path: root/Documentation/cdrom/cdrom-standard.tex

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\def\linux{{\sc Linux}}
\def\cdrom{{\sc cd-rom}}
\def\UCD{{\sc Uniform cd-rom Driver}}
\def\cdromc{{\tt {cdrom.c}}}
\def\cdromh{{\tt {cdrom.h}}}
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\begin{document}
\title{A \linux\ \cdrom\ standard}
\author{David van Leeuwen\\{\normalsize\tt david@ElseWare.cistron.nl}
\\{\footnotesize updated by Erik Andersen {\tt(andersee@debian.org)}}
\\{\footnotesize updated by Jens Axboe {\tt(axboe@image.dk)}}}
\date{12 March 1999}

\maketitle

\newsection{Introduction}

\linux\ is probably the Unix-like operating system that supports
the widest variety of hardware devices. The reasons for this are
presumably 
\begin{itemize} 
\item 
  The large list of hardware devices available for the many platforms
  that \linux\ now supports (\ie, i386-PCs, Sparc Suns, etc.)
\item 
  The open design of the operating system, such that anybody can write a
  driver for \linux.
\item 
  There is plenty of source code around as examples of how to write a driver.
\end{itemize}
The openness of \linux, and the many different types of available
hardware has allowed \linux\ to support many different hardware devices.
Unfortunately, the very openness that has allowed \linux\ to support
all these different devices has also allowed the behavior of each
device driver to differ significantly from one device to another.
This divergence of behavior has been very significant for \cdrom\
devices; the way a particular drive reacts to a `standard' $ioctl()$
call varies greatly from one device driver to another. To avoid making
their drivers totally inconsistent, the writers of \linux\ \cdrom\
drivers generally created new device drivers by understanding, copying,
and then changing an existing one. Unfortunately, this practice did not
maintain uniform behavior across all the \linux\ \cdrom\ drivers. 

This document describes an effort to establish Uniform behavior across
all the different \cdrom\ device drivers for \linux. This document also
defines the various $ioctl$s, and how the low-level \cdrom\ device
drivers should implement them. Currently (as of the \linux\ 2.1.$x$
development kernels) several low-level \cdrom\ device drivers, including
both IDE/ATAPI and SCSI, now use this Uniform interface.

When the \cdrom\ was developed, the interface between the \cdrom\ drive
and the computer was not specified in the standards. As a result, many
different \cdrom\ interfaces were developed. Some of them had their
own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
manufacturers adopted an existing electrical interface and changed
the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
adapted their drives to one or more of the already existing electrical
interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
most of the `NoName' manufacturers). In cases where a new drive really
brought its own interface or used its own command set and flow control
scheme, either a separate driver had to be written, or an existing
driver had to be enhanced. History has delivered us \cdrom\ support for
many of these different interfaces. Nowadays, almost all new \cdrom\
drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
manufacturer will create a new interface. Even finding drives for the
old proprietary interfaces is getting difficult.

When (in the 1.3.70's) I looked at the existing software interface,
which was expressed through \cdromh, it appeared to be a rather wild
set of commands and data formats.\footnote{I cannot recollect what
kernel version I looked at, then, presumably 1.2.13 and 1.3.34---the
latest kernel that I was indirectly involved in.} It seemed that many
features of the software interface had been added to accommodate the
capabilities of a particular drive, in an {\fo ad hoc\/} manner. More
importantly, it appeared that the behavior of the `standard' commands
was different for most of the different drivers: \eg, some drivers
close the tray if an $open()$ call occurs when the tray is open, while
others do not. Some drivers lock the door upon opening the device, to
prevent an incoherent file system, but others don't, to allow software
ejection. Undoubtedly, the capabilities of the different drives vary,
but even when two drives have the same capability their drivers'
behavior was usually different.

I decided to start a discussion on how to make all the \linux\ \cdrom\
drivers behave more uniformly. I began by contacting the developers of
the many \cdrom\ drivers found in the \linux\ kernel. Their reactions
encouraged me to write the \UCD\ which this document is intended to
describe. The implementation of the \UCD\ is in the file \cdromc. This
driver is intended to be an additional software layer that sits on top
of the low-level device drivers for each \cdrom\ drive. By adding this
additional layer, it is possible to have all the different \cdrom\
devices behave {\em exactly\/} the same (insofar as the underlying
hardware will allow).

The goal of the \UCD\ is {\em not\/} to alienate driver developers who
have not yet taken steps to support this effort. The goal of \UCD\ is
simply to give people writing application programs for \cdrom\ drives
{\em one\/} \linux\ \cdrom\ interface with consistent behavior for all
\cdrom\ devices. In addition, this also provides a consistent interface
between the low-level device driver code and the \linux\ kernel. Care
is taken that 100\,\% compatibility exists with the data structures and
programmer's interface defined in \cdromh. This guide was written to
help \cdrom\ driver developers adapt their code to use the \UCD\ code
defined in \cdromc.

Personally, I think that the most important hardware interfaces are
the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
of hardware drop continuously, it is also likely that people may have
more than one \cdrom\ drive, possibly of mixed types. It is important
that these drives behave in the same way. In December 1994, one of the
cheapest \cdrom\ drives was a Philips cm206, a double-speed proprietary
drive. In the months that I was busy writing a \linux\ driver for it,
proprietary drives became obsolete and IDE/ATAPI drives became the
standard. At the time of the last update to this document (November
1997) it is becoming difficult to even {\em find} anything less than a
16 speed \cdrom\ drive, and 24 speed drives are common.

\newsection{Standardizing through another software level}
\label{cdrom.c}

At the time this document was conceived, all drivers directly
implemented the \cdrom\ $ioctl()$ calls through their own routines. This
led to the danger of different drivers forgetting to do important things
like checking that the user was giving the driver valid data. More
importantly, this led to the divergence of behavior, which has already
been discussed.

For this reason, the \UCD\ was created to enforce consistent \cdrom\
drive behavior, and to provide a common set of services to the various
low-level \cdrom\ device drivers. The \UCD\ now provides another
software-level, that separates the $ioctl()$ and $open()$ implementation
from the actual hardware implementation. Note that this effort has
made few changes which will affect a user's application programs. The
greatest change involved moving the contents of the various low-level
\cdrom\ drivers' header files to the kernel's cdrom directory. This was
done to help ensure that the user is only presented with only one cdrom
interface, the interface defined in \cdromh.

\cdrom\ drives are specific enough (\ie, different from other
block-devices such as floppy or hard disc drives), to define a set
of common {\em \cdrom\ device operations}, $<cdrom-device>_dops$.
These operations are different from the classical block-device file
operations, $<block-device>_fops$.

The routines for the \UCD\ interface level are implemented in the file
\cdromc. In this file, the \UCD\ interfaces with the kernel as a block
device by registering the following general $struct\ file_operations$:
$$
\halign{$#$\ \hfil&$#$\ \hfil&$/*$ \rm# $*/$\hfil\cr
struct& file_operations\ cdrom_fops = \{\hidewidth\cr
        &NULL,                  & lseek \cr
        &block_read,            & read---general block-dev read \cr
        &block_write,           & write---general block-dev write \cr
        &NULL,                  & readdir \cr
        &NULL,                  & select \cr
        &cdrom_ioctl,           & ioctl \cr
        &NULL,                  & mmap \cr
        &cdrom_open,            & open \cr
        &cdrom_release,         & release \cr
        &NULL,                  & fsync \cr
        &NULL,                  & fasync \cr
        &cdrom_media_changed,   & media change \cr
        &NULL                   & revalidate \cr
\};\cr
}
$$ 

Every active \cdrom\ device shares this $struct$. The routines
declared above are all implemented in \cdromc, since this file is the
place where the behavior of all \cdrom-devices is defined and
standardized. The actual interface to the various types of \cdrom\ 
hardware is still performed by various low-level \cdrom-device
drivers. These routines simply implement certain {\em capabilities\/}
that are common to all \cdrom\ (and really, all removable-media
devices).

Registration of a low-level \cdrom\ device driver is now done through
the general routines in \cdromc, not through the Virtual File System
(VFS) any more. The interface implemented in \cdromc\ is carried out
through two general structures that contain information about the
capabilities of the driver, and the specific drives on which the
driver operates. The structures are:
\begin{description}
\item[$cdrom_device_ops$] 
  This structure contains information about the low-level driver for a
  \cdrom\ device. This structure is conceptually connected to the major
  number of the device (although some drivers may have different
  major numbers, as is the case for the IDE driver).
\item[$cdrom_device_info$] 
  This structure contains information about a particular \cdrom\ drive,
  such as its device name, speed, etc. This structure is conceptually
  connected to the minor number of the device.
\end{description}

Registering a particular \cdrom\ drive with the \UCD\ is done by the
low-level device driver though a call to:
$$register_cdrom(struct\ cdrom_device_info * <device>_info)  
$$
The device information structure, $<device>_info$, contains all the
information needed for the kernel to interface with the low-level
\cdrom\ device driver. One of the most important entries in this
structure is a pointer to the $cdrom_device_ops$ structure of the
low-level driver.

The device operations structure, $cdrom_device_ops$, contains a list
of pointers to the functions which are implemented in the low-level
device driver. When \cdromc\ accesses a \cdrom\ device, it does it
through the functions in this structure. It is impossible to know all
the capabilities of future \cdrom\ drives, so it is expected that this
list may need to be expanded from time to time as new technologies are
developed. For example, CD-R and CD-R/W drives are beginning to become
popular, and support will soon need to be added for them. For now, the
current $struct$ is:
$$
\halign{$#$\ \hfil&$#$\ \hfil&\hbox to 10em{$#$\hss}&
  $/*$ \rm# $*/$\hfil\cr
struct& cdrom_device_ops\ \{ \hidewidth\cr
  &int& (* open)(struct\ cdrom_device_info *