cdrom-standard.tex

Linux内核源代码 为压缩文件 是<<Linux内核>>一书中的源代码

\cdrom\ hardware and/or low-level \cdrom\ driver when a \cdrom\ drive
is registered with the \UCD. The value $n_minors$ should be a positive
value indicating the number of minor devices that are supported by
the low-level device driver, normally~1. Although these two variables
are `informative' rather than `operational,' they are included in
$cdrom_device_ops$ because they describe the capability of the {\em
driver\/} rather than the {\em drive}. Nomenclature has always been
difficult in computer programming.

Note that most functions have fewer parameters than their
$blkdev_fops$ counterparts. This is because very little of the
information in the structures $inode$ and $file$ is used. For most
drivers, the main parameter is the $struct$ $cdrom_device_info$, from
which the major and minor number can be extracted. (Most low-level
\cdrom\ drivers don't even look at the major and minor number though,
since many of them only support one device.) This will be available
through $dev$ in $cdrom_device_info$ described below.

The drive-specific, minor-like information that is registered with
\cdromc, currently contains the following fields:
$$
\halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}&
  $/*$ \rm# $*/$\hfil\cr
struct& cdrom_device_info\ \{ \hidewidth\cr
  & struct\ cdrom_device_ops *& ops;& device operations for this major\cr
  & struct\ cdrom_device_info *& next;& next device_info for this major\cr
  & void *&  handle;& driver-dependent data\cr
\noalign{\medskip}
  & kdev_t&  dev;& device number (incorporates minor)\cr
  & int& mask;& mask of capability: disables them \cr
  & int& speed;& maximum speed for reading data \cr
  & int& capacity;& number of discs in a jukebox \cr
\noalign{\medskip}
  &int& options : 30;& options flags \cr
  &unsigned& mc_flags : 2;& media-change buffer flags \cr
  & int& use_count;& number of times device is opened\cr
  & char& name[20];& name of the device type\cr
\}\cr
}$$
Using this $struct$, a linked list of the registered minor devices is
built, using the $next$ field. The device number, the device operations
struct and specifications of properties of the drive are stored in this
structure.

The $mask$ flags can be used to mask out some of the capabilities listed
in $ops\to capability$, if a specific drive doesn't support a feature
of the driver. The value $speed$ specifies the maximum head-rate of the
drive, measured in units of normal audio speed (176\,kB/sec raw data or
150\,kB/sec file system data). The value $n_discs$ should reflect the
number of discs the drive can hold simultaneously, if it is designed
as a juke-box, or otherwise~1. The parameters are declared $const$
because they describe properties of the drive, which don't change after
registration.

A few registers contain variables local to the \cdrom\ drive. The
flags $options$ are used to specify how the general \cdrom\ routines
should behave. These various flags registers should provide enough
flexibility to adapt to the different users' wishes (and {\em not\/} the
`arbitrary' wishes of the author of the low-level device driver, as is
the case in the old scheme). The register $mc_flags$ is used to buffer
the information from $media_changed()$ to two separate queues. Other
data that is specific to a minor drive, can be accessed through $handle$,
which can point to a data structure specific to the low-level driver.
The fields $use_count$, $next$, $options$ and $mc_flags$ need not be
initialized.

The intermediate software layer that \cdromc\ forms will perform some
additional bookkeeping. The use count of the device (the number of
processes that have the device opened) is registered in $use_count$. The
function $cdrom_ioctl()$ will verify the appropriate user-memory regions
for read and write, and in case a location on the CD is transferred,
it will `sanitize' the format by making requests to the low-level
drivers in a standard format, and translating all formats between the
user-software and low level drivers. This relieves much of the drivers'
memory checking and format checking and translation. Also, the necessary
structures will be declared on the program stack.

The implementation of the functions should be as defined in the
following sections. Two functions {\em must\/} be implemented, namely
$open()$ and $release()$. Other functions may be omitted, their
corresponding capability flags will be cleared upon registration.
Generally, a function returns zero on success and negative on error. A
function call should return only after the command has completed, but of
course waiting for the device should not use processor time.

\subsection{$Int\ open(struct\ cdrom_device_info * cdi, int\ purpose)$}

$Open()$ should try to open the device for a specific $purpose$, which
can be either:
\begin{itemize}
\item[0] Open for reading data, as done by {\tt {mount()}} (2), or the
user commands {\tt {dd}} or {\tt {cat}}.  
\item[1] Open for $ioctl$ commands, as done by audio-CD playing
programs.
\end{itemize}
In case the driver supports modules, the call $MOD_INC_USE_COUNT$
should be performed exactly once, if the $open()$ was successful. The
return value is negative on error, and zero on success. The
open-for-ioctl call can only fail if there is no hardware.

Notice that any strategic code (closing tray upon $open()$, etc.)\ is
done by the calling routine in \cdromc, so the low-level routine
should only be concerned with proper initialization, such as spinning
up the disc, etc. % and device-use count


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

In case of module support, a single call $MOD_DEC_USE_COUNT$ should be
coded here.  Possibly other device-specific actions should be taken
such as spinning down the device. However, strategic actions such as
ejection of the tray, or unlocking the door, should be left over to
the general routine $cdrom_release()$. Also, the invalidation of the
allocated buffers in the VFS is taken care of by the routine in
\cdromc.  This is the only function returning type $void$. 

\subsection{$Int\ drive_status(struct\ cdrom_device_info * cdi, int\ slot_nr)$}
\label{drive status}

The function $drive_status$, if implemented, should provide
information on the status of the drive (not the status of the disc,
which may or may not be in the drive). If the drive is not a changer,
$slot_nr$ should be ignored. In \cdromh\ the possibilities are listed: 
$$
\halign{$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
CDS_NO_INFO& no information available\cr
CDS_NO_DISC& no disc is inserted, tray is closed\cr
CDS_TRAY_OPEN& tray is opened\cr
CDS_DRIVE_NOT_READY& something is wrong, tray is moving?\cr
CDS_DISC_OK& a disc is loaded and everything is fine\cr
}
$$

\subsection{$Int\ media_changed(struct\ cdrom_device_info * cdi, int\ disc_nr)$}

This function is very similar to the original function in $struct\ 
file_operations$. It returns 1 if the medium of the device $cdi\to
dev$ has changed since the last call, and 0 otherwise. The parameter
$disc_nr$ identifies a specific slot in a juke-box, it should be
ignored for single-disc drives.  Note that by `re-routing' this
function through $cdrom_media_changed()$, we can implement separate
queues for the VFS and a new $ioctl()$ function that can report device
changes to software (\eg, an auto-mounting daemon).

\subsection{$Int\ tray_move(struct\ cdrom_device_info * cdi, int\ position)$}

This function, if implemented, should control the tray movement. (No
other function should control this.) The parameter $position$ controls
the desired direction of movement:
\begin{itemize}
\item[0] Close tray
\item[1] Open tray
\end{itemize}
This function returns 0 upon success, and a non-zero value upon
error. Note that if the tray is already in the desired position, no
action need be taken, and the return value should be 0. 

\subsection{$Int\ lock_door(struct\ cdrom_device_info * cdi, int\ lock)$}

This function (and no other code) controls locking of the door, if the
drive allows this. The value of $lock$ controls the desired locking
state:
\begin{itemize}
\item[0] Unlock door, manual opening is allowed
\item[1] Lock door, tray cannot be ejected manually
\end{itemize}
This function returns 0 upon success, and a non-zero value upon
error. Note that if the door is already in the requested state, no
action need be taken, and the return value should be 0. 

\subsection{$Int\ select_speed(struct\ cdrom_device_info * cdi, int\ speed)$}

Some \cdrom\ drives are capable of changing their head-speed. There
are several reasons for changing the speed of a \cdrom\ drive. Badly
pressed \cdrom s may benefit from less-than-maximum head rate. Modern
\cdrom\ drives can obtain very high head rates (up to $24\times$ is
common).  It has been reported that these drives can make reading
errors at these high speeds, reducing the speed can prevent data loss
in these circumstances.  Finally, some of these drives can
make an annoyingly loud noise, which a lower speed may reduce. %Finally,
%although the audio-low-pass filters probably aren't designed for it,
%more than real-time playback of audio might be used for high-speed
%copying of audio tracks.

This function specifies the speed at which data is read or audio is
played back. The value of $speed$ specifies the head-speed of the
drive, measured in units of standard cdrom speed (176\,kB/sec raw data
or 150\,kB/sec file system data). So to request that a \cdrom\ drive
operate at 300\,kB/sec you would call the CDROM_SELECT_SPEED $ioctl$
with $speed=2$. The special value `0' means `auto-selection', \ie,
maximum data-rate or real-time audio rate. If the drive doesn't have
this `auto-selection' capability, the decision should be made on the
current disc loaded and the return value should be positive. A negative
return value indicates an error.

\subsection{$Int\ select_disc(struct\ cdrom_device_info * cdi, int\ number)$}

If the drive can store multiple discs (a juke-box) this function
will perform disc selection. It should return the number of the
selected disc on success, a negative value on error. Currently, only
the ide-cd driver supports this functionality.

\subsection{$Int\ get_last_session(struct\ cdrom_device_info * cdi, struct\
  cdrom_multisession * ms_info)$}

This function should implement the old corresponding $ioctl()$. For
device $cdi\to dev$, the start of the last session of the current disc
should be returned in the pointer argument $ms_info$. Note that
routines in \cdromc\ have sanitized this argument: its requested
format will {\em always\/} be of the type $CDROM_LBA$ (linear block
addressing mode), whatever the calling software requested. But
sanitization goes even further: the low-level implementation may
return the requested information in $CDROM_MSF$ format if it wishes so
(setting the $ms_info\rightarrow addr_format$ field appropriately, of
course) and the routines in \cdromc\ will make the transformation if
necessary. The return value is 0 upon success.

\subsection{$Int\ get_mcn(struct\ cdrom_device_info * cdi, struct\
  cdrom_mcn * mcn)$}

Some discs carry a `Media Catalog Number' (MCN), also called
`Universal Product Code' (UPC). This number should reflect the number
that is generally found in the bar-code on the product. Unfortunately,
the few discs that carry such a number on the disc don't even use the
same format. The return argument to this function is a pointer to a
pre-declared memory region of type $struct\ cdrom_mcn$. The MCN is
expected as a 13-character string, terminated by a null-character.

\subsection{$Int\ reset(struct\ cdrom_device_info * cdi)$}

This call should perform a hard-reset on the drive (although in
circumstances that a hard-reset is necessary, a drive may very well not
listen to commands anymore). Preferably, control is returned to the
caller only after the drive has finished resetting. If the drive is no
longer listening, it may be wise for the underlying low-level cdrom
driver to time out.

\subsection{$Int\ audio_ioctl(struct\ cdrom_device_info * cdi, unsigned\
  int\ cmd, void * arg)$}

Some of the \cdrom-$ioctl$s defined in \cdromh\ can be
implemented by the routines described above, and hence the function
$cdrom_ioctl$ will use those. However, most $ioctl$s deal with
audio-control. We have decided to leave these to be accessed through a
single function, repeating the arguments $cmd$ and $arg$. Note that
the latter is of type $void*{}$, rather than $unsigned\ long\
int$. The routine $cdrom_ioctl()$ does do some useful things,
though. It sanitizes the address format type to $CDROM_MSF$ (Minutes,
Seconds, Frames) for all audio calls. It also verifies the memory
location of $arg$, and reserves stack-memory for the argument. This
makes implementation of the $audio_ioctl()$ much simpler than in the
old driver scheme. For example, you may look up the function
$cm206_audio_ioctl()$ in {\tt {cm206.c}} that should be updated with
this documentation. 

An unimplemented ioctl should return $-ENOSYS$, but a harmless request
(\eg, $CDROMSTART$) may be ignored by returning 0 (success). Other
errors should be according to the standards, whatever they are. When
an error is returned by the low-level driver, the \UCD\ tries whenever