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+\def\version{$Id: cdrom-standard.tex,v 1.9 1997/12/28 15:42:49 david Exp $}
+\topmargin=-\headheight \advance\topmargin by -\headsep
+\textwidth=15.99cm \textheight=24.62cm % normal A4, 1'' margin
+\def\linux{{\sc Linux}}
+\def\cdrom{{\sc cd-rom}}
+\def\UCD{{\sc Uniform cd-rom Driver}}
+\def\cdromc{{\tt {cdrom.c}}}
+\def\cdromh{{\tt {cdrom.h}}}
+\def\fo{\sl} % foreign words
+\def\ie{{\fo i.e.}}
+\def\eg{{\fo e.g.}}
+\everymath{\it} \everydisplay{\it}
+\catcode `\_=\active \def_{\_\penalty100 }
+\catcode`\<=\active \def<#1>{{\langle\hbox{\rm#1}\rangle}}
+\title{A \linux\ \cdrom\ standard}
+\author{David van Leeuwen\\{\normalsize\tt david@ElseWare.cistron.nl}
+\\{\footnotesize updated by Erik Andersen {\tt(andersee@debian.org)}}
+\\{\footnotesize updated by Jens Axboe {\tt(axboe@image.dk)}}}
+\date{12 March 1999}
+\linux\ is probably the Unix-like operating system that supports
+the widest variety of hardware devices. The reasons for this are
+ The large list of hardware devices available for the many platforms
+ that \linux\ now supports (\ie, 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 \cdrom\
+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\ \cdrom\
+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\ \cdrom\ drivers.
+This document describes an effort to establish Uniform behavior across
+all the different \cdrom\ device drivers for \linux. This document also
+defines the various $ioctl$s, and how the low-level \cdrom\ device
+drivers should implement them. Currently (as of the \linux\ 2.1.$x$
+development kernels) several low-level \cdrom\ device drivers, including
+both IDE/ATAPI and SCSI, now use this Uniform interface.
+When the \cdrom\ was developed, the interface between the \cdrom\ drive
+and the computer was not specified in the standards. As a result, many
+different \cdrom\ 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 \cdrom\ support for
+many of these different interfaces. Nowadays, almost all new \cdrom\
+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 \cdromh, it appeared to be a rather wild
+set of commands and data formats.\footnote{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.} It seemed that many
+features of the software interface had been added to accommodate the
+capabilities of a particular drive, in an {\fo ad hoc\/} manner. More
+importantly, it appeared that the behavior of the `standard' commands
+was different for most of the different drivers: \eg, 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.
+I decided to start a discussion on how to make all the \linux\ \cdrom\
+drivers behave more uniformly. I began by contacting the developers of
+the many \cdrom\ drivers found in the \linux\ kernel. Their reactions
+encouraged me to write the \UCD\ which this document is intended to
+describe. The implementation of the \UCD\ is in the file \cdromc. This
+driver is intended to be an additional software layer that sits on top
+of the low-level device drivers for each \cdrom\ drive. By adding this
+additional layer, it is possible to have all the different \cdrom\
+devices behave {\em exactly\/} the same (insofar as the underlying
+hardware will allow).
+The goal of the \UCD\ is {\em not\/} to alienate driver developers who
+have not yet taken steps to support this effort. The goal of \UCD\ is
+simply to give people writing application programs for \cdrom\ drives
+{\em one\/} \linux\ \cdrom\ interface with consistent behavior for all
+\cdrom\ 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 \cdromh. This guide was written to
+help \cdrom\ driver developers adapt their code to use the \UCD\ code
+defined in \cdromc.
+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 \cdrom\ drive, possibly of mixed types. It is important
+that these drives behave in the same way. In December 1994, one of the
+cheapest \cdrom\ 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 {\em find} anything less than a
+16 speed \cdrom\ drive, and 24 speed drives are common.
+\newsection{Standardizing through another software level}
+At the time this document was conceived, all drivers directly
+implemented the \cdrom\ $ioctl()$ 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 \UCD\ was created to enforce consistent \cdrom\
+drive behavior, and to provide a common set of services to the various
+low-level \cdrom\ device drivers. The \UCD\ 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
+\cdrom\ 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 \cdromh.
+\cdrom\ drives are specific enough (\ie, different from other
+block-devices such as floppy or hard disc drives), to define a set
+of common {\em \cdrom\ device operations}, $<cdrom-device>_dops$.
+These operations are different from the classical block-device file
+operations, $<block-device>_fops$.
+The routines for the \UCD\ interface level are implemented in the file
+\cdromc. In this file, the \UCD\ interfaces with the kernel as a block
+device by registering the following general $struct\ file_operations$:
+\halign{$#$\ \hfil&$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+struct& file_operations\ cdrom_fops = \{\hidewidth\cr
+ &NULL, & lseek \cr
+ &block_read, & read---general block-dev read \cr
+ &block_write, & write---general block-dev write \cr
+ &NULL, & readdir \cr
+ &NULL, & select \cr
+ &cdrom_ioctl, & ioctl \cr
+ &NULL, & mmap \cr
+ &cdrom_open, & open \cr
+ &cdrom_release, & release \cr
+ &NULL, & fsync \cr
+ &NULL, & fasync \cr
+ &cdrom_media_changed, & media change \cr
+ &NULL & revalidate \cr
+Every active \cdrom\ device shares this $struct$. The routines
+declared above are all implemented in \cdromc, since this file is the
+place where the behavior of all \cdrom-devices is defined and
+standardized. The actual interface to the various types of \cdrom\
+hardware is still performed by various low-level \cdrom-device
+drivers. These routines simply implement certain {\em capabilities\/}
+that are common to all \cdrom\ (and really, all removable-media
+Registration of a low-level \cdrom\ device driver is now done through
+the general routines in \cdromc, not through the Virtual File System
+(VFS) any more. The interface implemented in \cdromc\ 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:
+ This structure contains information about the low-level driver for a
+ \cdrom\ 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).
+ This structure contains information about a particular \cdrom\ drive,
+ such as its device name, speed, etc. This structure is conceptually
+ connected to the minor number of the device.
+Registering a particular \cdrom\ drive with the \UCD\ 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
+\cdrom\ 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 \cdromc\ accesses a \cdrom\ device, it does it
+through the functions in this structure. It is impossible to know all
+the capabilities of future \cdrom\ 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:
+\halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}&
+ $/*$ \rm# $*/$\hfil\cr
+struct& cdrom_device_ops\ \{ \hidewidth\cr
+ &int& (* open)(struct\ cdrom_device_info *, int)\cr
+ &void& (* release)(struct\ cdrom_device_info *);\cr
+ &int& (* drive_status)(struct\ cdrom_device_info *, int);\cr
+ &int& (* media_changed)(struct\ cdrom_device_info *, int);\cr
+ &int& (* tray_move)(struct\ cdrom_device_info *, int);\cr
+ &int& (* lock_door)(struct\ cdrom_device_info *, int);\cr
+ &int& (* select_speed)(struct\ cdrom_device_info *, int);\cr
+ &int& (* select_disc)(struct\ cdrom_device_info *, int);\cr
+ &int& (* get_last_session) (struct\ cdrom_device_info *,
+ struct\ cdrom_multisession *{});\cr
+ &int& (* get_mcn)(struct\ cdrom_device_info *, struct\ cdrom_mcn *{});\cr
+ &int& (* reset)(struct\ cdrom_device_info *);\cr
+ &int& (* audio_ioctl)(struct\ cdrom_device_info *, unsigned\ int,
+ void *{});\cr
+ &int& (* dev_ioctl)(struct\ cdrom_device_info *, unsigned\ int,
+ unsigned\ long);\cr
+ &const\ int& capability;& capability flags \cr
+ &int& n_minors;& number of active minor devices \cr
+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
+\cdrom\ hardware and/or low-level \cdrom\ driver when a \cdrom\ drive
+is registered with the \UCD. The value $n_minors$ should be a positive
+value indicating the number of minor devices that are supported by
+the low-level device driver, normally~1. Although these two variables
+are `informative' rather than `operational,' they are included in
+$cdrom_device_ops$ because they describe the capability of the {\em
+driver\/} rather than the {\em drive}. Nomenclature has always been
+difficult in computer programming.
+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
+\cdrom\ 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
+\cdromc, currently contains the following fields:
+\halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}&
+ $/*$ \rm# $*/$\hfil\cr
+struct& cdrom_device_info\ \{ \hidewidth\cr
+ & struct\ cdrom_device_ops *& ops;& device operations for this major\cr
+ & struct\ cdrom_device_info *& next;& next device_info for this major\cr
+ & void *& handle;& driver-dependent data\cr
+ & kdev_t& dev;& device number (incorporates minor)\cr
+ & int& mask;& mask of capability: disables them \cr
+ & int& speed;& maximum speed for reading data \cr
+ & int& capacity;& number of discs in a jukebox \cr
+ &int& options : 30;& options flags \cr
+ &unsigned& mc_flags : 2;& media-change buffer flags \cr
+ & int& use_count;& number of times device is opened\cr
+ & char& name[20];& name of the device type\cr
+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
+The $mask$ flags can be used to mask out some of the capabilities listed
+in $ops\to 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 (176\,kB/sec raw data or
+150\,kB/sec file system data). The value $n_discs$ should reflect the
+number of discs the drive can hold simultaneously, if it is designed
+as a juke-box, or otherwise~1. The parameters are declared $const$
+because they describe properties of the drive, which don't change after
+A few registers contain variables local to the \cdrom\ drive. The
+flags $options$ are used to specify how the general \cdrom\ routines
+should behave. These various flags registers should provide enough
+flexibility to adapt to the different users' wishes (and {\em 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
+The intermediate software layer that \cdromc\ 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 {\em 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.
+\subsection{$Int\ open(struct\ cdrom_device_info * cdi, int\ purpose)$}
+$Open()$ should try to open the device for a specific $purpose$, which
+can be either:
+\item[0] Open for reading data, as done by {\tt {mount()}} (2), or the
+user commands {\tt {dd}} or {\tt {cat}}.
+\item[1] Open for $ioctl$ commands, as done by audio-CD playing
+Notice that any strategic code (closing tray upon $open()$, etc.)\ is
+done by the calling routine in \cdromc, so the low-level routine
+should only be concerned with proper initialization, such as spinning
+up the disc, etc. % and device-use count
+\subsection{$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$.
+\subsection{$Int\ drive_status(struct\ cdrom_device_info * cdi, int\ slot_nr)$}
+\label{drive status}
+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 \cdromh\ the possibilities are listed:
+\halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+CDS_NO_INFO& no information available\cr
+CDS_NO_DISC& no disc is inserted, tray is closed\cr
+CDS_TRAY_OPEN& tray is opened\cr
+CDS_DRIVE_NOT_READY& something is wrong, tray is moving?\cr
+CDS_DISC_OK& a disc is loaded and everything is fine\cr
+\subsection{$Int\ media_changed(struct\ cdrom_device_info * cdi, int\ disc_nr)$}
+This function is very similar to the original function in $struct\
+file_operations$. It returns 1 if the medium of the device $cdi\to
+dev$ has changed since the last call, and 0 otherwise. The parameter
+$disc_nr$ identifies a specific slot in a juke-box, it should be
+ignored for single-disc drives. Note that by `re-routing' this
+function through $cdrom_media_changed()$, we can implement separate
+queues for the VFS and a new $ioctl()$ function that can report device
+changes to software (\eg, an auto-mounting daemon).
+\subsection{$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:
+\item[0] Close tray
+\item[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.
+\subsection{$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
+\item[0] Unlock door, manual opening is allowed
+\item[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.
+\subsection{$Int\ select_speed(struct\ cdrom_device_info * cdi, int\ speed)$}
+Some \cdrom\ drives are capable of changing their head-speed. There
+are several reasons for changing the speed of a \cdrom\ drive. Badly
+pressed \cdrom s may benefit from less-than-maximum head rate. Modern
+\cdrom\ drives can obtain very high head rates (up to $24\times$ 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. %Finally,
+%although the audio-low-pass filters probably aren't designed for it,
+%more than real-time playback of audio might be used for high-speed
+%copying of audio tracks.
+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 (176\,kB/sec raw data
+or 150\,kB/sec file system data). So to request that a \cdrom\ drive
+operate at 300\,kB/sec you would call the CDROM_SELECT_SPEED $ioctl$
+with $speed=2$. The special value `0' means `auto-selection', \ie,
+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.
+\subsection{$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.
+\subsection{$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\to dev$, the start of the last session of the current disc
+should be returned in the pointer argument $ms_info$. Note that
+routines in \cdromc\ have sanitized this argument: its requested
+format will {\em 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\rightarrow addr_format$ field appropriately, of
+course) and the routines in \cdromc\ will make the transformation if
+necessary. The return value is 0 upon success.
+\subsection{$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.
+\subsection{$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.
+\subsection{$Int\ audio_ioctl(struct\ cdrom_device_info * cdi, unsigned\
+ int\ cmd, void * arg)$}
+Some of the \cdrom-$ioctl$s defined in \cdromh\ 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()$ in {\tt {cm206.c}} that should be updated with
+this documentation.
+An unimplemented ioctl should return $-ENOSYS$, but a harmless request
+(\eg, $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 \UCD\ 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.)
+\subsection{$Int\ dev_ioctl(struct\ cdrom_device_info * cdi, unsigned\ int\
+ cmd, unsigned\ long\ arg)$}
+Some $ioctl$s seem to be specific to certain \cdrom\ 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 \cdromc.
+Because there are so many $ioctl$s that seem to be introduced to
+satisfy certain drivers,\footnote{Is there software around that
+ actually uses these? I'd be interested!} 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 \cdrom\ $ioctl$ number, {\tt {0x53}}. Currently the
+non-supported $ioctl$s are: {\it CDROMREADMODE1, CDROMREADMODE2,
+\subsection{\cdrom\ capabilities}
+Instead of just implementing some $ioctl$ calls, the interface in
+\cdromc\ supplies the possibility to indicate the {\em capabilities\/}
+of a \cdrom\ drive. This can be done by ORing any number of
+capability-constants that are defined in \cdromh\ at the registration
+phase. Currently, the capabilities are any of:
+\halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+CDC_CLOSE_TRAY& can close tray by software control\cr
+CDC_OPEN_TRAY& can open tray\cr
+CDC_LOCK& can lock and unlock the door\cr
+CDC_SELECT_SPEED& can select speed, in units of $\sim$150\,kB/s\cr
+CDC_SELECT_DISC& drive is juke-box\cr
+CDC_MULTI_SESSION& can read sessions $>\rm1$\cr
+CDC_MCN& can read Media Catalog Number\cr
+CDC_MEDIA_CHANGED& can report if disc has changed\cr
+CDC_PLAY_AUDIO& can perform audio-functions (play, pause, etc)\cr
+CDC_RESET& hard reset device\cr
+CDC_IOCTLS& driver has non-standard ioctls\cr
+CDC_DRIVE_STATUS& driver implements drive status\cr
+The capability flag is declared $const$, to prevent drivers from
+accidentally tampering with the contents. The capability fags actually
+inform \cdromc\ 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 \cdrom\
+driver has implemented the code for loading and ejecting \cdrom's, and
+hence its corresponding flags in $capability$ will be set. But a SCSI
+\cdrom\ 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 \cdromc\ you will encounter many constructions of the type
+if\ (cdo\rightarrow capability \mathrel\& \mathord{\sim} cdi\rightarrow mask
+ \mathrel{\&} CDC_<capability>) \ldots
+There is no $ioctl$ to set the mask\dots The reason is that
+I think it is better to control the {\em behavior\/} rather than the
+{\em capabilities}.
+A final flag register controls the {\em behavior\/} of the \cdrom\
+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:
+\halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+CDO_AUTO_CLOSE& try to close tray upon device $open()$\cr
+CDO_AUTO_EJECT& try to open tray on last device $close()$\cr
+CDO_USE_FFLAGS& use $file_pointer\rightarrow f_flags$ to indicate
+ purpose for $open()$\cr
+CDO_LOCK& try to lock door if device is opened\cr
+CDO_CHECK_TYPE& ensure disc type is data if opened for data\cr
+The initial value of this register is $CDO_AUTO_CLOSE \mathrel|
+CDO_USE_FFLAGS \mathrel| CDO_LOCK$, reflecting my own view on user
+interface and software standards. Before you protest, there are two
+new $ioctl$s implemented in \cdromc, that allow you to control the
+behavior by software. These are:
+\halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+CDROM_SET_OPTIONS& set options specified in $(int)\ arg$\cr
+CDROM_CLEAR_OPTIONS& clear options specified in $(int)\ arg$\cr
+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 {\tt setcd}, available from the Debian distribution
+and {\tt sunsite.unc.edu}, allows user level control of these flags.
+\newsection{The need to know the purpose of opening the \cdrom\ 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 \cdrom\ drives, is that they can be used for
+two entirely different purposes. One is to mount removable
+file systems, \cdrom 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 {\em 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 \cdrom 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 \cdrom\ 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 \cdrom\ on an empty drive occurs. This is not a
+particularly elegant way to find out that there is no \cdrom\ 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 {\em 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 \cdrom\ and its correct type (data), would be
+These two ways of using a \cdrom\ 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 {\em purpose\/} of opening the device is, is through the $flags$
+parameter (see {\tt {open(2)}}). For \cdrom\ 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
+\cdrom\ devices: $O_CREAT$, $O_NOCTTY$, $O_TRUNC$, $O_APPEND$, and
+$O_SYNC$ have no meaning to a \cdrom.
+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-\cdrom.'' Thus, our proposal of the
+implementation for the $open()$ call for \cdrom s is:
+\item 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 \cdrom, such as closing the tray.
+\item 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.
+\subsection{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.\footnote{Incidentally, I think that SUN's approach to
+mounting \cdrom s is very good in origin: under Solaris a
+volume-daemon automatically mounts a newly inserted \cdrom\ under {\tt
+{/cdrom/$<volume-name>$/}}. In my opinion they should have pushed this
+further and have {\em every\/} \cdrom\ on the local area network be
+mounted at the similar location, \ie, no matter in which particular
+machine you insert a \cdrom, 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,
+\subsection{The preferred strategy of $open()$}
+The routines in \cdromc\ are designed in such a way that run-time
+configuration of the behavior of \cdrom\ devices (of {\em any\/} type)
+can be carried out, by the $CDROM_SET/CLEAR_OPTIONS$ $ioctls$. Thus, various
+modes of operation can be set:
+\item[$CDO_AUTO_CLOSE \mathrel| CDO_USE_FFLAGS \mathrel| 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.
+\item[$CDO_AUTO_CLOSE \mathrel| CDO_AUTO_EJECT \mathrel| 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, \ie, if a \cdrom\ 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 \cdrom\
+driver scheme and option flag interpretation.
+\newsection{Description of routines in \cdromc}
+Only a few routines in \cdromc\ are exported to the drivers. In this
+new section we will discuss these, as well as the functions that `take
+over' the \cdrom\ interface to the kernel. The header file belonging
+to \cdromc\ is called \cdromh. Formerly, some of the contents of this
+file were placed in the file {\tt {ucdrom.h}}, but this file has now been
+merged back into \cdromh.
+\subsection{$Struct\ file_operations\ cdrom_fops$}
+The contents of this structure were described in section~\ref{cdrom.c}.
+A pointer to this structure is assigned to the $fops$ field
+of the $struct gendisk$.
+\subsection{$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 section~\ref{cdrom.c}, should be registered with the
+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 = \{\cr
+& <device>_dops;\cr
+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.
+\subsection{$Int\ unregister_cdrom(struct\ cdrom_device_info * cdi)$}
+Unregistering device $cdi$ with minor number $MINOR(cdi\to 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 \cdrom\ interface. This function returns zero upon
+success, and non-zero upon failure.
+\subsection{$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.
+\subsection{$Void\ cdrom_release(struct\ inode *ip, struct\ file
+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)$.
+\subsection{$Int\ cdrom_ioctl(struct\ inode *ip, struct\ file *fp,
+unsigned\ int\ cmd, unsigned\ long\ arg)$}
+This function handles all the standard $ioctl$ requests for \cdrom\
+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.
+\subsubsection{Directly implemented $ioctl$s}
+The following `old' \cdrom-$ioctl$s are implemented by directly
+calling device-operations in $cdrom_device_ops$, if implemented and
+not masked:
+\item[CDROMMULTISESSION] Requests the last session on a \cdrom.
+\item[CDROMEJECT] Open tray.
+\item[CDROMCLOSETRAY] Close tray.
+\item[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.
+\item[CDROM_GET_MCN] Get the Media Catalog Number from a CD.
+\subsubsection{$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.
+\item[CDROMSUBCHNL] Get sub-channel data in argument $arg$ of type $struct\
+cdrom_subchnl *{}$.
+\item[CDROMREADTOCHDR] Read Table of Contents header, in $arg$ of type
+$struct\ cdrom_tochdr *{}$.
+\item[CDROMREADTOCENTRY] Read a Table of Contents entry in $arg$ and
+specified by $arg$ of type $struct\ cdrom_tocentry *{}$.
+\item[CDROMPLAYMSF] Play audio fragment specified in Minute, Second,
+Frame format, delimited by $arg$ of type $struct\ cdrom_msf *{}$.
+\item[CDROMPLAYTRKIND] Play audio fragment in track-index format
+delimited by $arg$ of type $struct\ \penalty-1000 cdrom_ti *{}$.
+\item[CDROMVOLCTRL] Set volume specified by $arg$ of type $struct\
+cdrom_volctrl *{}$.
+\item[CDROMVOLREAD] Read volume into by $arg$ of type $struct\
+cdrom_volctrl *{}$.
+\item[CDROMSTART] Spin up disc.
+\item[CDROMSTOP] Stop playback of audio fragment.
+\item[CDROMPAUSE] Pause playback of audio fragment.
+\item[CDROMRESUME] Resume playing.
+\subsubsection{New $ioctl$s in \cdromc}
+The following $ioctl$s have been introduced to allow user programs to
+control the behavior of individual \cdrom\ devices. New $ioctl$
+commands can be identified by the underscores in their names.
+\item[CDROM_SET_OPTIONS] Set options specified by $arg$. Returns the
+option flag register after modification. Use $arg = \rm0$ for reading
+the current flags.
+\item[CDROM_CLEAR_OPTIONS] Clear options specified by $arg$. Returns
+ the option flag register after modification.
+\item[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
+ 150\,kB/sec file system data). The value 0 means `auto-select', \ie,
+ 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$.
+\item[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$.
+\item[CDROM_MEDIA_CHANGED] Returns 1 if a disc has been changed since
+ the last call. Note that calls to $cdrom_media_changed$ by the VFS
+ are treated by an independent queue, so both mechanisms will detect
+ a media change once. 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.
+\item[CDROM_DRIVE_STATUS] Returns the status of the drive by a call to
+ $drive_status()$. Return values are defined in section~\ref{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.
+\item[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 \emph {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 \UCD.
+ 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 \UCD\ 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:
+ $$
+ \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
+ CDS_NO_INFO& no information available\cr
+ CDS_NO_DISC& no disc is inserted, or tray is opened\cr
+ CDS_AUDIO& Audio disc (2352 audio bytes/frame)\cr
+ CDS_DATA_1& data disc, mode 1 (2048 user bytes/frame)\cr
+ CDS_XA_2_1& mixed data (XA), mode 2, form 1 (2048 user bytes)\cr
+ CDS_XA_2_2& mixed data (XA), mode 2, form 1 (2324 user bytes)\cr
+ CDS_MIXED& mixed audio/data disc\cr
+ }
+ $$
+ For some information concerning frame layout of the various disc
+ types, see a recent version of \cdromh.
+\item[CDROM_CHANGER_NSLOTS] Returns the number of slots in a
+ juke-box.
+\item[CDROMRESET] Reset the drive.
+\item[CDROM_GET_CAPABILITY] Returns the $capability$ flags for the
+ drive. Refer to section \ref{capability} for more information on
+ these flags.
+\item[CDROM_LOCKDOOR] Locks the door of the drive. $arg == \rm0$
+ unlocks the door, any other value locks it.
+\item[CDROM_DEBUG] Turns on debugging info. Only root is allowed
+ to do this. Same semantics as CDROM_LOCKDOOR.
+\subsubsection{Device dependent $ioctl$s}
+Finally, all other $ioctl$s are passed to the function $dev_ioctl()$,
+if implemented. No memory allocation or verification is carried out.
+\newsection{How to update your driver}
+\item Make a backup of your current driver.
+\item Get hold of the files \cdromc\ and \cdromh, they should be in
+ the directory tree that came with this documentation.
+\item Make sure you include \cdromh.
+\item Change the 3rd argument of $register_blkdev$ from
+$\&<your-drive>_fops$ to $\&cdrom_fops$.
+\item Just after that line, add the following to register with the \UCD:
+ $$register_cdrom(\&<your-drive>_info);$$
+ Similarly, add a call to $unregister_cdrom()$ at the appropriate place.
+\item Copy an example of the device-operations $struct$ to your
+ source, \eg, from {\tt {cm206.c}} $cm206_dops$, and change all
+ entries to names corresponding to your driver, or names you just
+ happen to like. If your driver doesn't support a certain function,
+ make the entry $NULL$. At the entry $capability$ you should list all
+ capabilities your driver currently supports. If your driver
+ has a capability that is not listed, please send me a message.
+\item Copy the $cdrom_device_info$ declaration from the same example
+ driver, and modify the entries according to your needs. If your
+ driver dynamically determines the capabilities of the hardware, this
+ structure should also be declared dynamically.
+\item Implement all functions in your $<device>_dops$ structure,
+ according to prototypes listed in \cdromh, and specifications given
+ in section~\ref{cdrom.c}. Most likely you have already implemented
+ the code in a large part, and you will almost certainly need to adapt the
+ prototype and return values.
+\item Rename your $<device>_ioctl()$ function to $audio_ioctl$ and
+ change the prototype a little. Remove entries listed in the first
+ part in section~\ref{cdrom-ioctl}, if your code was OK, these are
+ just calls to the routines you adapted in the previous step.
+\item You may remove all remaining memory checking code in the
+ $audio_ioctl()$ function that deals with audio commands (these are
+ listed in the second part of section~\ref{cdrom-ioctl}). There is no
+ need for memory allocation either, so most $case$s in the $switch$
+ statement look similar to:
+ $$
+ case\ CDROMREADTOCENTRY\colon get_toc_entry\bigl((struct\
+ cdrom_tocentry *{})\ arg\bigr);
+ $$
+\item 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!
+\item Change the prototypes of $<device>_open()$ and
+ $<device>_release()$, and remove any strategic code (\ie, tray
+ movement, door locking, etc.).
+\item Try to recompile the drivers. We advise you to use modules, both
+ for {\tt {cdrom.o}} and your driver, as debugging is much easier this
+ way.
+Thanks to all the people involved. First, Erik Andersen, who has
+taken over the torch in maintaining \cdromc\ and integrating much
+\cdrom-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 M\"onkeberg and Andrew
+Kroll, the \linux\ \cdrom\ 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.
+$ \version\ $