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

=======================
A Linux CD-ROM standard
=======================

:Author: David van Leeuwen <david@ElseWare.cistron.nl>
:Date: 12 March 1999
:Updated by: Erik Andersen (andersee@debian.org)
:Updated by: Jens Axboe (axboe@image.dk)


Introduction
============

Linux is probably the Unix-like operating system that supports
the widest variety of hardware devices. The reasons for this are
presumably

- The large list of hardware devices available for the many platforms
  that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
- The open design of the operating system, such that anybody can write a
  driver for Linux.
- There is plenty of source code around as examples of how to write a driver.

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 CD-ROM
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 CD-ROM
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 CD-ROM drivers.

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

When the CD-ROM was developed, the interface between the CD-ROM drive
and the computer was not specified in the standards. As a result, many
different CD-ROM 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 CD-ROM support for
many of these different interfaces. Nowadays, almost all new CD-ROM
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 `cdrom.h`, it appeared to be a rather wild
set of commands and data formats [#f1]_. It seemed that many
features of the software interface had been added to accommodate the
capabilities of a particular drive, in an *ad hoc* manner. More
importantly, it appeared that the behavior of the `standard` commands
was different for most of the different drivers: e. g., 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.

.. [#f1]
   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.

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

The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
Driver is simply to give people writing application programs for CD-ROM drives
**one** Linux CD-ROM interface with consistent behavior for all
CD-ROM 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 `cdrom.h`. This guide was written to
help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
Driver code defined in `cdrom.c`.

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 CD-ROM drive, possibly of mixed types. It is important
that these drives behave in the same way. In December 1994, one of the
cheapest CD-ROM 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 **find** anything less than a
16 speed CD-ROM drive, and 24 speed drives are common.

.. _cdrom_api:

Standardizing through another software level
============================================

At the time this document was conceived, all drivers directly
implemented the CD-ROM *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 Uniform CD-ROM Driver was created to enforce consistent
CD-ROM drive behavior, and to provide a common set of services to the various
low-level CD-ROM device drivers. The Uniform CD-ROM Driver 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
CD-ROM 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 `cdrom.h`.

CD-ROM drives are specific enough (i. e., different from other
block-devices such as floppy or hard disc drives), to define a set
of common **CD-ROM device operations**, *<cdrom-device>_dops*.
These operations are different from the classical block-device file
operations, *<block-device>_fops*.

The routines for the Uniform CD-ROM Driver interface level are implemented
in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
with the kernel as a block device by registering the following general
*struct file_operations*::

	struct file_operations cdrom_fops = {
		NULL,			/∗ lseek ∗/
		block _read ,		/∗ read—general block-dev read ∗/
		block _write,		/∗ write—general block-dev write ∗/
		NULL,			/∗ readdir ∗/
		NULL,			/∗ select ∗/
		cdrom_ioctl,		/∗ ioctl ∗/
		NULL,			/∗ mmap ∗/
		cdrom_open,		/∗ open ∗/
		cdrom_release,		/∗ release ∗/
		NULL,			/∗ fsync ∗/
		NULL,			/∗ fasync ∗/
		cdrom_media_changed,	/∗ media change ∗/
		NULL			/∗ revalidate ∗/
	};

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

Registration of a low-level CD-ROM device driver is now done through
the general routines in `cdrom.c`, not through the Virtual File System
(VFS) any more. The interface implemented in `cdrom.c` 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:

cdrom_device_ops
  This structure contains information about the low-level driver for a
  CD-ROM 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).

cdrom_device_info
  This structure contains information about a particular CD-ROM drive,
  such as its device name, speed, etc. This structure is conceptually
  connected to the minor number of the device.

Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
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
CD-ROM 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 `cdrom.c` accesses a CD-ROM device, it does it
through the functions in this structure. It is impossible to know all
the capabilities of future CD-ROM 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::

	struct cdrom_device_ops {
		int (*open)(struct cdrom_device_info *, int)
		void (*release)(struct cdrom_device_info *);
		int (*drive_status)(struct cdrom_device_info *, int);
		unsigned int (*check_events)(struct cdrom_device_info *,
					     unsigned int, int);
		int (*media_changed)(struct cdrom_device_info *, int);
		int (*tray_move)(struct cdrom_device_info *, int);
		int (*lock_door)(struct cdrom_device_info *, int);
		int (*select_speed)(struct cdrom_device_info *, int);
		int (*select_disc)(struct cdrom_device_info *, int);
		int (*get_last_session) (struct cdrom_device_info *,
					 struct cdrom_multisession *);
		int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
		int (*reset)(struct cdrom_device_info *);
		int (*audio_ioctl)(struct cdrom_device_info *,
				   unsigned int, void *);
		const int capability;		/* capability flags */
		int (*generic_packet)(struct cdrom_device_info *,
				      struct packet_command *);
	};

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
CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
is registered with the Uniform CD-ROM Driver.

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
CD-ROM 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
`cdrom.c`, currently contains the following fields::

  struct cdrom_device_info {
	const struct cdrom_device_ops * ops; 	/* device operations for this major */
	struct list_head list;			/* linked list of all device_info */
	struct gendisk * disk;			/* matching block layer disk */
	void *  handle;				/* driver-dependent data */

	int mask; 				/* mask of capability: disables them */
	int speed;				/* maximum speed for reading data */
	int capacity;				/* number of discs in a jukebox */

	unsigned int options:30;		/* options flags */
	unsigned mc_flags:2;			/*  media-change buffer flags */
	unsigned int vfs_events;		/*  cached events for vfs path */
	unsigned int ioctl_events;		/*  cached events for ioctl path */
	int use_count;				/*  number of times device is opened */
	char name[20];				/*  name of the device type */

	__u8 sanyo_slot : 2;			/*  Sanyo 3-CD changer support */
	__u8 keeplocked : 1;			/*  CDROM_LOCKDOOR status */
	__u8 reserved : 5;			/*  not used yet */
	int cdda_method;			/*  see CDDA_* flags */
	__u8 last_sense;			/*  saves last sense key */
	__u8 media_written;			/*  dirty flag, DVD+RW bookkeeping */
	unsigned short mmc3_profile;		/*  current MMC3 profile */
	int for_data;				/*  unknown:TBD */
	int (*exit)(struct cdrom_device_info *);/*  unknown:TBD */
	int mrw_mode_page;			/*  which MRW mode page is in use */
  };

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->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 (176kB/sec raw data or
150kB/sec file system data). 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 CD-ROM drive. The
flags *options* are used to specify how the general CD-ROM routines
should behave. These various flags registers should provide enough
flexibility to adapt to the different users' wishes (and **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 `cdrom.c` 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 **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.

::

	int open(struct cdrom_device_info *cdi, int purpose)

*Open()* should try to open the device for a specific *purpose*, which
can be either:

- Open for reading da