just calls to the routines you adapted in the previous step.
995 \item You may remove all remaining memory checking code in the
996 $audio_ioctl()$ function that deals with audio commands (these are
997 listed in the second part of section~\ref{cdrom-ioctl}). There is no
998 need for memory allocation either, so most $case$s in the $switch$
999 statement look similar to:
1000 $$
1001 case\ CDROMREADTOCENTRY\colon get_toc_entry\bigl((struct\
1002 cdrom_tocentry *{})\ arg\bigr);
1003 $$
1004 \item All remaining $ioctl$ cases must be moved to a separate
1005 function, $<device>_ioctl$, the device-dependent $ioctl$s. Note that
1006 memory checking and allocation must be kept in this code!
1007 \item Change the prototypes of $<device>_open()$ and
1008 $<device>_release()$, and remove any strategic code (\ie, tray
1009 movement, door locking, etc.).
1010 \item Try to recompile the drivers. We advise you to use modules, both
1011 for {\tt {cdrom.o}} and your driver, as debugging is much easier this
1012 way.
1013 \end{enumerate}
1015 \newsection{Thanks}
1017 Thanks to all the people involved. First, Erik Andersen, who has
1018 taken over the torch in maintaining \cdromc\ and integrating much
1019 \cdrom-related code in the 2.1-kernel. Thanks to Scott Snyder and
1020 Gerd Knorr, who were the first to implement this interface for SCSI
1021 and IDE-CD drivers and added many ideas for extension of the data
1022 structures relative to kernel~2.0. Further thanks to Heiko Eissfeldt,
1023 Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard M\"onkeberg and Andrew
1024 Kroll, the \linux\ \cdrom\ device driver developers who were kind
1025 enough to give suggestions and criticisms during the writing. Finally
1026 of course, I want to thank Linus Torvalds for making this possible in
1027 the first place.
1029 \vfill
1030 $ \version\ $
1031 \eject
1032 \end{document}