DISK(9) MidnightBSD Kernel Developer’s Manual DISK(9)


disk — kernel disk storage API


#include <geom/geom_disk.h>

struct disk *



disk_create(struct disk *disk, int version);


disk_destroy(struct disk *disk);


The disk storage API permits kernel device drivers providing access to disk-like storage devices to advertise the device to other kernel components, including GEOM(4) and devfs(5).

Each disk device is described by a struct disk structure, which contains a variety of parameters for the disk device, function pointers for various methods that may be performed on the device, as well as private data storage for the device driver. In addition, some fields are reserved for use by GEOM in managing access to the device and its statistics.

GEOM has the ownership of struct disk, and drivers must allocate storage for it with the disk_alloc() function, fill in the fields and call disk_create() when the device is ready to service requests. After calling disk_destroy(), the device driver is not allowed to access the contents of struct disk anymore.

The disk_create() function takes a second parameter, version, which must always be passed DISK_VERSION. If GEOM detects that the driver is compiled against an unsupported version, it will ignore the device and print a warning on the console.

Descriptive Fields
The following fields identify the disk device described by the structure instance, and must be filled in prior to submitting the structure to disk_create() and may not be subsequently changed:

u_int d_flags

Optional flags indicating to the storage framework what optional features or descriptions the storage device driver supports. Currently supported flags are DISKFLAG_NEEDSGIANT (maintained by device driver), DISKFLAG_OPEN (maintained by storage framework), DISKFLAG_CANDELETE (maintained by device driver), and DISKFLAG_CANFLUSHCACHE (maintained by device driver).

const char * d_name

Holds the name of the storage device class, e.g., ‘‘ahd’’. This value typically uniquely identifies a particular driver device, and must not conflict with devices serviced by other device drivers.

u_int d_unit

Holds the instance of the storage device class, e.g., ‘‘4’’. This namespace is managed by the device driver, and assignment of unit numbers might be a property of probe order, or in some cases topology. Together, the d_name and d_unit values will uniquely identify a disk storage device.

Disk Device Methods
The following fields identify various disk device methods, if implemented:

disk_open_t * d_open

Optional: invoked when the disk device is opened. If no method is provided, open will always succeed.

disk_close_t * d_close

Optional: invoked when the disk device is closed. Although an error code may be returned, the call should always terminate any state setup by the corresponding open method call.

disk_strategy_t * d_strategy

Mandatory: invoked when a new struct bio is to be initiated on the disk device.

disk_ioctl_t * d_ioctl

Optional: invoked when an I/O control operation is initiated on the disk device. Please note that for security reasons these operations should not be able to affect other devices than the one on which they are performed.

dumper_t * d_dump

Optional: if configured with dumpon(8), this function is invoked from a very restricted system state after a kernel panic to record a copy of the system RAM to the disk.

Mandatory Media Properties
The following fields identify the size and granularity of the disk device. These fields must stay stable from return of the drivers open method until the close method is called, but it is perfectly legal to modify them in the open method before returning.

u_int d_sectorsize

The sector size of the disk device in bytes.

off_t d_mediasize

The size of the disk device in bytes.

u_int d_maxsize

The maximum supported size in bytes of an I/O request. Requests larger than this size will be chopped up by GEOM.

Optional Media Properties
These optional fields can provide extra information about the disk device. Do not initialize these fields if the field/concept does not apply. These fields must stay stable from return of the drivers open method until the close method is called, but it is perfectly legal to modify them in the open method before returning.

u_int d_fwsectors, u_int d_fwheads

The number of sectors and heads advertised on the disk device by the firmware or BIOS. These values are almost universally bogus, but on some architectures necessary for the correct calculation of disk partitioning.

u_int d_stripeoffset, u_int d_stripesize

These two fields can be used to describe the width and location of natural performance boundaries for most disk technologies. Please see src/sys/geom/notes for details.

char d_ident[DISK_IDENT_SIZE]

This field can and should be used to store disk’s serial number.

Driver Private Data
This field may be used by the device driver to store a pointer to private data to implement the disk service.

void * d_drv1

Private data pointer. Typically used to store a pointer to the drivers softc structure for this disk device.


GEOM(4), devfs(5)


This manual page was written by Robert Watson.

MidnightBSD 0.3 February 18, 2004 MidnightBSD 0.3