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\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}}}

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\begin{document}

\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}

\maketitle

\newsection{Introduction}

\linux\ is probably the Unix-like operating system that supports

the widest variety of hardware devices. The reasons for this are

presumably

\begin{itemize}

\item

  The large list of hardware devices available for the many platforms

  that \linux\ now supports (\ie, i386-PCs, Sparc Suns, etc.)

\item

  The open design of the operating system, such that anybody can write a

  driver for \linux.

\item

  There is plenty of source code around as examples of how to write a driver.

\end{itemize}

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}

\label{cdrom.c}

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

\};\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

devices).

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:

\begin{description}

\item[$cdrom_device_ops$]

  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).

\item[$cdrom_device_info$]

  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.

\end{description}

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

\noalign{\medskip}

  &const\ int& capability;& capability flags \cr

  &int& n_minors;& number of active minor devices \cr

\};\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

\noalign{\medskip}

  & 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

\noalign{\medskip}

  &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

\}\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

structure.

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

registration.

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

initialized.

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:

\begin{itemize}

\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

programs.

\end{itemize}

In case the driver supports modules, the call $MOD_INC_USE_COUNT$

should be performed exactly once, if the $open()$ was successful. The

return value is negative on error, and zero on success. The

open-for-ioctl call can only fail if there is no hardware.

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{$Void\ release(struct\ cdrom_device_info * cdi)$}

In case of module support, a single call $MOD_DEC_USE_COUNT$ should be

coded here.  Possibly other 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 $cdrom_release()$. Also, the invalidation of the

allocated buffers in the VFS is taken care of by the routine in

\cdromc.  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:

\begin{itemize}

\item[0] Close tray

\item[1] Open tray

\end{itemize}

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

state:

\begin{itemize}

\item[0] Unlock door, manual opening is allowed

\item[1] Lock door, tray cannot be ejected manually

\end{itemize}

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,

  CDROMREADAUDIO, CDROMREADRAW, CDROMREADCOOKED, CDROMSEEK,

  CDROMPLAY\-BLK and CDROM\-READALL}.

\subsection{\cdrom\ capabilities}

\label{capability}

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

$$\it

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}.

\subsection{Options}

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

desirable.

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:

\begin{itemize}

\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.

\end{itemize}

\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,

CDO_USE_FFLAGS)$.

\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:

\begin{description}

\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.

\end{description}

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}.

As already stated, this structure should be used to register block

devices with the kernel:

$$

register_blkdev(major, <name>, \&cdrom_fops);

$$

\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

\UCD:

$$

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

$$

\vbox{\halign{&$#$\hfil\cr

struct\ &cdrom_device_info\ <device>_info = \{\cr

& <device>_dops;\cr

&\ldots\cr

\}\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

*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 $invalidate_buffers(dev)$.

\subsection{$Int\ cdrom_ioctl(struct\ inode *ip, struct\ file *fp,

unsigned\ int\ cmd, unsigned\ long\ arg)$}

\label{cdrom-ioctl}

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}

\label{ioctl-direct}

The following `old' \cdrom-$ioctl$s are implemented by directly

calling device-operations in $cdrom_device_ops$, if implemented and

not masked:

\begin{description}

\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.

\end{description}

\subsubsection{$Ioctl$s routed through $audio_ioctl()$}

\label{ioctl-audio}

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.

\begin{description}

\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.

\end{description}

\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.

\begin{description}

\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