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diff --git a/Documentation/cdrom/cdrom-standard.tex b/Documentation/cdrom/cdrom-standard.tex new file mode 100644 index 00000000000..92f94e59758 --- /dev/null +++ b/Documentation/cdrom/cdrom-standard.tex @@ -0,0 +1,1022 @@ +\documentclass{article} +\def\version{$Id: cdrom-standard.tex,v 1.9 1997/12/28 15:42:49 david Exp $} +\newcommand{\newsection}[1]{\newpage\section{#1}} + +\evensidemargin=0pt +\oddsidemargin=0pt +\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}} + +\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} +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: +\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}. +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 +\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: + $$ + \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr + CDS_NO_INFO& no information available\cr + CDS_NO_DISC& no disc is inserted, or tray is opened\cr + CDS_AUDIO& Audio disc (2352 audio bytes/frame)\cr + CDS_DATA_1& data disc, mode 1 (2048 user bytes/frame)\cr + CDS_XA_2_1& mixed data (XA), mode 2, form 1 (2048 user bytes)\cr + CDS_XA_2_2& mixed data (XA), mode 2, form 1 (2324 user bytes)\cr + CDS_MIXED& mixed audio/data disc\cr + } + $$ + 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. +\end{description} + +\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} + +\begin{enumerate} +\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. +\end{enumerate} + +\newsection{Thanks} + +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. + +\vfill +$ \version\ $ +\eject +\end{document} |