\documentclass{article} \def\version{$Id: cdrom-standard.tex,v 1.6 1996/12/29 20:45:18 davem Exp $} \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 CDrom}} \def\cdromc{{\tt cdrom.c}} \def\cdromh{{\tt cdrom.h}} \def\ucdrom{{\tt ucdrom.h}} \def\fo{\sl} \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@tm.tno.nl}} \maketitle \section{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 different hardware devices available for the popular IBM PC-architecture, \item The open design of the operating system, such that everybody can write a driver for Linux (source code examples). \end{itemize} The vast choice and openness has lead not only to a wide support of hardware devices, but also to a certain divergence in behavior. Especially for \cdrom\ devices, the way a particular drive reacts to a `standard' $ioctl()$ call varies a lot from one brand to another; however, the \linux\ \cdrom\ driver writers kept away from wilderness by the practice of evolving a new driver by copying, understanding and changing an existing one. Since the beginning of the \cdrom, many different interfaces 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 his own interface or used his own command set and flow control scheme, either a separate driver had to be written, or an existing driver had to get enhanced. Nowadays, almost all new \cdrom\ types are either ATAPI/IDE or SCSI; it is very unlikely that any manufacturer still will create a new interface, and new drives for the existing proprietary interfaces are getting rare. But history has delivered us \cdrom\ support for many different interfaces. When (in the 1.3.70's) I looked at the existing interface which is 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 interface have been added to include certain specific capabilities of certain drives, in an {\fo ad hoc\/} manner. More importantly, it appeared that actual execution of the commands is different for most of the different drivers: e.g., some drivers close the tray if an $open()$ call occurs while the tray is unloaded, 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 the driver behavior may be different. I decided to start a discussion on how to improve uniformity, addressing all \cdrom-driver developers found in the various driver files. The reactions encouraged me to write a uniform (compatible) software level \cdromc\ to which this document is the documentation. In the mean time, the data structure definitions in \cdromh\ had been cleaned up a lot---which was very helpful for the new code. \begin{quote} \small [Apparently, not all \cdrom\ developers support this initiative. They---mainly those who used the already existing drivers not only as a coding example, but also as a `user interface' reference during their own development---have taken care that \cdromh\ reflects a software interface to `user programs' which is unique between all drivers as much as possible; these driver writers will continue to refine the existing \cdromh\ where it seems necessary, and they tend to look if any newly requested functionality isn't already there before they are ready to define new structures. The {\tt sbpcd} driver gives an example that it is possible to let a robot arm play juke box---either with audio or with data CDs---and that it is possible to let the juke box work on even if a disk has fallen upon the floor and the drive door has closed without having a disk inside; without any new software layer or any structures which are not already present in \cdromh. This `other' group of \linux\ \cdrom\ driver writers explicitly does {\em not\/} support the idea to define an additional software layer between driver and user program.]\parfillskip=0pt \end{quote} The effort (\cdromc) of which this is the documentation is {\em not\/} meant to drive a wedge between two groups of driver developers, but rather to enable sharing of `strategic code' among drivers. The code should {\em not\/} be viewed as a new interface to user-level programs, but rather as a new interface between driver code and kernel. Care is taken that 100\,\% compatibility exists with the data structures and programmer's interface defined in \cdromh, and in order not to interfere with ongoing development in \cdromh, any `new' data structures have been put in a separate header file called \ucdrom. Because the data structures of \cdromh\ are fully supported, this documentation may also be of help to the programmers using the interface defined in \cdromh, but this guide is primarily written to help \cdrom\ driver developers adapt their code to use the `common \cdrom' code in \cdromc. Personally, I think that the most important hardware interfaces will be the IDE/ATAPI drives and of course the SCSI drives, but as prices of hardware drop continuously, it is not unlikely that people will 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 old-fashioned and IDE/ATAPI drives became standard. At the time of writing (December 1996) the cheapest drive is quadruple speed IDE and at less than half the price of its predecessor. Twelve speed drives are available now.) This document defines the various $ioctl$s, and the way the drivers should implement this. Currently (in the kernel 2.1.$n$ development line) three low-level \cdrom\ drivers use this interface, among which are the most important drivers for IDE and SCSI. \section{Standardizing through another software level} \label{cdrom.c} At the time this document was conceived, all drivers directly implement the $ioctl()$ calls through their own routines, with the danger of forgetting calls to $verify_area()$ and the risk of divergence in implementation. For this reason, we\footnote{The writing style is such that `we' is used when (at least part of) the \cdrom-device driver authors support the idea, an `I' is used for personal opinions} propose to define another software-level, that separates the $ioctl()$ and $open()$ implementation from the actual hardware implementation. Note that we do not wish to alter the existing application interface defined in \cdromh, but rather want to re-root the hardware-implementation through some common code. We believe that \cdrom\ drives are specific enough (i.e., different from other block-devices such as floppy or hard disc drives), to define a set of {\em \cdrom\ device operations}, $_dops$. These are of a different nature than the classical block-device file operations $_fops$. The extra interfacing level routines are implemented in a file \cdromc, and a low-level \cdrom\ driver hands over the interfacing to the kernel 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, and this is the place where the {\em behavior\/} of all \cdrom-devices is defined, and hence standardized. The implementation of the interfacing to the various types of hardware still is done by the various \cdrom-device drivers, but these routines only implement certain {\em capabilities\/} that are typical to \cdrom\ (removable-media) devices. Registration of the \cdrom\ device driver should now be to the general routines in \cdromc, not to the VFS any more. The interfacing with \cdromc\ is implemented trough two general structures, that contain information about the capabilities of the driver, and the specific drives on which the driver operates. The structures are separated to contain information about \begin{description} \item[the low-level driver] It lists the routines that actually implement \cdrom\ operations, and hence the structure is called $cdrom_device_ops$. The structure is conceptually connected to the major number of the device (although some drivers may have have different major numbers, as is the case for the IDE driver). \item[the specific drive] It lists the variables informative of the drive that is driven, and hence the structure is called $cdrom_device_info$. The structure is conceptually connected to the minor number of the device. \end{description} The registration is done for each drive found by the driver (and hence for each minor number) though the call $$register_cdrom(struct\ cdrom_device_info * _info, char * name) $$ This device information structure $_info$ (described shortly) contains all information needed for the kernel to interface with the low-level \cdrom\ device driver. One of the main entries of this structure is a pointer to the $cdrom_device_ops$ structure of the driver. This device operations structure $cdrom_device_ops$ lists the implemented routines for interfacing to the hardware. [It is impossible to come up with a complete list of all capabilities of (future) \cdrom\ drives, as the developments in technology follow-up at an incredible rate. Maybe write-operation (WORM devices) will become very popular in the future.] The list now is: $$ \halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}& $/*$ \rm# $*/$\hfil\cr struct& cdrom_device_ops\ \{ \hidewidth\cr &int& (* open)(struct\ cdrom_device_info *, int)\cr &void& (* release)(struct\ cdrom_device_info *);\cr &int& (* drive_status)(struct\ cdrom_device_info *);\cr &int& (* disc_status)(struct\ cdrom_device_info *);\cr &int& (* media_changed)(struct\ cdrom_device_info *, int);\cr &int& (* tray_move)(struct\ cdrom_device_info *, int);\cr &int& (* lock_door)(struct\ cdrom_device_info *, int);\cr &int& (* select_speed)(struct\ cdrom_device_info *, int);\cr &int& (* select_disc)(struct\ cdrom_device_info *, int);\cr &int& (* get_last_session) (struct\ cdrom_device_info *, struct\ cdrom_multisession *{});\cr &int& (* get_mcn)(struct\ cdrom_device_info *, struct\ cdrom_mcn *{});\cr &int& (* reset)(struct\ cdrom_device_info *);\cr &int& (* audio_ioctl)(struct\ cdrom_device_info *, unsigned\ int, void *{});\cr &int& (* dev_ioctl)(struct\ cdrom_device_info *, unsigned\ int, unsigned\ long);\cr \noalign{\medskip} &\llap{const\ }int& capability;& capability flags \cr &int& n_minors;& number of active minor devices \cr } $$ The \cdrom-driver should simply implement (some of) these functions, and register the functions to the global \cdrom\ driver, which performs interfacing with the Virtual File System and system $ioctl$s. The flags $capability$ specify the hardware-capabilities on registration of the device. The value $n_minors$ should be a positive value indicating the number of minor devices that are supported by the 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 $file$ are used, the main parameter is the first, from which the major and minor number can be extracted. (Most low-level \cdrom\ drivers don't even look at that value as only one device is supported.) This will be available through $dev$ in $cdrom_device_info$ described below. The drive-specific, minor-like information that is registered to \cdromc, contains the following fields: $$ \halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}& $/*$ \rm# $*/$\hfil\cr struct& cdrom_device_info\ \{ \hidewidth\cr & struct\ cdrom_device_ops *& ops;& device operations for this major\cr & struct\ cdrom_device_info *& next;& next device_info for this major\cr & void *& handle;& driver-dependent data\cr \noalign{\medskip} & kdev_t& dev;& device number (incorporates minor)/\cr & int& mask;& mask of capability: disables them \cr &\llap{$const\ $}int& speed;& maximum speed for reading data \cr &\llap{$const\ $}int& n_discs;& number of discs in jukebox \cr \noalign{\medskip} &int& options : 30;& options flags \cr &long& mc_flags : 2;& media-change buffer flags \cr & int& use_count;& number of times devices is opened\cr \}\cr }$$ With this $struct$, a linked list of minor devices registered with the same low-level driver is built, though the field $next$. The device number, the device operations struct and specifications of properties of the drive are stored in this structure. The flags $mask$ 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 user's 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 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 performs 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{$Open(struct\ cdrom_device_info * cdi, int\ purpose)$} $Open()$ should try to open the device for a specific $purpose$, which can be either: \begin{itemize} \item[0] Open for data read, as is used by {\tt mount()} (2), or the user commands {\tt dd} or {\tt cat}. \item[1] Open for $ioctl$ commanding, as is used for audio-CD playing programs mostly. \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 and device-use count. \subsection{$Release(struct\ cdrom_device_info * cdi)$} 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. \subsection{$Drive_status(struct\ cdrom_device_info * cdi)$} \label{drive status} The function $drive_status$, if implemented, should provide information of the status of the drive (not the status of the disc, which may or may not be in the drive). In \ucdrom\ the possibilities are listed: $$ \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr CDS_NO_INFO& no information available\cr CDS_NO_DISC& no disc is inserted, tray is closed\cr CDS_TRAY_OPEN& tray is opened\cr CDS_DRIVE_NOT_READY& something is wrong, tray is moving?\cr CDS_DISC_OK& a disc is loaded and everything is fine\cr } $$ %For a juke-box, the second argument $drive_nr$ specifies information %is requested for another than the default disc ($drive_nr=0$), %possibly only a subset of the return values can be returned. \subsection{$Disc_status(struct\ cdrom_device_info * cdi)$} \label{disc status} As a complement to $drive_status()$, this function can provide the general \cdrom-routines with information about the current disc that is inserted in the drive represented by $cdi\to dev$. The history of development of the CD's use as a carrier medium for various digital information has lead to many different disc types, hence this function can return: $$ \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr CDS_NO_INFO& no information available\cr CDS_NO_DISC& no disc is inserted, or tray is opened\cr CDS_AUDIO& Audio disc (2352 audio bytes/frame)\cr CDS_DATA_1& data disc, mode 1 (2048 user bytes/frame)\cr CDS_DATA_2& data disc, mode 2 (2336 user bytes/frame)\cr CDS_XA_2_1& mixed data (XA), mode 2, form 1 (2048 user bytes)\cr CDS_XA_2_2& mixed data (XA), mode 2, form 1 (2324 user bytes)\cr } $$ As far as I know, \cdrom s are always of type $CDS_DATA_1$. For some information concerning frame layout of the various disc types, see a recent version of {\tt cdrom.h}. \subsection{$Media_changed(struct\ cdrom_device_info * cdi, int disc_nr)$} 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 (e.g., an auto-mounting daemon). \subsection{$Tray_move(struct\ cdrom_device_info * cdi, int\ position)$} 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{$Lock_door(struct\ cdrom_device_info * cdi, int\ lock)$} 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} Return values are as for $tray_move()$. \subsection{$Select_speed(struct\ cdrom_device_info * cdi, int\ speed)$} Although none of the drivers has implemented this function so far, some drives are capable of head-speed selection, and hence this is a capability that should be standardized through a function in the device-operations structure. This function should select the speed at which data is read or audio is played back. The special value `0' means `auto-selection', i.e., maximum data-rate or real-time audio rate. If the drive doesn't have this `auto-selection' capability, the decision should be made on the current disc loaded and the return value should be positive. A negative return value indicates an error. There are a few reasons for having the speed to be selectable. Badly pressed \cdrom s may benefit from less-than-maximum head rate. Modern \cdrom\ drives can obtain very high head rates (up to twelve times audio speed is common), but these drives tend to make an annoyingly loud noise. A lower speed may reduce this. 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. \subsection{$Select_disc(struct\ cdrom_device_info * cdi, int\ number)$} If the drive can store multiple discs (a juke-box) this function should 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 such functionality. \subsection{$Get_last_session(struct\ cdrom_device_info * cdi, struct\ cdrom_multisession * ms_info)$} This function should implement the old corresponding $ioctl()$. For device $cdi->dev$, the start of the last session of the current disc should be returned in the pointer argument $ms_info$. Note that routines in \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 transform if necessary. The return value is 0 upon success. \subsection{$Get_mcn(struct\ cdrom_device_info * cdi, struct\ cdrom_mcn * mcn)$} 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{$Reset(struct\ cdrom_device_info * cdi)$} This call should implement hard-resetting 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. \subsection{$Audio_ioctl(struct\ cdrom_device_info * cdi, unsigned\ int\ cmd, void * arg)$} Some of the \cdrom-$ioctl$s defined in {\tt cdrom.h} can be implemented by the routines described above, and hence the function $cdrom_ioctl$ will use those. However, most $ioctl$s deal with audio-control. We have decided to leave these 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 an 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 $-EINVAL$, but a harmless request (e.g., $CDROMSTART$) may be ignored by returning 0 (success). Other errors should be according to the standards, whatever they are. (We may decide to sanitize the return value in $cdrom_ioctl()$, in order to guarantee a uniform interface to the audio-player software.) \subsection{$Dev_ioctl(struct\ cdrom_device_info * cdi, unsigned\ int\ cmd, unsigned\ long\ arg)$} 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). Once this question is 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 CDROMREADALL}. \subsection{\cdrom\ capabilities} Instead of just implementing some $ioctl$ calls, the interface in \cdromc\ supplies the possibility to indicate the {\em capabilities\/} of a \cdrom\ drive. This can be done by ORing any number of capability-constants that are defined in \ucdrom\ at the registration phase. Currently, the capabilities are any of: $$ \halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr CDC_CLOSE_TRAY& can close tray by software control\cr CDC_OPEN_TRAY& can open tray\cr CDC_LOCK& can lock and unlock the door\cr CDC_SELECT_SPEED& can select speed, in units of $\sim$150\,kB/s\cr CDC_SELECT_DISC& drive is juke-box\cr CDC_MULTI_SESSION& can read sessions $>\rm1$\cr CDC_MCN& can read Medium Catalog Number\cr CDC_MEDIA_CHANGED& can report if disc has changed\cr CDC_PLAY_AUDIO& can perform audio-functions (play, pause, etc)\cr } $$ The capability flag is declared $const$, to prevent drivers from accidentally tampering with the contents. The capability fags actually inform \cdromc\ on what the driver is capable of. 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 $$\it if\ (cdo\rightarrow capability \mathrel\& \mathord{\sim} cdi\rightarrow mask \mathrel{\&} CDC_) \ldots $$ 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 section we explain what the need for this option is. \section{The need to know the purpose of opening} 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/$$/}. 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, i.e., 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 a 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 it is opened for data ($O_NONBLOCK$ is not set) and the tray is found 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 its type is `data mode 1.' Only if all tests are passed, the return value is zero. The door is locked to prevent file system corruption. If opened for audio ($O_NONBLOCK$ is set), no actions are taken and a value of 0 will be returned. \item[0] $Open()$ will always be successful, the option flags are ignored. Neither actions are undertaken, nor any integrity checks are made. \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, i.e., 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 adapt to the new \cdrom\ driver scheme and option flag interpretation. \section{Description of routines in \cdromc} Only a few routines in \cdromc\ are exported to the drivers. In this section we will treat these, as well as the functioning of the routines that `take over' the interface to the kernel. The header file belonging to \cdromc\ is called \ucdrom, but may be included in {\tt cdrom.h} in the future. \subsection{$Struct\ file_operations\ cdrom_fops$} The contents of this structure has been described in section~\ref{cdrom.c}, and this structure should be used in registering the block device to the kernel: $$ register_blkdev(major, , \&cdrom_fops); $$ \subsection{$Int\ register_cdrom( struct\ cdrom_device_info\ * cdi, char * name,)$} Similar to registering $cdrom_fops$ to the kernel, the device operations and information structures, as described in section~\ref{cdrom.c}, should be registered to the general \cdrom\ interface: $$ register_cdrom(\&_info), ); $$ This function returns zero upon success, and non-zero upon failure. The structure $_info$ should have a pointer the driver's $_dops$, as in $$ \vbox{\halign{&$#$\hfil\cr struct\ &cdrom_device_info\ _info = \{\cr & _dops;\cr &\ldots\cr \}\cr }}$$ Note that a drivers has one static structure, $_dops$, while it has as many structures $_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 minor for the 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 logic 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 in the 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 $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 or CDROM_GET_UPC] Get the Medium Catalog Number from a CD. \end{description} \subsubsection{$Ioctl$s rooted 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$. The value 0 means `auto-select', i.e., play audio discs at real time and data disc 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 is returned. \item[CDROM_DRIVE_STATUS] Returns the status of the drive by a call to $drive_status()$. Return values are as 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 is returned. \item[CDROM_DISC_STATUS] Returns the type of the disc currently in the drive by a call to $disc_status()$. Return values are as defined in section~\ref{disc status}. \item[CDROM_CHANGER_NSLOTS] Returns the number of slots in a juke-box. \end{description} \subsubsection{Device dependent $ioct$s} Finally, all other $ioctl$s are passed to the function $dev_ioctl()$, if implemented. No memory allocation or verification is carried out. \subsection{How to update your driver} \begin{enumerate} \item Make a backup of your current driver. \item Get hold of the files \cdromc\ and \ucdrom, they should be in the directory tree that came with this documentation. \item Include {\tt \char`\} just after {\tt cdrom.h}. \item Change the 3rd argument of $register_blkdev$ from $\&_fops$ to $\&cdrom_fops$. \item Just after that line, add a line to register to the \cdrom\ routines: $$register_cdrom(\&_info, );$$ Similarly, add a call to $unregister_cdrom()$. \item Copy an example of the device-operations $struct$ to your source, e.g., from {\tt cm206.c} $cm206_dops$, and change all entries to names corresponding to your driver, or names you just happen to like. If your driver doesn't support a certain function, make the entry $NULL$. At the entry $capability$ you should list all capabilities your drive could support, in principle. If your drive has a capability that is not listed, please send me a message. \item Copy the $cdrom_device_info$ declaration from the same example driver, and modify the entries according to your needs. If your driver dynamically determines the capabilities of the hardware, this structure should also be declared dynamically. \item Implement all functions in your $_dops$ structure, according to prototypes listed in \ucdrom, and specifications given in section~\ref{cdrom.c}. Most likely you have already implemented the code in a large part, and you may just have to adapt the prototype and return values. \item Rename your $_ioctl()$ function to $audio_ioctl$ and change the prototype a little. Remove entries listed in the first part in section~\ref{cdrom-ioctl}, if your code was OK, these are just calls to the routines you adapted in the previous step. \item You may remove all remaining memory checking code in the $audio_ioctl()$ function that deals with audio commands (these are listed in the second part of section~\ref{cdrom-ioctl}). There is no need for memory allocation either, so most $case$s in the $switch$ statement look similar to: $$ case\ CDROMREADTOCENTRY\colon get_toc_entry\bigl((struct\ cdrom_tocentry *{})\ arg\bigr); $$ \item All remaining $ioctl$ cases must be moved to a separate function, $_ioctl$, the device-dependent $ioctl$s. Note that memory checking and allocation must be kept in this code! \item Change the prototypes of $_open()$ and $_release()$, and remove any strategic code (i.e., tray movement, door locking, etc.). \item Try to recompile the drivers. We advice you to use modules, both for {\tt cdrom.o} and your driver, as debugging is much easier this way. \end{enumerate} \section{Thanks} Thanks to all the people involved. 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 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} \def\versionlog{ $Log: cdrom-standard.tex,v $ Revision 1.6 1996/12/29 20:45:18 davem Merge to 2.1.18, versioned module symbols are disabled until new modutils is released. Revision 1.6 1996/12/23 21:17:44 david Added reasons for speed selection. Revision 1.5 1996/12/22 21:54:25 david Repared version definition. Revision 1.4 1996/12/22 21:40:26 david Added older version log at end of text. Revision 1.3 1996/12/22 21:31:58 david Adapted text to fit kernel changes up to 2.1.15. Sun Dec 22 21:31:58 1996 David A. van Leeuwen * cdrom-standard.tex: Adapted text to fit kernel changes up to 2.1.15. Sun Sep 22 20:18:00 1996 David * cdrom-standard.tex: Documentation adapted to align with developments started by Scott Snyder, Gerd Knorr and myself. Split _ops in _info (minor stuff) and _ops (major stuff). Moved capability back to _ops, added n_minors. Implemented use_count in _info. Removed open_files(). Sat Aug 10 10:57:16 1996 David * cdrom-standard.tex: Weakened Eberhard's annoying comments by making it a quotation and having it appear smaal-type. He doesn't react to my email-messages. Changed float speed to int speed. Tue May 21 15:27:10 1996 David * cdrom-standard.tex: Typographic errors introduced by Eberhard Moenkeberg were corrected, as well one of my own typos. I deepfully respect the person that corrects all typos i make in code and documentation! Linux version 1.99.6. * cdrom-standard.tex: Changes made by Eberhard Moenkeberg, much to my annoyance. The contents of the introduction were altered completely so that it now says the opposite of what I wrote before: that there is no need for an additional unform cdrom layer. This makes the whole document and the project for that matter worthless. ---david * cdrom-standard.tex: Version as it appeared first in the official kernel documentation tree, Linux 1.99.2, Documentation/cdrom/cdrom-standard.tex. Some improvements in use of English language have been made by people that know better English than me. Wed Apr 17 20:46:34 1996 David * cdrom-standard.tex: changed #minors from 4 to 16. Sun Apr 14 20:53:17 1996 David * cdrom-standard.tex: Update to go with cdrom.c version 0.3. More ioctl stuff. Wed Apr 10 18:00:28 1996 David * cdrom-standard.tex: Version as first distributed among CDrom device driver authors. Mon Apr 8 18:25:21 1996 David A. van Leeuwen * cdrom-standard.tex: *** empty log message *** }