buf Structure

The following buf structure members are important to block drivers:

     int          b_flags;     /* Buffer Status */
     struct buf       *av_forw;    /* Driver work list link */
     struct buf       *av_back;    /* Driver work list link */
     size_t       b_bcount;    /* # of bytes to transfer */
     union {
     caddr_t      b_addr;      /* Buffer's virtual address */
     } b_un;
     daddr_t      b_blkno;     /* Block number on device */
     diskaddr_t       b_lblkno;    /* Expanded block number on device */
     size_t       b_resid;     /* # of bytes not transferred */
                       /* after error */
     int          b_error;     /* Expanded error field */
     void         *b_private;      /* “opaque” driver private area */
     dev_t        b_edev;      /* expanded dev field */

where:

av_forw and av_back

Pointers that the driver can use to manage a list of buffers by the driver. See Asynchronous Data Transfers (Block Drivers) for a discussion of the av_forw and av_back pointers.

b_bcount

Specifies the number of bytes to be transferred by the device.

b_un.b_addr

The kernel virtual address of the data buffer. Only valid after bp_mapin(9F) call.

b_blkno

The starting 32-bit logical block number on the device for the data transfer, which is expressed in 512-byte DEV_BSIZE units. The driver should use either b_blkno or b_lblkno but not both.

b_lblkno

The starting 64-bit logical block number on the device for the data transfer, which is expressed in 512-byte DEV_BSIZE units. The driver should use either b_blkno or b_lblkno but not both.

b_resid

Set by the driver to indicate the number of bytes that were not transferred because of an error. See Example 16–7 for an example of setting b_resid. The b_resid member is overloaded. b_resid is also used by disksort(9F).

b_error

Set to an error number by the driver when a transfer error occurs. b_error is set in conjunction with the b_flags B_ERROR bit. See the Intro(9E) man page for details about error values. Drivers should use bioerror(9F) rather than setting b_error directly.

b_flags

Flags with status and transfer attributes of the buf structure. If B_READ is set, the buf structure indicates a transfer from the device to memory. Otherwise, this structure indicates a transfer from memory to the device. If the driver encounters an error during data transfer, the driver should set the B_ERROR field in the b_flags member. In addition, the driver should provide a more specific error value in b_error. Drivers should use bioerror(9F) rather than setting B_ERROR.

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Caution –

Drivers should never clear b_flags.

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b_private

For exclusive use by the driver to store driver-private data.

b_edev

Contains the device number of the device that was used in the transfer.

bp_mapin Structure

A buf structure pointer can be passed into the device driver's strategy(9E) routine. However, the data buffer referred to by b_un.b_addr is not necessarily mapped in the kernel's address space. Therefore, the driver cannot directly access the data. Most block-oriented devices have DMA capability and therefore do not need to access the data buffer directly. Instead, these devices use the DMA mapping routines to enable the device's DMA engine to do the data transfer. For details about using DMA, see Chapter 9, Direct Memory Access (DMA).

If a driver needs to access the data buffer directly, that driver must first map the buffer into the kernel's address space by using bp_mapin(9F). bp_mapout(9F) should be used when the driver no longer needs to access the data directly.

bp_mapout(9F) should only be called on buffers that have been allocated and are owned by the device driver. bp_mapout() must not be called on buffers that are passed to the driver through the strategy(9E) entry point, such as a file system. bp_mapin(9F) does not keep a reference count. bp_mapout(9F) removes any kernel mapping on which a layer over the device driver might rely.