Allocating and Initializing Lists

Each sglist object consists of a header structure and a variable-length array of scatter/gather list elements. The sglist_alloc function allocates a new list that contains a header and nsegs scatter/gather list elements. The mflags argument can be set to either M_NOWAIT or M_WAITOK.

The sglist_count function returns the number of scatter/gather list elements needed to describe the physical address ranges mapped by a single kernel virtual address range. The kernel virtual address range starts at buf and is len bytes long.

The sglist_build function allocates a new scatter/gather list object that describes the physical address ranges mapped by a single kernel virtual address range. The kernel virtual address range starts at buf and is len bytes long. The mflags argument can be set to either M_NOWAIT or M_WAITOK.

The sglist_clone function returns a copy of an existing scatter/gather list object sg. The mflags argument can be set to either M_NOWAIT or M_WAITOK. This can be used to obtain a private copy of a scatter/gather list before modifying it.

The sglist_init function initializes a scatter/gather list header. The header is pointed to by sg and is initialized to manage an array of maxsegs scatter/gather list elements pointed to by segs. This can be used to initialize a scatter/gather list header whose storage is not provided by sglist_alloc. In that case, the caller should not call sglist_free to release its own reference and is responsible for ensuring all other references to the list are dropped before it releases the storage for sg and segs.

Constructing Scatter/Gather Lists

The sglist API provides several routines for building a scatter/gather list to describe one or more objects. Specifically, the sglist_append family of routines can be used to append the physical address ranges described by an object to the end of a scatter/gather list. All of these routines return 0 on success or an error on failure. If a request to append an address range to a scatter/gather list fails, the scatter/gather list will remain unchanged.

The sglist_append function appends the physical address ranges described by a single kernel virtual address range to the scatter/gather list sg. The kernel virtual address range starts at buf and is len bytes long.

The sglist_append_bio function appends the physical address ranges described by a single bio bp to the scatter/gather list sg.

The sglist_append_mbuf function appends the physical address ranges described by an entire mbuf chain m to the scatter/gather list sg.

The sglist_append_phys function appends a single physical address range to the scatter/gather list sg. The physical address range starts at paddr and is len bytes long.

The sglist_append_uio function appends the physical address ranges described by a uio(9) object to the scatter/gather list sg. Note that it is the caller's responsibility to ensure that the pages backing the I/O request are wired for the lifetime of sg. Note also that this routine does not modify uio.

The sglist_append_user function appends the physical address ranges described by a single user virtual address range to the scatter/gather list sg. The user virtual address range is relative to the address space of the thread td. It starts at buf and is len bytes long. Note that it is the caller's responsibility to ensure that the pages backing the user buffer are wired for the lifetime of sg.

The sglist_consume_uio function is a variation of sglist_append_uio. As with sglist_append_uio, it appends the physical address ranges described by uio to the scatter/gather list sg. Unlike sglist_append_uio, however, sglist_consume_uio modifies the I/O request to indicate that the appended address ranges have been processed similar to calling uiomove(9). This routine will only append ranges that describe up to resid total bytes in length. If the available segments in the scatter/gather list are exhausted before resid bytes are processed, then the uio structure will be updated to reflect the actual number of bytes processed, and sglist_consume_io will return zero to indicate success. In effect, this function will perform partial reads or writes. The caller can compare the uio_resid member of uio before and after calling sglist_consume_uio to determine the actual number of bytes processed.

Manipulating Scatter/Gather Lists

The sglist_join function appends physical address ranges from the scatter/gather list second onto first and then resets second to an empty list. It returns zero on success or an error on failure.

The sglist_split function splits an existing scatter/gather list into two lists. The first length bytes described by the list original are moved to a new list *head. If original describes a total address range that is smaller than length bytes, then all of the address ranges will be moved to the new list at *head and original will be an empty list. The caller may supply an existing scatter/gather list in *head. If so, the list must be empty. Otherwise, the caller may set *head to NULL in which case a new scatter/gather list will be allocated. In that case, mflags may be set to either M_NOWAIT or M_WAITOK. Note that since the original list is modified by this call, it must be a private list with no other references. The sglist_split function returns zero on success or an error on failure.

The sglist_slice function generates a new scatter/gather list from a sub-range of an existing scatter/gather list original. The sub-range to extract is specified by the offset and length parameters. The new scatter/gather list is stored in *slice. As with head for sglist_join, the caller may either provide an empty scatter/gather list, or it may set *slice to NULL in which case sglist_slice will allocate a new list subject to mflags. Unlike sglist_split, sglist_slice does not modify original and does not require it to be a private list. The sglist_split function returns zero on success or an error on failure.

Miscellaneous Routines

The sglist_reset function clears the scatter/gather list sg so that it no longer maps any address ranges. This can allow reuse of a single scatter/gather list object for multiple requests.

The sglist_length function returns the total length of the physical address ranges described by the scatter/gather list sg.