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CD(4) Device Drivers Manual CD(4)

cd — SCSI CD-ROM driver

device cd
device cd1 at scbus0 target 4 unit 0
options CHANGER_MIN_BUSY_SECONDS=3
options CHANGER_MAX_BUSY_SECONDS=11

The cd driver provides support for a SCSI CD-ROM (Compact Disc-Read Only Memory) drive. In an attempt to look like a regular disk, the cd driver synthesizes a partition table, with one partition covering the entire CD-ROM. It is possible to modify this partition table using disklabel(8), but it will only last until the CD-ROM is unmounted. In general the interfaces are similar to those described by ad(4) and da(4).

As the SCSI adapter is probed during boot, the SCSI bus is scanned for devices. Any devices found which answer as CDROM (type 5) or WORM (type 4) type devices will be `attached' to the cd driver. Prior to FreeBSD 2.1, the first device found will be attached as cd0 the next, cd1, etc. Beginning in FreeBSD 2.1 it is possible to specify what cd unit a device should come on line as; refer to scsi(4) for details on kernel configuration.

The target string is followed by the device's standard SCSI device ID number. The unit string is followed by the Logical Unit Number (LUN) of the SCSI device's sub-device, if any, or zero.

The system utility disklabel(8) may be used to read the synthesized disk label structure, which will contain correct figures for the size of the CD-ROM should that information be required.

Any number of CD-ROM devices may be attached to the system regardless of system configuration as all resources are dynamically allocated.

The following ioctl(2) calls which apply to SCSI CD-ROM drives are defined in the header files <sys/cdio.h> and <sys/disklabel.h>.

 
(struct disklabel) Read or write the in-core copy of the disklabel for the drive. The disklabel is initialized with information read from the scsi inquiry commands, and should be the same as the information printed at boot. This structure is defined in disklabel(5).
(struct ioc_play_track) Start audio playback given a track address and length. The structure is defined as follows: 
struct ioc_play_track
{
	u_char	start_track;
	u_char	start_index;
	u_char	end_track;
	u_char	end_index;
};
(struct ioc_play_blocks) Start audio playback given a block address and length. The structure is defined as follows: 
struct ioc_play_blocks
{
	int	blk;
	int	len;
};
(struct ioc_play_msf) Start audio playback given a `minutes-seconds-frames' address and length. The structure is defined as follows: 
struct ioc_play_msf
{
	u_char	start_m;
	u_char	start_s;
	u_char	start_f;
	u_char	end_m;
	u_char	end_s;
	u_char	end_f;
};
(struct ioc_read_subchannel) Read information from the subchannel at the location specified by this structure: 
struct ioc_read_subchannel {
	u_char address_format;
#define CD_LBA_FORMAT	1
#define CD_MSF_FORMAT	2
	u_char data_format;
#define CD_SUBQ_DATA		0
#define CD_CURRENT_POSITION	1
#define CD_MEDIA_CATALOG	2
#define CD_TRACK_INFO		3
	u_char track;
	int	data_len;
	struct  cd_sub_channel_info *data;
};
(struct ioc_toc_header) Return summary information about the table of contents for the mounted CD-ROM. The information is returned into the following structure: 
struct ioc_toc_header {
	u_short len;
	u_char  starting_track;
	u_char  ending_track;
};
(struct ioc_read_toc_entry) Return information from the table of contents entries mentioned. (Yes, this command name is misspelled.) The argument structure is defined as follows: 
struct ioc_read_toc_entry {
	u_char	address_format;
	u_char	starting_track;
	u_short	data_len;
	struct  cd_toc_entry *data;
};
The requested data is written into an area of size data_len and pointed to by data.
(struct ioc_patch) Attach various audio channels to various output channels. The argument structure is defined thusly: 
struct ioc_patch {
	u_char	patch[4];
	/* one for each channel */
};
 
(struct ioc_vol) Get (set) information about the volume settings of the output channels. The argument structure is as follows: 
struct	ioc_vol
{
	u_char	vol[4];
	/* one for each channel */
};
Patch all output channels to all source channels.
Patch left source channel to the left output channel and the right source channel to the right output channel.
Mute output without changing the volume settings.
 
Attach both output channels to the left (right) source channel.
 
Turn on (off) debugging for the appropriate device.
 
Pause (resume) audio play, without resetting the location of the read-head.
Reset the drive.
 
Tell the drive to spin-up (-down) the CD-ROM.
 
Tell the drive to allow (prevent) manual ejection of the CD-ROM disc. Not all drives support this feature.
Eject the CD-ROM.
Tell the drive to close its door and load the media. Not all drives support this feature.

When a CD-ROM is changed in a drive controlled by the cd driver, then the act of changing the media will invalidate the disklabel and information held within the kernel. To stop corruption, all accesses to the device will be discarded until there are no more open file descriptors referencing the device. During this period, all new open attempts will be rejected. When no more open file descriptors reference the device, the first next open will load a new set of parameters (including disklabel) for the drive.

The audio code in the cd driver only support SCSI-2 standard audio commands. Because many CD-ROM manufacturers have not followed the standard, there are many CD-ROM drives for which audio will not work. Some work is planned to support some of the more common `broken' CD-ROM drives; however, this is not yet under way.

This driver has built-in support for LUN-based CD changers. A LUN-based CD changer is a drive that can hold two or more CDs, but only has one CD player mechanism. Each CD in the drive shows up as a separate logical unit on the SCSI bus. The cd driver automatically recognizes LUN-based changers, and routes commands for changers through an internal scheduler. The scheduler prevents changer "thrashing", which is caused by sending commands to different LUNs in the changer at the same time.

The scheduler honors minimum and maximum time quanta that the driver will spend on a particular LUN. The minimum time is the guaranteed minimum amount of time that the driver will spend on a given LUN, even if there is no outstanding I/O for that LUN. The maximum time is the maximum amount of time the changer will spend on a LUN if there is outstanding I/O for another LUN. If there is no outstanding I/O for another LUN, the driver will allow indefinite access to a given LUN.

The minimum and maximum time quanta are configurable via kernel options and also via sysctl variables. The kernel options are:

The sysctl variables are:

It is suggested that the user try experimenting with the minimum and maximum timeouts via the sysctl variables to arrive at the proper values for your changer. Once you have settled on the proper timeouts for your changer, you can then put them in your kernel config file.

If your system does have a LUN-based changer, you may notice that the probe messages for the various LUNs of the changer will continue to appear while the boot process is going on. This is normal, and is caused by the changer scheduling code.

/dev/cd[0-9][a-h]
raw mode CD-ROM devices

None.

da(4), scsi(4), disklabel(5), disklabel(8), cd(9)

This cd driver is based upon the cd driver written by Julian Elischer, which appeared in 386BSD-0.1. The CAM version of the cd driver was written by Kenneth Merry and first appeared in FreeBSD 3.0.

The names of the structures used for the third argument to ioctl() were poorly chosen, and a number of spelling errors have survived in the names of the ioctl() commands.

There is no mechanism currently to set different minimum and maximum timeouts for different CD changers; the timeout values set by the kernel options or the sysctl variables apply to all LUN-based CD changers in the system. It is possible to implement such support, but the sysctl implementation at least would be rather inelegant, because of the current inability of the sysctl code to handle the addition of nodes after compile time. Thus, it would take one dynamically sized sysctl variable and a userland utility to get/set the timeout values. Implementation of separate timeouts for different CD devices in the kernel config file would likely require modification of config(8) to support the two timeouts when hardwiring cd devices.

February 18, 2019 DragonFly-5.6.1