include/linux/scatterlist.h

#ifndef _LINUX_SCATTERLIST_H
#define _LINUX_SCATTERLIST_H

#include <asm/types.h>
#include <asm/scatterlist.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <asm/io.h>

struct sg_table {
	struct scatterlist *sgl;	/* the list */
	unsigned int nents;		/* number of mapped entries */
	unsigned int orig_nents;	/* original size of list */
};

/*
 * Notes on SG table design.
 *
 * Architectures must provide an unsigned long page_link field in the
 * scatterlist struct. We use that to place the page pointer AND encode
 * information about the sg table as well. The two lower bits are reserved
 * for this information.
 *
 * If bit 0 is set, then the page_link contains a pointer to the next sg
 * table list. Otherwise the next entry is at sg + 1.
 *
 * If bit 1 is set, then this sg entry is the last element in a list.
 *
 * See sg_next().
 *
 */

#define SG_MAGIC	0x87654321

/*
 * We overload the LSB of the page pointer to indicate whether it's
 * a valid sg entry, or whether it points to the start of a new scatterlist.
 * Those low bits are there for everyone! (thanks mason :-)
 */
#define sg_is_chain(sg)		((sg)->page_link & 0x01)
#define sg_is_last(sg)		((sg)->page_link & 0x02)
#define sg_chain_ptr(sg)	\
	((struct scatterlist *) ((sg)->page_link & ~0x03))

/**
 * sg_assign_page - Assign a given page to an SG entry
 * @sg:		    SG entry
 * @page:	    The page
 *
 * Description:
 *   Assign page to sg entry. Also see sg_set_page(), the most commonly used
 *   variant.
 *
 **/
static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
{
	unsigned long page_link = sg->page_link & 0x3;

	/*
	 * In order for the low bit stealing approach to work, pages
	 * must be aligned at a 32-bit boundary as a minimum.
	 */
	BUG_ON((unsigned long) page & 0x03);
#ifdef CONFIG_DEBUG_SG
	BUG_ON(sg->sg_magic != SG_MAGIC);
	BUG_ON(sg_is_chain(sg));
#endif
	sg->page_link = page_link | (unsigned long) page;
}

/**
 * sg_set_page - Set sg entry to point at given page
 * @sg:		 SG entry
 * @page:	 The page
 * @len:	 Length of data
 * @offset:	 Offset into page
 *
 * Description:
 *   Use this function to set an sg entry pointing at a page, never assign
 *   the page directly. We encode sg table information in the lower bits
 *   of the page pointer. See sg_page() for looking up the page belonging
 *   to an sg entry.
 *
 **/
static inline void sg_set_page(struct scatterlist *sg, struct page *page,
			       unsigned int len, unsigned int offset)
{
	sg_assign_page(sg, page);
	sg->offset = offset;
	sg->length = len;
}

static inline struct page *sg_page(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
	BUG_ON(sg->sg_magic != SG_MAGIC);
	BUG_ON(sg_is_chain(sg));
#endif
	return (struct page *)((sg)->page_link & ~0x3);
}

/**
 * sg_set_buf - Set sg entry to point at given data
 * @sg:		 SG entry
 * @buf:	 Data
 * @buflen:	 Data length
 *
 **/
static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
			      unsigned int buflen)
{
	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
}

/*
 * Loop over each sg element, following the pointer to a new list if necessary
 */
#define for_each_sg(sglist, sg, nr, __i)	\
	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))

/**
 * sg_chain - Chain two sglists together
 * @prv:	First scatterlist
 * @prv_nents:	Number of entries in prv
 * @sgl:	Second scatterlist
 *
 * Description:
 *   Links @prv@ and @sgl@ together, to form a longer scatterlist.
 *
 **/
static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
			    struct scatterlist *sgl)
{
#ifndef ARCH_HAS_SG_CHAIN
	BUG();
#endif

	/*
	 * offset and length are unused for chain entry.  Clear them.
	 */
	prv[prv_nents - 1].offset = 0;
	prv[prv_nents - 1].length = 0;

	/*
	 * Set lowest bit to indicate a link pointer, and make sure to clear
	 * the termination bit if it happens to be set.
	 */
	prv[prv_nents - 1].page_link = ((unsigned long) sgl | 0x01) & ~0x02;
}

/**
 * sg_mark_end - Mark the end of the scatterlist
 * @sg:		 SG entryScatterlist
 *
 * Description:
 *   Marks the passed in sg entry as the termination point for the sg
 *   table. A call to sg_next() on this entry will return NULL.
 *
 **/
static inline void sg_mark_end(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
	BUG_ON(sg->sg_magic != SG_MAGIC);
#endif
	/*
	 * Set termination bit, clear potential chain bit
	 */
	sg->page_link |= 0x02;
	sg->page_link &= ~0x01;
}

/**
 * sg_phys - Return physical address of an sg entry
 * @sg:	     SG entry
 *
 * Description:
 *   This calls page_to_phys() on the page in this sg entry, and adds the
 *   sg offset. The caller must know that it is legal to call page_to_phys()
 *   on the sg page.
 *
 **/
static inline dma_addr_t sg_phys(struct scatterlist *sg)
{
	return page_to_phys(sg_page(sg)) + sg->offset;
}

/**
 * sg_virt - Return virtual address of an sg entry
 * @sg:      SG entry
 *
 * Description:
 *   This calls page_address() on the page in this sg entry, and adds the
 *   sg offset. The caller must know that the sg page has a valid virtual
 *   mapping.
 *
 **/
static inline void *sg_virt(struct scatterlist *sg)
{
	return page_address(sg_page(sg)) + sg->offset;
}

struct scatterlist *sg_next(struct scatterlist *);
struct scatterlist *sg_last(struct scatterlist *s, unsigned int);
void sg_init_table(struct scatterlist *, unsigned int);
void sg_init_one(struct scatterlist *, const void *, unsigned int);

typedef struct scatterlist *(sg_alloc_fn)(unsigned int, gfp_t);
typedef void (sg_free_fn)(struct scatterlist *, unsigned int);

void __sg_free_table(struct sg_table *, unsigned int, sg_free_fn *);
void sg_free_table(struct sg_table *);
int __sg_alloc_table(struct sg_table *, unsigned int, unsigned int, gfp_t,
		     sg_alloc_fn *);
int sg_alloc_table(struct sg_table *, unsigned int, gfp_t);

size_t sg_copy_from_buffer(struct scatterlist *sgl, unsigned int nents,
			   void *buf, size_t buflen);
size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
			 void *buf, size_t buflen);

/*
 * Maximum number of entries that will be allocated in one piece, if
 * a list larger than this is required then chaining will be utilized.
 */
#define SG_MAX_SINGLE_ALLOC		(PAGE_SIZE / sizeof(struct scatterlist))


/*
 * Mapping sg iterator
 *
 * Iterates over sg entries mapping page-by-page.  On each successful
 * iteration, @miter->page points to the mapped page and
 * @miter->length bytes of data can be accessed at @miter->addr.  As
 * long as an interation is enclosed between start and stop, the user
 * is free to choose control structure and when to stop.
 *
 * @miter->consumed is set to @miter->length on each iteration.  It
 * can be adjusted if the user can't consume all the bytes in one go.
 * Also, a stopped iteration can be resumed by calling next on it.
 * This is useful when iteration needs to release all resources and
 * continue later (e.g. at the next interrupt).
 */

#define SG_MITER_ATOMIC		(1 << 0)	 /* use kmap_atomic */
#define SG_MITER_TO_SG		(1 << 1)	/* flush back to phys on unmap */
#define SG_MITER_FROM_SG	(1 << 2)	/* nop */

struct sg_mapping_iter {
	/* the following three fields can be accessed directly */
	struct page		*page;		/* currently mapped page */
	void			*addr;		/* pointer to the mapped area */
	size_t			length;		/* length of the mapped area */
	size_t			consumed;	/* number of consumed bytes */

	/* these are internal states, keep away */
	struct scatterlist	*__sg;		/* current entry */
	unsigned int		__nents;	/* nr of remaining entries */
	unsigned int		__offset;	/* offset within sg */
	unsigned int		__flags;
};

void sg_miter_start(struct sg_mapping_iter *miter, struct scatterlist *sgl,
		    unsigned int nents, unsigned int flags);
bool sg_miter_next(struct sg_mapping_iter *miter);
void sg_miter_stop(struct sg_mapping_iter *miter);

#endif /* _LINUX_SCATTERLIST_H */