NetBSD/share/man/man9/disk.9

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.Dd May 3, 2008
.Dt DISK 9
.Os
.Sh NAME
.Nm disk ,
.Nm disk_init ,
.Nm disk_attach ,
.Nm disk_detach ,
.Nm disk_destroy ,
.Nm disk_busy ,
.Nm disk_unbusy ,
.Nm disk_find ,
.Nm disk_blocksize
.Nd generic disk framework
.Sh SYNOPSIS
.In sys/types.h
.In sys/disklabel.h
.In sys/disk.h
.Ft void
.Fn disk_init "struct disk *" "const char *name" "const struct dkdriver *driver"
.Ft void
.Fn disk_attach "struct disk *"
.Ft void
.Fn disk_detach "struct disk *"
.Ft void
.Fn disk_destroy "struct disk *"
.Ft void
.Fn disk_busy "struct disk *"
.Ft void
.Fn disk_unbusy "struct disk *" "long bcount" "int read"
.Ft struct disk *
.Fn disk_find "const char *"
.Ft void
.Fn disk_blocksize "struct disk *" "int blocksize"
.Sh DESCRIPTION
The
.Nx
generic disk framework is designed to provide flexible,
scalable, and consistent handling of disk state and metrics information.
The fundamental component of this framework is the
.Nm disk
structure, which is defined as follows:
.Bd -literal
struct disk {
	TAILQ_ENTRY(disk) dk_link;	/* link in global disklist */
	const char	*dk_name;	/* disk name */
	prop_dictionary_t dk_info;	/* reference to disk-info dictionary */
	int		dk_bopenmask;	/* block devices open */
	int		dk_copenmask;	/* character devices open */
	int		dk_openmask;	/* composite (bopen|copen) */
	int		dk_state;	/* label state   ### */
	int		dk_blkshift;	/* shift to convert DEV_BSIZE to blks */
	int		dk_byteshift;	/* shift to convert bytes to blks */

	/*
	 * Metrics data; note that some metrics may have no meaning
	 * on certain types of disks.
	 */
	struct io_stats	*dk_stats;

	const struct dkdriver *dk_driver;	/* pointer to driver */

	/*
	 * Information required to be the parent of a disk wedge.
	 */
	kmutex_t	dk_rawlock;	/* lock on these fields */
	u_int		dk_rawopens;	/* # of openes of rawvp */
	struct vnode	*dk_rawvp;	/* vnode for the RAW_PART bdev */

	kmutex_t	dk_openlock;	/* lock on these and openmask */
	u_int		dk_nwedges;	/* # of configured wedges */
					/* all wedges on this disk */
	LIST_HEAD(, dkwedge_softc) dk_wedges;

	/*
	 * Disk label information.  Storage for the in-core disk label
	 * must be dynamically allocated, otherwise the size of this
	 * structure becomes machine-dependent.
	 */
	daddr_t		dk_labelsector;		/* sector containing label */
	struct disklabel *dk_label;	/* label */
	struct cpu_disklabel *dk_cpulabel;
};
.Ed
.Pp
The system maintains a global linked-list of all disks attached to the
system.
This list, called
.Nm disklist ,
may grow or shrink over time as disks are dynamically added and removed
from the system.
Drivers which currently make use of the detachment
capability of the framework are the
.Nm ccd
and
.Nm vnd
pseudo-device drivers.
.Pp
The following is a brief description of each function in the framework:
.Bl -tag -width ".Fn disk_blocksize"
.It Fn disk_init
Initialize the disk structure.
.It Fn disk_attach
Attach a disk; allocate storage for the disklabel, set the
.Dq attached time
timestamp, insert the disk into the disklist, and increment the
system disk count.
.It Fn disk_detach
Detach a disk; free storage for the disklabel, remove the disk
from the disklist, and decrement the system disk count.
If the count drops below zero, panic.
.It Fn disk_destroy
Release resources used by the disk structure when it is no longer
required.
.It Fn disk_busy
Increment the disk's
.Dq busy counter .
If this counter goes from 0 to 1, set the timestamp corresponding to
this transfer.
.It Fn disk_unbusy
Decrement a disk's busy counter.
If the count drops below zero, panic.
Get the current time, subtract it from the disk's timestamp, and add
the difference to the disk's running total.
Set the disk's timestamp to the current time.
If the provided byte count is greater than 0, add it to the disk's
running total and increment the number of transfers performed by the disk.
The third argument
.Ar read
specifies the direction of I/O;
if non-zero it means reading from the disk,
otherwise it means writing to the disk.
.It Fn disk_find
Return a pointer to the disk structure corresponding to the name provided,
or NULL if the disk does not exist.
.It Fn disk_blocksize
Initialize
.Fa dk_blkshift
and
.Fa dk_byteshift
members of
.Fa struct disk
with suitable values derived from the supplied physical blocksize.
It is only necessary to call this function if the device's physical blocksize
is not
.Dv DEV_BSIZE .
.El
.Pp
The functions typically called by device drivers are
.Fn disk_init
.Fn disk_attach ,
.Fn disk_detach ,
.Fn disk_destroy,
.Fn disk_busy ,
.Fn disk_unbusy ,
and
.Fn disk_blocksize .
The function
.Fn disk_find
is provided as a utility function.
.Sh USING THE FRAMEWORK
This section includes a description on basic use of the framework
and example usage of its functions.
Actual implementation of a device driver which uses the framework
may vary.
.Pp
Each device in the system uses a
.Dq softc
structure which contains autoconfiguration and state information for that
device.
In the case of disks, the softc should also contain one instance
of the disk structure, e.g.:
.Bd -literal
struct foo_softc {
	device_t	sc_dev;		/* generic device information */
	struct	disk	sc_dk;		/* generic disk information */
	[ . . . more . . . ]
};
.Ed
.Pp
In order for the system to gather metrics data about a disk, the disk must
be registered with the system.
The
.Fn disk_attach
routine performs all of the functions currently required to register a disk
with the system including allocation of disklabel storage space,
recording of the time since boot that the disk was attached, and insertion
into the disklist.
Note that since this function allocates storage space for the disklabel,
it must be called before the disklabel is read from the media or used in
any other way.
Before
.Fn disk_attach
is called, a portions of the disk structure must be initialized with
data specific to that disk.
For example, in the
.Dq foo
disk driver, the following would be performed in the autoconfiguration
.Dq attach
routine:
.Bd -literal
void
fooattach(device_t parent, device_t self, void *aux)
{
	struct foo_softc *sc = device_private(self);
	[ . . . ]

	/* Initialize and attach the disk structure. */
	disk_init(\*[Am]sc-\*[Gt]sc_dk, device_xname(self), \*[Am]foodkdriver);
	disk_attach(\*[Am]sc-\*[Gt]sc_dk);

	/* Read geometry and fill in pertinent parts of disklabel. */
	[ . . . ]
	disk_blocksize(\*[Am]sc-\*[Gt]sc_dk, bytes_per_sector);
}
.Ed
.Pp
The
.Nm foodkdriver
above is the disk's
.Dq driver
switch.
This switch currently includes a pointer to the disk's
.Dq strategy
routine.
This switch needs to have global scope and should be initialized as follows:
.Bd -literal
void foostrategy(struct buf *);

const struct dkdriver foodkdriver = {
	.d_strategy = foostrategy,
};
.Ed
.Pp
Once the disk is attached, metrics may be gathered on that disk.
In order to gather metrics data, the driver must tell the framework when
the disk starts and stops operations.
This functionality is provided by the
.Fn disk_busy
and
.Fn disk_unbusy
routines.
The
.Fn disk_busy
routine should be called immediately before a command to the disk is
sent, e.g.:
.Bd -literal
void
foostart(sc)
	struct foo_softc *sc;
{
	[ . . . ]

	/* Get buffer from drive's transfer queue. */
	[ . . . ]

	/* Build command to send to drive. */
	[ . . . ]

	/* Tell the disk framework we're going busy. */
	disk_busy(\*[Am]sc-\*[Gt]sc_dk);

	/* Send command to the drive. */
	[ . . . ]
}
.Ed
.Pp
When
.Fn disk_busy
is called, a timestamp is taken if the disk's busy counter moves from
0 to 1, indicating the disk has gone from an idle to non-idle state.
Note that
.Fn disk_busy
must be called at
.Fn splbio .
At the end of a transaction, the
.Fn disk_unbusy
routine should be called.
This routine performs some consistency checks,
such as ensuring that the calls to
.Fn disk_busy
and
.Fn disk_unbusy
are balanced.
This routine also performs the actual metrics calculation.
A timestamp is taken, and the difference from the timestamp taken in
.Fn disk_busy
is added to the disk's total running time.
The disk's timestamp is then updated in case there is more than one
pending transfer on the disk.
A byte count is also added to the disk's running total, and if greater than
zero, the number of transfers the disk has performed is incremented.
The third argument
.Ar read
specifies the direction of I/O;
if non-zero it means reading from the disk,
otherwise it means writing to the disk.
.Bd -literal
void
foodone(xfer)
	struct foo_xfer *xfer;
{
	struct foo_softc = (struct foo_softc *)xfer-\*[Gt]xf_softc;
	struct buf *bp = xfer-\*[Gt]xf_buf;
	long nbytes;
	[ . . . ]

	/*
	 * Get number of bytes transfered.  If there is no buf
	 * associated with the xfer, we are being called at the
	 * end of a non-I/O command.
	 */
	if (bp == NULL)
		nbytes = 0;
	else
		nbytes = bp-\*[Gt]b_bcount - bp-\*[Gt]b_resid;

	[ . . . ]

	/* Notify the disk framework that we've completed the transfer. */
	disk_unbusy(\*[Am]sc-\*[Gt]sc_dk, nbytes,
	    bp != NULL ? bp-\*[Gt]b_flags \*[Am] B_READ : 0);

	[ . . . ]
}
.Ed
.Pp
Like
.Fn disk_busy ,
.Fn disk_unbusy
must be called at
.Fn splbio .
.Sh CODE REFERENCES
This section describes places within the
.Nx
source tree where actual
code implementing or using the disk framework can be found.
All pathnames are relative to
.Pa /usr/src .
.Pp
The disk framework itself is implemented within the file
.Pa sys/kern/subr_disk.c .
Data structures and function prototypes for the framework are located in
.Pa sys/sys/disk.h .
.Pp
The
.Nx
machine-independent SCSI disk and CD-ROM drivers use the
disk framework.
They are located in
.Pa sys/scsi/sd.c
and
.Pa sys/scsi/cd.c .
.Pp
The
.Nx
.Nm ccd
and
.Nm vnd
drivers use the detachment capability of the framework.
They are located in
.Pa sys/dev/ccd.c
and
.Pa sys/dev/vnd.c .
.Sh SEE ALSO
.Xr ccd 4 ,
.Xr vnd 4 ,
.Xr spl 9
.Sh HISTORY
The
.Nx
generic disk framework appeared in
.Nx 1.2 .
.Sh AUTHORS
The
.Nx
generic disk framework was architected and implemented by
.An Jason R. Thorpe
.Aq thorpej@NetBSD.org .