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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 (*select_disc)(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 select_disc(struct cdrom_device_info *cdi, int number)
+
+If the drive can store multiple discs (a juke-box) this function
+will perform disc selection. It should return the number of the
+selected disc on success, a negative value on error. Currently, only
+the ide-cd driver supports this functionality.
+
+::
+
+	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_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.