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 =======================
 A Linux CD-ROM standard
 =======================
 
 :Author: David van Leeuwen <david@ElseWare.cistron.nl>
 :Date: 12 March 1999
 :Updated by: Erik Andersen (andersee@debian.org)
 :Updated by: Jens Axboe (axboe@image.dk)
 
 
 Introduction
 ============
 
 Linux is probably the Unix-like operating system that supports
 the widest variety of hardware devices. The reasons for this are
 presumably
 
 - The large list of hardware devices available for the many platforms
   that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
 - The open design of the operating system, such that anybody can write a
   driver for Linux.
 - There is plenty of source code around as examples of how to write a driver.
 
 The openness of Linux, and the many different types of available
 hardware has allowed Linux to support many different hardware devices.
 Unfortunately, the very openness that has allowed Linux to support
 all these different devices has also allowed the behavior of each
 device driver to differ significantly from one device to another.
 This divergence of behavior has been very significant for CD-ROM
 devices; the way a particular drive reacts to a `standard` *ioctl()*
 call varies greatly from one device driver to another. To avoid making
 their drivers totally inconsistent, the writers of Linux CD-ROM
 drivers generally created new device drivers by understanding, copying,
 and then changing an existing one. Unfortunately, this practice did not
 maintain uniform behavior across all the Linux CD-ROM drivers.
 
 This document describes an effort to establish Uniform behavior across
 all the different CD-ROM device drivers for Linux. This document also
 defines the various *ioctl()'s*, and how the low-level CD-ROM device
 drivers should implement them. Currently (as of the Linux 2.1.\ *x*
 development kernels) several low-level CD-ROM device drivers, including
 both IDE/ATAPI and SCSI, now use this Uniform interface.
 
 When the CD-ROM was developed, the interface between the CD-ROM drive
 and the computer was not specified in the standards. As a result, many
 different CD-ROM interfaces were developed. Some of them had their
 own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
 manufacturers adopted an existing electrical interface and changed
 the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
 adapted their drives to one or more of the already existing electrical
 interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
 most of the `NoName` manufacturers). In cases where a new drive really
 brought its own interface or used its own command set and flow control
 scheme, either a separate driver had to be written, or an existing
 driver had to be enhanced. History has delivered us CD-ROM support for
 many of these different interfaces. Nowadays, almost all new CD-ROM
 drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
 manufacturer will create a new interface. Even finding drives for the
 old proprietary interfaces is getting difficult.
 
 When (in the 1.3.70's) I looked at the existing software interface,
 which was expressed through `cdrom.h`, it appeared to be a rather wild
 set of commands and data formats [#f1]_. It seemed that many
 features of the software interface had been added to accommodate the
 capabilities of a particular drive, in an *ad hoc* manner. More
 importantly, it appeared that the behavior of the `standard` commands
 was different for most of the different drivers: e. g., some drivers
 close the tray if an *open()* call occurs when the tray is open, while
 others do not. Some drivers lock the door upon opening the device, to
 prevent an incoherent file system, but others don't, to allow software
 ejection. Undoubtedly, the capabilities of the different drives vary,
 but even when two drives have the same capability their drivers'
 behavior was usually different.
 
 .. [#f1]
    I cannot recollect what kernel version I looked at, then,
    presumably 1.2.13 and 1.3.34 --- the latest kernel that I was
    indirectly involved in.
 
 I decided to start a discussion on how to make all the Linux CD-ROM
 drivers behave more uniformly. I began by contacting the developers of
 the many CD-ROM drivers found in the Linux kernel. Their reactions
 encouraged me to write the Uniform CD-ROM Driver which this document is
 intended to describe. The implementation of the Uniform CD-ROM Driver is
 in the file `cdrom.c`. This driver is intended to be an additional software
 layer that sits on top of the low-level device drivers for each CD-ROM drive.
 By adding this additional layer, it is possible to have all the different
 CD-ROM devices behave **exactly** the same (insofar as the underlying
 hardware will allow).
 
 The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
 whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
 Driver is simply to give people writing application programs for CD-ROM drives
 **one** Linux CD-ROM interface with consistent behavior for all
 CD-ROM devices. In addition, this also provides a consistent interface
 between the low-level device driver code and the Linux kernel. Care
 is taken that 100% compatibility exists with the data structures and
 programmer's interface defined in `cdrom.h`. This guide was written to
 help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
 Driver code defined in `cdrom.c`.
 
 Personally, I think that the most important hardware interfaces are
 the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
 of hardware drop continuously, it is also likely that people may have
 more than one CD-ROM drive, possibly of mixed types. It is important
 that these drives behave in the same way. In December 1994, one of the
 cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary
 drive. In the months that I was busy writing a Linux driver for it,
 proprietary drives became obsolete and IDE/ATAPI drives became the
 standard. At the time of the last update to this document (November
 1997) it is becoming difficult to even **find** anything less than a
 16 speed CD-ROM drive, and 24 speed drives are common.
 
 .. _cdrom_api:
 
 Standardizing through another software level
 ============================================
 
 At the time this document was conceived, all drivers directly
 implemented the CD-ROM *ioctl()* calls through their own routines. This
 led to the danger of different drivers forgetting to do important things
 like checking that the user was giving the driver valid data. More
 importantly, this led to the divergence of behavior, which has already
 been discussed.
 
 For this reason, the Uniform CD-ROM Driver was created to enforce consistent
 CD-ROM drive behavior, and to provide a common set of services to the various
 low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another
 software-level, that separates the *ioctl()* and *open()* implementation
 from the actual hardware implementation. Note that this effort has
 made few changes which will affect a user's application programs. The
 greatest change involved moving the contents of the various low-level
 CD-ROM drivers\' header files to the kernel's cdrom directory. This was
 done to help ensure that the user is only presented with only one cdrom
 interface, the interface defined in `cdrom.h`.
 
 CD-ROM drives are specific enough (i. e., different from other
 block-devices such as floppy or hard disc drives), to define a set
 of common **CD-ROM device operations**, *<cdrom-device>_dops*.
 These operations are different from the classical block-device file
 operations, *<block-device>_fops*.
 
 The routines for the Uniform CD-ROM Driver interface level are implemented
 in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
 with the kernel as a block device by registering the following general
 *struct file_operations*::
 
         struct file_operations cdrom_fops = {
                 NULL,                   /* lseek */
                 block _read ,           /* read--general block-dev read */
                 block _write,           /* write--general block-dev write */
                 NULL,                   /* readdir */
                 NULL,                   /* select */
                 cdrom_ioctl,            /* ioctl */
                 NULL,                   /* mmap */
                 cdrom_open,             /* open */
                 cdrom_release,          /* release */
                 NULL,                   /* fsync */
                 NULL,                   /* fasync */
                 NULL                    /* revalidate */
         };
 
 Every active CD-ROM device shares this *struct*. The routines
 declared above are all implemented in `cdrom.c`, since this file is the
 place where the behavior of all CD-ROM-devices is defined and
 standardized. The actual interface to the various types of CD-ROM
 hardware is still performed by various low-level CD-ROM-device
 drivers. These routines simply implement certain **capabilities**
 that are common to all CD-ROM (and really, all removable-media
 devices).
 
 Registration of a low-level CD-ROM device driver is now done through
 the general routines in `cdrom.c`, not through the Virtual File System
 (VFS) any more. The interface implemented in `cdrom.c` is carried out
 through two general structures that contain information about the
 capabilities of the driver, and the specific drives on which the
 driver operates. The structures are:
 
 cdrom_device_ops
   This structure contains information about the low-level driver for a
   CD-ROM device. This structure is conceptually connected to the major
   number of the device (although some drivers may have different
   major numbers, as is the case for the IDE driver).
 
 cdrom_device_info
   This structure contains information about a particular CD-ROM drive,
   such as its device name, speed, etc. This structure is conceptually
   connected to the minor number of the device.
 
 Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
 is done by the low-level device driver though a call to::
 
         register_cdrom(struct cdrom_device_info * <device>_info)
 
 The device information structure, *<device>_info*, contains all the
 information needed for the kernel to interface with the low-level
 CD-ROM device driver. One of the most important entries in this
 structure is a pointer to the *cdrom_device_ops* structure of the
 low-level driver.
 
 The device operations structure, *cdrom_device_ops*, contains a list
 of pointers to the functions which are implemented in the low-level
 device driver. When `cdrom.c` accesses a CD-ROM device, it does it
 through the functions in this structure. It is impossible to know all
 the capabilities of future CD-ROM drives, so it is expected that this
 list may need to be expanded from time to time as new technologies are
 developed. For example, CD-R and CD-R/W drives are beginning to become
 popular, and support will soon need to be added for them. For now, the
 current *struct* is::
 
         struct cdrom_device_ops {
                 int (*open)(struct cdrom_device_info *, int)
                 void (*release)(struct cdrom_device_info *);
                 int (*drive_status)(struct cdrom_device_info *, int);
                 unsigned int (*check_events)(struct cdrom_device_info *,
                                              unsigned int, int);
                 int (*media_changed)(struct cdrom_device_info *, int);
                 int (*tray_move)(struct cdrom_device_info *, int);
                 int (*lock_door)(struct cdrom_device_info *, int);
                 int (*select_speed)(struct cdrom_device_info *, int);
                 int (*get_last_session) (struct cdrom_device_info *,
                                          struct cdrom_multisession *);
                 int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
                 int (*reset)(struct cdrom_device_info *);
                 int (*audio_ioctl)(struct cdrom_device_info *,
                                    unsigned int, void *);
                 const int capability;           /* capability flags */
                 int (*generic_packet)(struct cdrom_device_info *,
                                       struct packet_command *);
         };
 
 When a low-level device driver implements one of these capabilities,
 it should add a function pointer to this *struct*. When a particular
 function is not implemented, however, this *struct* should contain a
 NULL instead. The *capability* flags specify the capabilities of the
 CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
 is registered with the Uniform CD-ROM Driver.
 
 Note that most functions have fewer parameters than their
 *blkdev_fops* counterparts. This is because very little of the
 information in the structures *inode* and *file* is used. For most
 drivers, the main parameter is the *struct* *cdrom_device_info*, from
 which the major and minor number can be extracted. (Most low-level
 CD-ROM drivers don't even look at the major and minor number though,
 since many of them only support one device.) This will be available
 through *dev* in *cdrom_device_info* described below.
 
 The drive-specific, minor-like information that is registered with
 `cdrom.c`, currently contains the following fields::
 
   struct cdrom_device_info {
         const struct cdrom_device_ops * ops;    /* device operations for this major */
         struct list_head list;                  /* linked list of all device_info */
         struct gendisk * disk;                  /* matching block layer disk */
         void *  handle;                         /* driver-dependent data */
 
         int mask;                               /* mask of capability: disables them */
         int speed;                              /* maximum speed for reading data */
         int capacity;                           /* number of discs in a jukebox */
 
         unsigned int options:30;                /* options flags */
         unsigned mc_flags:2;                    /*  media-change buffer flags */
         unsigned int vfs_events;                /*  cached events for vfs path */
         unsigned int ioctl_events;              /*  cached events for ioctl path */
         int use_count;                          /*  number of times device is opened */
         char name[20];                          /*  name of the device type */
 
         __u8 sanyo_slot : 2;                    /*  Sanyo 3-CD changer support */
         __u8 keeplocked : 1;                    /*  CDROM_LOCKDOOR status */
         __u8 reserved : 5;                      /*  not used yet */
         int cdda_method;                        /*  see CDDA_* flags */
         __u8 last_sense;                        /*  saves last sense key */
         __u8 media_written;                     /*  dirty flag, DVD+RW bookkeeping */
         unsigned short mmc3_profile;            /*  current MMC3 profile */
         int for_data;                           /*  unknown:TBD */
         int (*exit)(struct cdrom_device_info *);/*  unknown:TBD */
         int mrw_mode_page;                      /*  which MRW mode page is in use */
   };
 
 Using this *struct*, a linked list of the registered minor devices is
 built, using the *next* field. The device number, the device operations
 struct and specifications of properties of the drive are stored in this
 structure.
 
 The *mask* flags can be used to mask out some of the capabilities listed
 in *ops->capability*, if a specific drive doesn't support a feature
 of the driver. The value *speed* specifies the maximum head-rate of the
 drive, measured in units of normal audio speed (176kB/sec raw data or
 150kB/sec file system data). The parameters are declared *const*
 because they describe properties of the drive, which don't change after
 registration.
 
 A few registers contain variables local to the CD-ROM drive. The
 flags *options* are used to specify how the general CD-ROM routines
 should behave. These various flags registers should provide enough
 flexibility to adapt to the different users' wishes (and **not** the
 `arbitrary` wishes of the author of the low-level device driver, as is
 the case in the old scheme). The register *mc_flags* is used to buffer
 the information from *media_changed()* to two separate queues. Other
 data that is specific to a minor drive, can be accessed through *handle*,
 which can point to a data structure specific to the low-level driver.
 The fields *use_count*, *next*, *options* and *mc_flags* need not be
 initialized.
 
 The intermediate software layer that `cdrom.c` forms will perform some
 additional bookkeeping. The use count of the device (the number of
 processes that have the device opened) is registered in *use_count*. The
 function *cdrom_ioctl()* will verify the appropriate user-memory regions
 for read and write, and in case a location on the CD is transferred,
 it will `sanitize` the format by making requests to the low-level
 drivers in a standard format, and translating all formats between the
 user-software and low level drivers. This relieves much of the drivers'
 memory checking and format checking and translation. Also, the necessary
 structures will be declared on the program stack.
 
 The implementation of the functions should be as defined in the
 following sections. Two functions **must** be implemented, namely
 *open()* and *release()*. Other functions may be omitted, their
 corresponding capability flags will be cleared upon registration.
 Generally, a function returns zero on success and negative on error. A
 function call should return only after the command has completed, but of
 course waiting for the device should not use processor time.
 
 ::
 
         int open(struct cdrom_device_info *cdi, int purpose)
 
 *Open()* should try to open the device for a specific *purpose*, which
 can be either:
 
 - Open for reading data, as done by `mount()` (2), or the
   user commands `dd` or `cat`.
 - Open for *ioctl* commands, as done by audio-CD playing programs.
 
 Notice that any strategic code (closing tray upon *open()*, etc.) is
 done by the calling routine in `cdrom.c`, so the low-level routine
 should only be concerned with proper initialization, such as spinning
 up the disc, etc.
 
 ::
 
         void release(struct cdrom_device_info *cdi)
 
 Device-specific actions should be taken such as spinning down the device.
 However, strategic actions such as ejection of the tray, or unlocking
 the door, should be left over to the general routine *cdrom_release()*.
 This is the only function returning type *void*.
 
 .. _cdrom_drive_status:
 
 ::
 
         int drive_status(struct cdrom_device_info *cdi, int slot_nr)
 
 The function *drive_status*, if implemented, should provide
 information on the status of the drive (not the status of the disc,
 which may or may not be in the drive). If the drive is not a changer,
 *slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::
 
 
         CDS_NO_INFO             /* no information available */
         CDS_NO_DISC             /* no disc is inserted, tray is closed */
         CDS_TRAY_OPEN           /* tray is opened */
         CDS_DRIVE_NOT_READY     /* something is wrong, tray is moving? */
         CDS_DISC_OK             /* a disc is loaded and everything is fine */
 
 ::
 
         int tray_move(struct cdrom_device_info *cdi, int position)
 
 This function, if implemented, should control the tray movement. (No
 other function should control this.) The parameter *position* controls
 the desired direction of movement:
 
 - 0 Close tray
 - 1 Open tray
 
 This function returns 0 upon success, and a non-zero value upon
 error. Note that if the tray is already in the desired position, no
 action need be taken, and the return value should be 0.
 
 ::
 
         int lock_door(struct cdrom_device_info *cdi, int lock)
 
 This function (and no other code) controls locking of the door, if the
 drive allows this. The value of *lock* controls the desired locking
 state:
 
 - 0 Unlock door, manual opening is allowed
 - 1 Lock door, tray cannot be ejected manually
 
 This function returns 0 upon success, and a non-zero value upon
 error. Note that if the door is already in the requested state, no
 action need be taken, and the return value should be 0.
 
 ::
 
         int select_speed(struct cdrom_device_info *cdi, int speed)
 
 Some CD-ROM drives are capable of changing their head-speed. There
 are several reasons for changing the speed of a CD-ROM drive. Badly
 pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
 CD-ROM drives can obtain very high head rates (up to *24x* is
 common). It has been reported that these drives can make reading
 errors at these high speeds, reducing the speed can prevent data loss
 in these circumstances. Finally, some of these drives can
 make an annoyingly loud noise, which a lower speed may reduce.
 
 This function specifies the speed at which data is read or audio is
 played back. The value of *speed* specifies the head-speed of the
 drive, measured in units of standard cdrom speed (176kB/sec raw data
 or 150kB/sec file system data). So to request that a CD-ROM drive
 operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
 with *speed=2*. The special value `0` means `auto-selection`, i. e.,
 maximum data-rate or real-time audio rate. If the drive doesn't have
 this `auto-selection` capability, the decision should be made on the
 current disc loaded and the return value should be positive. A negative
 return value indicates an error.
 
 ::
 
         int get_last_session(struct cdrom_device_info *cdi,
                              struct cdrom_multisession *ms_info)
 
 This function should implement the old corresponding *ioctl()*. For
 device *cdi->dev*, the start of the last session of the current disc
 should be returned in the pointer argument *ms_info*. Note that
 routines in `cdrom.c` have sanitized this argument: its requested
 format will **always** be of the type *CDROM_LBA* (linear block
 addressing mode), whatever the calling software requested. But
 sanitization goes even further: the low-level implementation may
 return the requested information in *CDROM_MSF* format if it wishes so
 (setting the *ms_info->addr_format* field appropriately, of
 course) and the routines in `cdrom.c` will make the transformation if
 necessary. The return value is 0 upon success.
 
 ::
 
         int get_mcn(struct cdrom_device_info *cdi,
                     struct cdrom_mcn *mcn)
 
 Some discs carry a `Media Catalog Number` (MCN), also called
 `Universal Product Code` (UPC). This number should reflect the number
 that is generally found in the bar-code on the product. Unfortunately,
 the few discs that carry such a number on the disc don't even use the
 same format. The return argument to this function is a pointer to a
 pre-declared memory region of type *struct cdrom_mcn*. The MCN is
 expected as a 13-character string, terminated by a null-character.
 
 ::
 
         int reset(struct cdrom_device_info *cdi)
 
 This call should perform a hard-reset on the drive (although in
 circumstances that a hard-reset is necessary, a drive may very well not
 listen to commands anymore). Preferably, control is returned to the
 caller only after the drive has finished resetting. If the drive is no
 longer listening, it may be wise for the underlying low-level cdrom
 driver to time out.
 
 ::
 
         int audio_ioctl(struct cdrom_device_info *cdi,
                         unsigned int cmd, void *arg)
 
 Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
 implemented by the routines described above, and hence the function
 *cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
 audio-control. We have decided to leave these to be accessed through a
 single function, repeating the arguments *cmd* and *arg*. Note that
 the latter is of type *void*, rather than *unsigned long int*.
 The routine *cdrom_ioctl()* does do some useful things,
 though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
 Seconds, Frames) for all audio calls. It also verifies the memory
 location of *arg*, and reserves stack-memory for the argument. This
 makes implementation of the *audio_ioctl()* much simpler than in the
 old driver scheme. For example, you may look up the function
 *cm206_audio_ioctl()* `cm206.c` that should be updated with
 this documentation.
 
 An unimplemented ioctl should return *-ENOSYS*, but a harmless request
 (e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
 errors should be according to the standards, whatever they are. When
 an error is returned by the low-level driver, the Uniform CD-ROM Driver
 tries whenever possible to return the error code to the calling program.
 (We may decide to sanitize the return value in *cdrom_ioctl()* though, in
 order to guarantee a uniform interface to the audio-player software.)
 
 ::
 
         int dev_ioctl(struct cdrom_device_info *cdi,
                       unsigned int cmd, unsigned long arg)
 
 Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
 they are introduced to service some capabilities of certain drives. In
 fact, there are 6 different *ioctl()'s* for reading data, either in some
 particular kind of format, or audio data. Not many drives support
 reading audio tracks as data, I believe this is because of protection
 of copyrights of artists. Moreover, I think that if audio-tracks are
 supported, it should be done through the VFS and not via *ioctl()'s*. A
 problem here could be the fact that audio-frames are 2352 bytes long,
 so either the audio-file-system should ask for 75264 bytes at once
 (the least common multiple of 512 and 2352), or the drivers should
 bend their backs to cope with this incoherence (to which I would be
 opposed). Furthermore, it is very difficult for the hardware to find
 the exact frame boundaries, since there are no synchronization headers
 in audio frames. Once these issues are resolved, this code should be
 standardized in `cdrom.c`.
 
 Because there are so many *ioctl()'s* that seem to be introduced to
 satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
 are routed through the call *dev_ioctl()*. In principle, `private`
 *ioctl()*\ 's should be numbered after the device's major number, and not
 the general CD-ROM *ioctl* number, `0x53`. Currently the
 non-supported *ioctl()'s* are:
 
         CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW,
         CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL
 
 .. [#f2]
 
    Is there software around that actually uses these? I'd be interested!
 
 .. _cdrom_capabilities:
 
 CD-ROM capabilities
 -------------------
 
 Instead of just implementing some *ioctl* calls, the interface in
 `cdrom.c` supplies the possibility to indicate the **capabilities**
 of a CD-ROM drive. This can be done by ORing any number of
 capability-constants that are defined in `cdrom.h` at the registration
 phase. Currently, the capabilities are any of::
 
         CDC_CLOSE_TRAY          /* can close tray by software control */
         CDC_OPEN_TRAY           /* can open tray */
         CDC_LOCK                /* can lock and unlock the door */
         CDC_SELECT_SPEED        /* can select speed, in units of * sim*150 ,kB/s */
         CDC_SELECT_DISC         /* drive is juke-box */
         CDC_MULTI_SESSION       /* can read sessions *> rm1* */
         CDC_MCN                 /* can read Media Catalog Number */
         CDC_MEDIA_CHANGED       /* can report if disc has changed */
         CDC_PLAY_AUDIO          /* can perform audio-functions (play, pause, etc) */
         CDC_RESET               /* hard reset device */
         CDC_IOCTLS              /* driver has non-standard ioctls */
         CDC_DRIVE_STATUS        /* driver implements drive status */
 
 The capability flag is declared *const*, to prevent drivers from
 accidentally tampering with the contents. The capability flags actually
 inform `cdrom.c` of what the driver can do. If the drive found
 by the driver does not have the capability, is can be masked out by
 the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
 driver has implemented the code for loading and ejecting CD-ROM's, and
 hence its corresponding flags in *capability* will be set. But a SCSI
 CD-ROM drive might be a caddy system, which can't load the tray, and
 hence for this drive the *cdrom_device_info* struct will have set
 the *CDC_CLOSE_TRAY* bit in *mask*.
 
 In the file `cdrom.c` you will encounter many constructions of the type::
 
         if (cdo->capability & ~cdi->mask & CDC _<capability>) ...
 
 There is no *ioctl* to set the mask... The reason is that
 I think it is better to control the **behavior** rather than the
 **capabilities**.
 
 Options
 -------
 
 A final flag register controls the **behavior** of the CD-ROM
 drives, in order to satisfy different users' wishes, hopefully
 independently of the ideas of the respective author who happened to
 have made the drive's support available to the Linux community. The
 current behavior options are::
 
         CDO_AUTO_CLOSE  /* try to close tray upon device open() */
         CDO_AUTO_EJECT  /* try to open tray on last device close() */
         CDO_USE_FFLAGS  /* use file_pointer->f_flags to indicate purpose for open() */
         CDO_LOCK        /* try to lock door if device is opened */
         CDO_CHECK_TYPE  /* ensure disc type is data if opened for data */
 
 The initial value of this register is
 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
 interface and software standards. Before you protest, there are two
 new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
 behavior by software. These are::
 
         CDROM_SET_OPTIONS       /* set options specified in (int)arg */
         CDROM_CLEAR_OPTIONS     /* clear options specified in (int)arg */
 
 One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
 newsection we explain what the need for this option is.
 
 A software package `setcd`, available from the Debian distribution
 and `sunsite.unc.edu`, allows user level control of these flags.
 
 
 The need to know the purpose of opening the CD-ROM device
 =========================================================
 
 Traditionally, Unix devices can be used in two different `modes`,
 either by reading/writing to the device file, or by issuing
 controlling commands to the device, by the device's *ioctl()*
 call. The problem with CD-ROM drives, is that they can be used for
 two entirely different purposes. One is to mount removable
 file systems, CD-ROM's, the other is to play audio CD's. Audio commands
 are implemented entirely through *ioctl()\'s*, presumably because the
 first implementation (SUN?) has been such. In principle there is
 nothing wrong with this, but a good control of the `CD player` demands
 that the device can **always** be opened in order to give the
 *ioctl* commands, regardless of the state the drive is in.
 
 On the other hand, when used as a removable-media disc drive (what the
 original purpose of CD-ROM s is) we would like to make sure that the
 disc drive is ready for operation upon opening the device. In the old
 scheme, some CD-ROM drivers don't do any integrity checking, resulting
 in a number of i/o errors reported by the VFS to the kernel when an
 attempt for mounting a CD-ROM on an empty drive occurs. This is not a
 particularly elegant way to find out that there is no CD-ROM inserted;
 it more-or-less looks like the old IBM-PC trying to read an empty floppy
 drive for a couple of seconds, after which the system complains it
 can't read from it. Nowadays we can **sense** the existence of a
 removable medium in a drive, and we believe we should exploit that
 fact. An integrity check on opening of the device, that verifies the
 availability of a CD-ROM and its correct type (data), would be
 desirable.
 
 These two ways of using a CD-ROM drive, principally for data and
 secondarily for playing audio discs, have different demands for the
 behavior of the *open()* call. Audio use simply wants to open the
 device in order to get a file handle which is needed for issuing
 *ioctl* commands, while data use wants to open for correct and
 reliable data transfer. The only way user programs can indicate what
 their *purpose* of opening the device is, is through the *flags*
 parameter (see `open(2)`). For CD-ROM devices, these flags aren't
 implemented (some drivers implement checking for write-related flags,
 but this is not strictly necessary if the device file has correct
 permission flags). Most option flags simply don't make sense to
 CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
 *O_SYNC* have no meaning to a CD-ROM.
 
 We therefore propose to use the flag *O_NONBLOCK* to indicate
 that the device is opened just for issuing *ioctl*
 commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
 subsequent calls to the device don't cause the calling process to
 wait. We could interpret this as don't wait until someone has
 inserted some valid data-CD-ROM. Thus, our proposal of the
 implementation for the *open()* call for CD-ROM s is:
 
 - If no other flags are set than *O_RDONLY*, the device is opened
   for data transfer, and the return value will be 0 only upon successful
   initialization of the transfer. The call may even induce some actions
   on the CD-ROM, such as closing the tray.
 - If the option flag *O_NONBLOCK* is set, opening will always be
   successful, unless the whole device doesn't exist. The drive will take
   no actions whatsoever.
 
 And what about standards?
 -------------------------
 
 You might hesitate to accept this proposal as it comes from the
 Linux community, and not from some standardizing institute. What
 about SUN, SGI, HP and all those other Unix and hardware vendors?
 Well, these companies are in the lucky position that they generally
 control both the hardware and software of their supported products,
 and are large enough to set their own standard. They do not have to
 deal with a dozen or more different, competing hardware
 configurations\ [#f3]_.
 
 .. [#f3]
 
    Incidentally, I think that SUN's approach to mounting CD-ROM s is very
    good in origin: under Solaris a volume-daemon automatically mounts a
    newly inserted CD-ROM under `/cdrom/*<volume-name>*`.
 
    In my opinion they should have pushed this
    further and have **every** CD-ROM on the local area network be
    mounted at the similar location, i. e., no matter in which particular
    machine you insert a CD-ROM, it will always appear at the same
    position in the directory tree, on every system. When I wanted to
    implement such a user-program for Linux, I came across the
    differences in behavior of the various drivers, and the need for an
    *ioctl* informing about media changes.
 
 We believe that using *O_NONBLOCK* to indicate that a device is being opened
 for *ioctl* commands only can be easily introduced in the Linux
 community. All the CD-player authors will have to be informed, we can
 even send in our own patches to the programs. The use of *O_NONBLOCK*
 has most likely no influence on the behavior of the CD-players on
 other operating systems than Linux. Finally, a user can always revert
 to old behavior by a call to
 *ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.
 
 The preferred strategy of *open()*
 ----------------------------------
 
 The routines in `cdrom.c` are designed in such a way that run-time
 configuration of the behavior of CD-ROM devices (of **any** type)
 can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
 modes of operation can be set:
 
 `CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`
    This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
    the future.) If the device is not yet opened by any other process, and if
    the device is being opened for data (*O_NONBLOCK* is not set) and the
    tray is found to be open, an attempt to close the tray is made. Then,
    it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
    set, that it contains tracks of type `data mode 1`. Only if all tests
    are passed is the return value zero. The door is locked to prevent file
    system corruption. If the drive is opened for audio (*O_NONBLOCK* is
    set), no actions are taken and a value of 0 will be returned.
 
 `CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK`
    This mimics the behavior of the current sbpcd-driver. The option flags are
    ignored, the tray is closed on the first open, if necessary. Similarly,
    the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
    it is automatically ejected, such that the user can replace it.
 
 We hope that these option can convince everybody (both driver
 maintainers and user program developers) to adopt the new CD-ROM
 driver scheme and option flag interpretation.
 
 Description of routines in `cdrom.c`
 ====================================
 
 Only a few routines in `cdrom.c` are exported to the drivers. In this
 new section we will discuss these, as well as the functions that `take
 over` the CD-ROM interface to the kernel. The header file belonging
 to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
 file were placed in the file `ucdrom.h`, but this file has now been
 merged back into `cdrom.h`.
 
 ::
 
         struct file_operations cdrom_fops
 
 The contents of this structure were described in cdrom_api_.
 A pointer to this structure is assigned to the *fops* field
 of the *struct gendisk*.
 
 ::
 
         int register_cdrom(struct cdrom_device_info *cdi)
 
 This function is used in about the same way one registers *cdrom_fops*
 with the kernel, the device operations and information structures,
 as described in cdrom_api_, should be registered with the
 Uniform CD-ROM Driver::
 
         register_cdrom(&<device>_info);
 
 
 This function returns zero upon success, and non-zero upon
 failure. The structure *<device>_info* should have a pointer to the
 driver's *<device>_dops*, as in::
 
         struct cdrom_device_info <device>_info = {
                 <device>_dops;
                 ...
         }
 
 Note that a driver must have one static structure, *<device>_dops*, while
 it may have as many structures *<device>_info* as there are minor devices
 active. *Register_cdrom()* builds a linked list from these.
 
 
 ::
 
         void unregister_cdrom(struct cdrom_device_info *cdi)
 
 Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
 the minor device from the list. If it was the last registered minor for
 the low-level driver, this disconnects the registered device-operation
 routines from the CD-ROM interface. This function returns zero upon
 success, and non-zero upon failure.
 
 ::
 
         int cdrom_open(struct inode * ip, struct file * fp)
 
 This function is not called directly by the low-level drivers, it is
 listed in the standard *cdrom_fops*. If the VFS opens a file, this
 function becomes active. A strategy is implemented in this routine,
 taking care of all capabilities and options that are set in the
 *cdrom_device_ops* connected to the device. Then, the program flow is
 transferred to the device_dependent *open()* call.
 
 ::
 
         void cdrom_release(struct inode *ip, struct file *fp)
 
 This function implements the reverse-logic of *cdrom_open()*, and then
 calls the device-dependent *release()* routine. When the use-count has
 reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
 and *invalidate_buffers(dev)*.
 
 
 .. _cdrom_ioctl:
 
 ::
 
         int cdrom_ioctl(struct inode *ip, struct file *fp,
                         unsigned int cmd, unsigned long arg)
 
 This function handles all the standard *ioctl* requests for CD-ROM
 devices in a uniform way. The different calls fall into three
 categories: *ioctl()'s* that can be directly implemented by device
 operations, ones that are routed through the call *audio_ioctl()*, and
 the remaining ones, that are presumable device-dependent. Generally, a
 negative return value indicates an error.
 
 Directly implemented *ioctl()'s*
 --------------------------------
 
 The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
 calling device-operations in *cdrom_device_ops*, if implemented and
 not masked:
 
 `CDROMMULTISESSION`
         Requests the last session on a CD-ROM.
 `CDROMEJECT`
         Open tray.
 `CDROMCLOSETRAY`
         Close tray.
 `CDROMEJECT_SW`
         If *arg\not=0*, set behavior to auto-close (close
         tray on first open) and auto-eject (eject on last release), otherwise
         set behavior to non-moving on *open()* and *release()* calls.
 `CDROM_GET_MCN`
         Get the Media Catalog Number from a CD.
 
 *Ioctl*s routed through *audio_ioctl()*
 ---------------------------------------
 
 The following set of *ioctl()'s* are all implemented through a call to
 the *cdrom_fops* function *audio_ioctl()*. Memory checks and
 allocation are performed in *cdrom_ioctl()*, and also sanitization of
 address format (*CDROM_LBA*/*CDROM_MSF*) is done.
 
 `CDROMSUBCHNL`
         Get sub-channel data in argument *arg* of type
         `struct cdrom_subchnl *`.
 `CDROMREADTOCHDR`
         Read Table of Contents header, in *arg* of type
         `struct cdrom_tochdr *`.
 `CDROMREADTOCENTRY`
         Read a Table of Contents entry in *arg* and specified by *arg*
         of type `struct cdrom_tocentry *`.
 `CDROMPLAYMSF`
         Play audio fragment specified in Minute, Second, Frame format,
         delimited by *arg* of type `struct cdrom_msf *`.
 `CDROMPLAYTRKIND`
         Play audio fragment in track-index format delimited by *arg*
         of type `struct cdrom_ti *`.
 `CDROMVOLCTRL`
         Set volume specified by *arg* of type `struct cdrom_volctrl *`.
 `CDROMVOLREAD`
         Read volume into by *arg* of type `struct cdrom_volctrl *`.
 `CDROMSTART`
         Spin up disc.
 `CDROMSTOP`
         Stop playback of audio fragment.
 `CDROMPAUSE`
         Pause playback of audio fragment.
 `CDROMRESUME`
         Resume playing.
 
 New *ioctl()'s* in `cdrom.c`
 ----------------------------
 
 The following *ioctl()'s* have been introduced to allow user programs to
 control the behavior of individual CD-ROM devices. New *ioctl*
 commands can be identified by the underscores in their names.
 
 `CDROM_SET_OPTIONS`
         Set options specified by *arg*. Returns the option flag register
         after modification. Use *arg = \rm0* for reading the current flags.
 `CDROM_CLEAR_OPTIONS`
         Clear options specified by *arg*. Returns the option flag register
         after modification.
 `CDROM_SELECT_SPEED`
         Select head-rate speed of disc specified as by *arg* in units
         of standard cdrom speed (176\,kB/sec raw data or
         150kB/sec file system data). The value 0 means `auto-select`,
         i. e., play audio discs at real time and data discs at maximum speed.
         The value *arg* is checked against the maximum head rate of the
         drive found in the *cdrom_dops*.
 `CDROM_SELECT_DISC`
         Select disc numbered *arg* from a juke-box.
 
         First disc is numbered 0. The number *arg* is checked against the
         maximum number of discs in the juke-box found in the *cdrom_dops*.
 `CDROM_MEDIA_CHANGED`
         Returns 1 if a disc has been changed since the last call.
         For juke-boxes, an extra argument *arg*
         specifies the slot for which the information is given. The special
         value *CDSL_CURRENT* requests that information about the currently
         selected slot be returned.
 `CDROM_TIMED_MEDIA_CHANGE`
         Checks whether the disc has been changed since a user supplied time
         and returns the time of the last disc change.
 
         *arg* is a pointer to a *cdrom_timed_media_change_info* struct.
         *arg->last_media_change* may be set by calling code to signal
         the timestamp of the last known media change (by the caller).
         Upon successful return, this ioctl call will set
         *arg->last_media_change* to the latest media change timestamp (in ms)
         known by the kernel/driver and set *arg->has_changed* to 1 if
         that timestamp is more recent than the timestamp set by the caller.
 `CDROM_DRIVE_STATUS`
         Returns the status of the drive by a call to
         *drive_status()*. Return values are defined in cdrom_drive_status_.
         Note that this call doesn't return information on the
         current playing activity of the drive; this can be polled through
         an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
         *arg* specifies the slot for which (possibly limited) information is
         given. The special value *CDSL_CURRENT* requests that information
         about the currently selected slot be returned.
 `CDROM_DISC_STATUS`
         Returns the type of the disc currently in the drive.
         It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
         This *ioctl* can provide *some* information about the current
         disc that is inserted in the drive. This functionality used to be
         implemented in the low level drivers, but is now carried out
         entirely in Uniform CD-ROM Driver.
 
         The history of development of the CD's use as a carrier medium for
         various digital information has lead to many different disc types.
         This *ioctl* is useful only in the case that CDs have \emph {only
         one} type of data on them. While this is often the case, it is
         also very common for CDs to have some tracks with data, and some
         tracks with audio. Because this is an existing interface, rather
         than fixing this interface by changing the assumptions it was made
         under, thereby breaking all user applications that use this
         function, the Uniform CD-ROM Driver implements this *ioctl* as
         follows: If the CD in question has audio tracks on it, and it has
         absolutely no CD-I, XA, or data tracks on it, it will be reported
         as *CDS_AUDIO*. If it has both audio and data tracks, it will
         return *CDS_MIXED*. If there are no audio tracks on the disc, and
         if the CD in question has any CD-I tracks on it, it will be
         reported as *CDS_XA_2_2*. Failing that, if the CD in question
         has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
         Finally, if the CD in question has any data tracks on it,
         it will be reported as a data CD (*CDS_DATA_1*).
 
         This *ioctl* can return::
 
                 CDS_NO_INFO     /* no information available */
                 CDS_NO_DISC     /* no disc is inserted, or tray is opened */
                 CDS_AUDIO       /* Audio disc (2352 audio bytes/frame) */
                 CDS_DATA_1      /* data disc, mode 1 (2048 user bytes/frame) */
                 CDS_XA_2_1      /* mixed data (XA), mode 2, form 1 (2048 user bytes) */
                 CDS_XA_2_2      /* mixed data (XA), mode 2, form 1 (2324 user bytes) */
                 CDS_MIXED       /* mixed audio/data disc */
 
         For some information concerning frame layout of the various disc
         types, see a recent version of `cdrom.h`.
 
 `CDROM_CHANGER_NSLOTS`
         Returns the number of slots in a juke-box.
 `CDROMRESET`
         Reset the drive.
 `CDROM_GET_CAPABILITY`
         Returns the *capability* flags for the drive. Refer to section
         cdrom_capabilities_ for more information on these flags.
 `CDROM_LOCKDOOR`
          Locks the door of the drive. `arg == 0` unlocks the door,
          any other value locks it.
 `CDROM_DEBUG`
          Turns on debugging info. Only root is allowed to do this.
          Same semantics as CDROM_LOCKDOOR.
 
 
 Device dependent *ioctl()'s*
 ----------------------------
 
 Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
 if implemented. No memory allocation or verification is carried out.
 
 How to update your driver
 =========================
 
 - Make a backup of your current driver.
 - Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
   the directory tree that came with this documentation.
 - Make sure you include `cdrom.h`.
 - Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
   to `&cdrom_fops`.
 - Just after that line, add the following to register with the Uniform
   CD-ROM Driver::
 
         register_cdrom(&<your-drive>_info);*
 
   Similarly, add a call to *unregister_cdrom()* at the appropriate place.
 - Copy an example of the device-operations *struct* to your
   source, e. g., from `cm206.c` *cm206_dops*, and change all
   entries to names corresponding to your driver, or names you just
   happen to like. If your driver doesn't support a certain function,
   make the entry *NULL*. At the entry *capability* you should list all
   capabilities your driver currently supports. If your driver
   has a capability that is not listed, please send me a message.
 - Copy the *cdrom_device_info* declaration from the same example
   driver, and modify the entries according to your needs. If your
   driver dynamically determines the capabilities of the hardware, this
   structure should also be declared dynamically.
 - Implement all functions in your `<device>_dops` structure,
   according to prototypes listed in  `cdrom.h`, and specifications given
   in cdrom_api_. Most likely you have already implemented
   the code in a large part, and you will almost certainly need to adapt the
   prototype and return values.
 - Rename your `<device>_ioctl()` function to *audio_ioctl* and
   change the prototype a little. Remove entries listed in the first
   part in cdrom_ioctl_, if your code was OK, these are
   just calls to the routines you adapted in the previous step.
 - You may remove all remaining memory checking code in the
   *audio_ioctl()* function that deals with audio commands (these are
   listed in the second part of cdrom_ioctl_. There is no
   need for memory allocation either, so most *case*s in the *switch*
   statement look similar to::
 
         case CDROMREADTOCENTRY:
                 get_toc_entry\bigl((struct cdrom_tocentry *) arg);
 
 - All remaining *ioctl* cases must be moved to a separate
   function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
   memory checking and allocation must be kept in this code!
 - Change the prototypes of *<device>_open()* and
   *<device>_release()*, and remove any strategic code (i. e., tray
   movement, door locking, etc.).
 - Try to recompile the drivers. We advise you to use modules, both
   for `cdrom.o` and your driver, as debugging is much easier this
   way.
 
 Thanks
 ======
 
 Thanks to all the people involved. First, Erik Andersen, who has
 taken over the torch in maintaining `cdrom.c` and integrating much
 CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
 Gerd Knorr, who were the first to implement this interface for SCSI
 and IDE-CD drivers and added many ideas for extension of the data
 structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt,
 Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll,
 the Linux CD-ROM device driver developers who were kind
 enough to give suggestions and criticisms during the writing. Finally
 of course, I want to thank Linus Torvalds for making this possible in
 the first place.

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