litmus-rt.git - The LITMUS^RT kernel.

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author	Jonathan Cameron <jic23@cam.ac.uk>	2010-03-30 12:45:04 -0400
committer	Greg Kroah-Hartman <gregkh@suse.de>	2010-04-30 12:30:09 -0400
commit	4845187b39b79e3c1d4474c2767ddc7fb493eb05 (patch)
tree	78743d925597e96220b6a1d63fdd3f72bac1f766 /tools/perf/scripts/python
parent	a8947fcf3bbeda9fc6ff1205e51f8ff1b0128cc4 (diff)

staging: iio: ring_sw: Fix incorrect test on successful read of last value, causes infinite loop

This is a bad one. The test means that almost no reads of the last value ever succeed! Result is an infinite loop. Another one for the 'oops' category. Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Diffstat (limited to 'tools/perf/scripts/python')

0 files changed, 0 insertions, 0 deletions

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\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


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