DISK(9)

NAME
disk, disk_init, disk_attach, disk_detach, disk_destroy, disk_busy, disk_unbusy, disk_find, disk_blocksize - generic disk framework

SYNOPSIS
#include <sys/types.h>
#include <sys/disklabel.h>
#include <sys/disk.h>
void
disk_init(struct disk *, const char *name, const struct dkdriver *driver);
void
disk_attach(struct disk *);
void
disk_detach(struct disk *);
void
disk_destroy(struct disk *);
void
disk_busy(struct disk *);
void
disk_unbusy(struct disk *, long bcount, int read);
struct disk *
disk_find(const char *);
void
disk_blocksize(struct disk *, int blocksize);

DESCRIPTION
The NetBSD 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 disk structure, which is defined as follows:

    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;
    };

The system maintains a global linked-list of all disks attached to the system. This list, called 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 ccd and vnd pseudo-device drivers.

The following is a brief description of each function in the framework:

disk_init()
    Initialize the disk structure.

disk_attach()
    Attach a disk; allocate storage for the disklabel, set the “attached time” timestamp, insert the disk into the disklist, and increment the system disk count.

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.

disk_destroy()
    Release resources used by the disk structure when it is no longer required.

disk_busy()
    Increment the disk's “busy counter”. If this counter goes from 0 to 1, set the timestamp corresponding to this transfer.

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 read specifies the direction of I/O; if non-zero it means reading from the disk, otherwise it means writing to the disk.

disk_find()
    Return a pointer to the disk structure corresponding to the name provided, or NULL if the disk does not exist.

disk_blocksize()
    Initialize dk_blkshift and dk_byteshift members of 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 DEV_BSIZE.

The functions typically called by device drivers are disk_init() disk_attach(), disk_detach(), disk_destroy,() disk_busy(), disk_unbusy(), and disk_blocksize(). The function disk_find() is provided as a utility function.

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.

Each device in the system uses a “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.:

    struct foo_softc {
    	device_t	sc_dev;		/* generic device information */
    	struct	disk	sc_dk;		/* generic disk information */
    	[ . . . more . . . ]
    };

In order for the system to gather metrics data about a disk, the disk must be registered with the system. The 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 disk_attach() is called, a portions of the disk structure must be initialized with data specific to that disk. For example, in the “foo” disk driver, the following would be performed in the autoconfiguration “attach” routine:

    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(sc-sc_dk, device_xname(self), foodkdriver);
    	disk_attach(sc-sc_dk);
    	/* Read geometry and fill in pertinent parts of disklabel. */
    	[ . . . ]
    	disk_blocksize(sc-sc_dk, bytes_per_sector);
    }

The foodkdriver above is the disk's “driver” switch. This switch currently includes a pointer to the disk's “strategy” routine. This switch needs to have global scope and should be initialized as follows:

    void foostrategy(struct buf *);
    const struct dkdriver foodkdriver = {
    	.d_strategy = foostrategy,
    };

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 disk_busy() and disk_unbusy() routines. The disk_busy() routine should be called immediately before a command to the disk is sent, e.g.:

    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(sc-sc_dk);
    	/* Send command to the drive. */
    	[ . . . ]
    }

When 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 disk_busy() must be called at splbio(). At the end of a transaction, the disk_unbusy() routine should be called. This routine performs some consistency checks, such as ensuring that the calls to disk_busy() and disk_unbusy() are balanced. This routine also performs the actual metrics calculation. A timestamp is taken, and the difference from the timestamp taken in 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 read specifies the direction of I/O; if non-zero it means reading from the disk, otherwise it means writing to the disk.

    void
    foodone(xfer)
    	struct foo_xfer *xfer;
    {
    	struct foo_softc = (struct foo_softc *)xfer-xf_softc;
    	struct buf *bp = xfer-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-b_bcount - bp-b_resid;
    	[ . . . ]
    	/* Notify the disk framework that we've completed the transfer. */
    	disk_unbusy(sc-sc_dk, nbytes,
    	    bp != NULL ? bp-b_flags  B_READ : 0);
    	[ . . . ]
    }

Like disk_busy(), disk_unbusy() must be called at splbio().

CODE REFERENCES
This section describes places within the NetBSD source tree where actual code implementing or using the disk framework can be found. All pathnames are relative to /usr/src.

The disk framework itself is implemented within the file sys/kern/subr_disk.c. Data structures and function prototypes for the framework are located in sys/sys/disk.h.

The NetBSD machine-independent SCSI disk and CD-ROM drivers use the disk framework. They are located in sys/scsi/sd.c and sys/scsi/cd.c.

The NetBSD ccd and vnd drivers use the detachment capability of the framework. They are located in sys/dev/ccd.c and sys/dev/vnd.c.

SEE ALSO
ccd(4), vnd(4), spl(9)

HISTORY
The NetBSD generic disk framework appeared in NetBSD 1.2.

AUTHORS
The NetBSD generic disk framework was architected and implemented by Jason R. Thorpe <thorpej@NetBSD.org>.