man/man9/disk.9

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.Dd April 30, 2020
.Dt DISK 9
.Os
.Sh NAME
.Nm disk
.Nd kernel disk storage API
.Sh SYNOPSIS
.In geom/geom_disk.h
.Ft struct disk *
.Fn disk_alloc void
.Ft void
.Fn disk_create "struct disk *disk" "int version"
.Ft void
.Fn disk_gone "struct disk *disk"
.Ft void
.Fn disk_destroy "struct disk *disk"
.Ft int
.Fn disk_resize "struct disk *disk" "int flags"
.Ft void
.Fn disk_add_alias "struct disk *disk" "const char *alias"
.Sh DESCRIPTION
The disk storage API permits kernel device drivers providing access to
disk-like storage devices to advertise the device to other kernel
components, including
.Xr GEOM 4
and
.Xr devfs 5 .
.Pp
Each disk device is described by a
.Vt "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.
.Pp
GEOM has the ownership of
.Vt "struct disk" ,
and drivers must allocate storage for it with the
.Fn disk_alloc
function,
fill in the fields and call
.Fn disk_create
when the device is ready to service requests.
.Fn disk_add_alias
adds an alias for the disk and must be called before
.Fn disk_create ,
but may be called multiple times.
For each alias added, a device node will be created with
.Xr make_dev_alias 9
in the same way primary device nodes are created with
.Xr make_dev 9
for
.Va d_name
and
.Va d_unit .
Care should be taken to ensure that only one driver creates aliases
for any given name.
.Fn disk_resize
can be called by the driver after modifying
.Va d_mediasize
to notify GEOM about the disk capacity change.
The
.Fa flags
field should be set to either M_WAITOK, or M_NOWAIT.
.Fn 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
.Fn disk_destroy ,
the device driver is not allowed to access the contents of
.Vt "struct disk"
anymore.
.Pp
The
.Fn disk_create
function
takes a second parameter,
.Fa version ,
which must always be passed
.Dv 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.
.Ss 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
.Fn disk_create
and may not be subsequently changed:
.Bl -tag -width indent
.It Vt u_int Va d_flags
Optional flags indicating to the storage framework what optional features
or descriptions the storage device driver supports.
Currently supported flags are
.Dv DISKFLAG_OPEN
(maintained by storage framework),
.Dv DISKFLAG_CANDELETE
(maintained by device driver),
and
.Dv DISKFLAG_CANFLUSHCACHE
(maintained by device driver).
.It Vt "const char *" Va d_name
Holds the name of the storage device class, e.g.,
.Dq Li ahd .
This value typically uniquely identifies a particular driver device,
and must not conflict with devices serviced by other device drivers.
.It Vt u_int Va d_unit
Holds the instance of the storage device class, e.g.,
.Dq Li 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
.Va d_name
and
.Va d_unit
values will uniquely identify a disk storage device.
.El
.Ss Disk Device Methods
The following fields identify various disk device methods, if implemented:
.Bl -tag -width indent
.It Vt "disk_open_t *" Va d_open
Optional: invoked when the disk device is opened.
If no method is provided, open will always succeed.
.It Vt "disk_close_t *" Va 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.
.It Vt "disk_strategy_t *" Va d_strategy
Mandatory: invoked when a new
.Vt "struct bio"
is to be initiated on the disk device.
.It Vt "disk_ioctl_t *" Va 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.
.It Vt "dumper_t *" Va d_dump
Optional: if configured with
.Xr 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.
.It Vt "disk_getattr_t *" Va 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
.Fn g_io_deliver .
.It Vt "disk_gone_t *" Va d_gone
Optional: if this method is provided, it will be called after
.Fn 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.
.El
.Ss 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.
.Bl -tag -width indent
.It Vt u_int Va d_sectorsize
The sector size of the disk device in bytes.
.It Vt off_t Va d_mediasize
The size of the disk device in bytes.
.It Vt u_int Va d_maxsize
The maximum supported size in bytes of an I/O request.
Requests larger than this size will be chopped up by GEOM.
.El
.Ss 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.
.Bl -tag -width indent
.It Vt u_int Va d_fwsectors , Vt u_int Va 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.
.It Vt u_int Va d_stripeoffset , Vt u_int Va d_stripesize
These two fields can be used to describe the width and location of
natural performance boundaries for most disk technologies.
Please see
.Pa src/sys/geom/notes
for details.
.It Vt char Va d_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.
.It Vt char Va d_descr[DISK_IDENT_SIZE]
This field can be used to store the disk vendor and product description.
.It Vt uint16_t Va d_hba_vendor
This field can be used to store the PCI vendor ID for the HBA connected to
the disk.
.It Vt uint16_t Va d_hba_device
This field can be used to store the PCI device ID for the HBA connected to
the disk.
.It Vt uint16_t Va d_hba_subvendor
This field can be used to store the PCI subvendor ID for the HBA connected to
the disk.
.It Vt uint16_t Va d_hba_subdevice
This field can be used to store the PCI subdevice ID for the HBA connected to
the disk.
.El
.Ss Driver Private Data
This field may be used by the device driver to store a pointer to
private data to implement the disk service.
.Bl -tag -width indent
.It Vt "void *" Va d_drv1
Private data pointer.
Typically used to store a pointer to the drivers
.Vt softc
structure for this disk device.
.El
.Sh SEE ALSO
.Xr GEOM 4 ,
.Xr devfs 5
.Sh HISTORY
The
.Nm kernel disk storage API
first appeared in
.Fx 4.9 .
.Sh AUTHORS
This manual page was written by
.An Robert Watson .
.Sh BUGS
Disk aliases are not a general purpose aliasing mechanism, but are
intended only to ease the transition from one name to another.
They can be used to ensure that nvd0 and nda0 are the same thing.
They cannot be used to implement the diskX concept from macOS.