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 structdisk 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 structdisk, 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.
disk_add_alias() adds an alias for the disk and must be called before disk_create(), but may be called
multiple times. For each alias added, a device node will be created with make_dev_alias(9) in the same
way primary device nodes are created with make_dev(9) for d_name and d_unit. Care should be taken to
ensure that only one driver creates aliases for any given name. disk_resize() can be called by the
driver after modifying d_mediasize to notify GEOM about the disk capacity change. The flags field should
be set to either M_WAITOK, or M_NOWAIT. disk_gone() orphans all of the providers associated with the
drive, setting an error condition of ENXIO in each one. In addition, it prevents a re-taste on last
close for writing if an error condition has been set in the provider. After calling disk_destroy(), the
device driver is not allowed to access the contents of structdisk 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.
DescriptiveFields
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_intd_flags
Optional flags indicating to the storage framework what optional features or descriptions the
storage device driver supports. Currently supported flags are DISKFLAG_OPEN (maintained by
storage framework), DISKFLAG_CANDELETE (maintained by device driver), and DISKFLAG_CANFLUSHCACHE
(maintained by device driver).
constchar*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_intd_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.
DiskDeviceMethods
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 structbio 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.
disk_getattr_t*d_getattr
Optional: if this method is provided, it gives the disk driver the opportunity to override the
default GEOM response to BIO_GETATTR requests. This function should return -1 if the attribute
is not handled, 0 if the attribute is handled, or an errno to be passed to g_io_deliver().
disk_gone_t*d_gone
Optional: if this method is provided, it will be called after disk_gone() is called, once GEOM
has finished its cleanup process. Once this callback is called, it is safe for the disk driver
to free all of its resources, as it will not be receiving further calls from GEOM.
MandatoryMediaProperties
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_intd_sectorsize
The sector size of the disk device in bytes.
off_td_mediasize
The size of the disk device in bytes.
u_intd_maxsize
The maximum supported size in bytes of an I/O request. Requests larger than this size will be
chopped up by GEOM.
OptionalMediaProperties
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_intd_fwsectors, u_intd_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_intd_stripeoffset, u_intd_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.
chard_ident[DISK_IDENT_SIZE]
This field can and should be used to store disk's serial number if the d_getattr method described
above isn't implemented, or if it does not support the GEOM::ident attribute.
chard_descr[DISK_IDENT_SIZE]
This field can be used to store the disk vendor and product description.
uint16_td_hba_vendor
This field can be used to store the PCI vendor ID for the HBA connected to the disk.
uint16_td_hba_device
This field can be used to store the PCI device ID for the HBA connected to the disk.
uint16_td_hba_subvendor
This field can be used to store the PCI subvendor ID for the HBA connected to the disk.
uint16_td_hba_subdevice
This field can be used to store the PCI subdevice ID for the HBA connected to the disk.
DriverPrivateData
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.
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