kernel / include / linux / scatterlist.h

#ifndef _LINUX_SCATTERLIST_H
#define _LINUX_SCATTERLIST_H

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

/*
 * 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

/**
 * sg_set_page - Set sg entry to point at given page
 * @sg:		 SG entry
 * @page:	 The 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 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);
#endif
	sg->page_link = page_link | (unsigned long) page;
}

#define sg_page(sg)	((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));
	sg->offset = offset_in_page(buf);
	sg->length = buflen;
}

/*
 * 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_next - return the next scatterlist entry in a list
 * @sg:		The current sg entry
 *
 * Description:
 *   Usually the next entry will be @sg@ + 1, but if this sg element is part
 *   of a chained scatterlist, it could jump to the start of a new
 *   scatterlist array.
 *
 **/
static inline struct scatterlist *sg_next(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
	BUG_ON(sg->sg_magic != SG_MAGIC);
#endif
	if (sg_is_last(sg))
		return NULL;

	sg++;
	if (unlikely(sg_is_chain(sg)))
		sg = sg_chain_ptr(sg);

	return sg;
}

/*
 * 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_last - return the last scatterlist entry in a list
 * @sgl:	First entry in the scatterlist
 * @nents:	Number of entries in the scatterlist
 *
 * Description:
 *   Should only be used casually, it (currently) scan the entire list
 *   to get the last entry.
 *
 *   Note that the @sgl@ pointer passed in need not be the first one,
 *   the important bit is that @nents@ denotes the number of entries that
 *   exist from @sgl@.
 *
 **/
static inline struct scatterlist *sg_last(struct scatterlist *sgl,
					  unsigned int nents)
{
#ifndef ARCH_HAS_SG_CHAIN
	struct scatterlist *ret = &sgl[nents - 1];
#else
	struct scatterlist *sg, *ret = NULL;
	int i;

	for_each_sg(sgl, sg, nents, i)
		ret = sg;

#endif
#ifdef CONFIG_DEBUG_SG
	BUG_ON(sgl[0].sg_magic != SG_MAGIC);
	BUG_ON(!sg_is_last(ret));
#endif
	return ret;
}

/**
 * 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
	prv[prv_nents - 1].page_link = (unsigned long) sgl | 0x01;
}

/**
 * sg_mark_end - Mark the end of the scatterlist
 * @sgl:	Scatterlist
 * @nents:	Number of entries in sgl
 *
 * Description:
 *   Marks the last entry as the termination point for sg_next()
 *
 **/
static inline void sg_mark_end(struct scatterlist *sgl, unsigned int nents)
{
	sgl[nents - 1].page_link = 0x02;
}

static inline void __sg_mark_end(struct scatterlist *sg)
{
	sg->page_link |= 0x02;
}

/**
 * sg_init_one - Initialize a single entry sg list
 * @sg:		 SG entry
 * @buf:	 Virtual address for IO
 * @buflen:	 IO length
 *
 * Notes:
 *   This should not be used on a single entry that is part of a larger
 *   table. Use sg_init_table() for that.
 *
 **/
static inline void sg_init_one(struct scatterlist *sg, const void *buf,
			       unsigned int buflen)
{
	memset(sg, 0, sizeof(*sg));
#ifdef CONFIG_DEBUG_SG
	sg->sg_magic = SG_MAGIC;
#endif
	sg_mark_end(sg, 1);
	sg_set_buf(sg, buf, buflen);
}

/**
 * sg_init_table - Initialize SG table
 * @sgl:	   The SG table
 * @nents:	   Number of entries in table
 *
 * Notes:
 *   If this is part of a chained sg table, sg_mark_end() should be
 *   used only on the last table part.
 *
 **/
static inline void sg_init_table(struct scatterlist *sgl, unsigned int nents)
{
	memset(sgl, 0, sizeof(*sgl) * nents);
	sg_mark_end(sgl, nents);
#ifdef CONFIG_DEBUG_SG
	{
		int i;
		for (i = 0; i < nents; i++)
			sgl[i].sg_magic = SG_MAGIC;
	}
#endif
}

/**
 * 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 unsigned long 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;
}

#endif /* _LINUX_SCATTERLIST_H */