mtd: introduce the mtd_pairing_scheme concept

MLC and TLC NAND devices are using NAND cells exposing more than one bit,
but instead of attaching all the bits in a given cell to a single NAND
page, each bit is usually attached to a different page. This concept is
called 'page pairing', and has significant impacts on the flash storage
usage.
The main problem showed by these devices is that interrupting a page
program operation may not only corrupt the page we are programming
but also the page it is paired with, hence the need to expose to MTD
users the pairing scheme information.

The pairing APIs allows one to query pairing information attached to a
given page (here called wunit), or the other way around (the wunit
pointed by pairing information).
It also provides several helpers to help the conversion between absolute
offsets and wunits, and query the number of pairing groups.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>

2 files changed, 105 insertions, 0 deletions

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index e3936b847c6b..4f270482cfd0 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -376,6 +376,110 @@ static int mtd_reboot_notifier(struct notifier_block *n, unsigned long state,
 }
 
 /**
+ * mtd_wunit_to_pairing_info - get pairing information of a wunit
+ * @mtd: pointer to new MTD device info structure
+ * @wunit: write unit we are interested in
+ * @info: returned pairing information
+ *
+ * Retrieve pairing information associated to the wunit.
+ * This is mainly useful when dealing with MLC/TLC NANDs where pages can be
+ * paired together, and where programming a page may influence the page it is
+ * paired with.
+ * The notion of page is replaced by the term wunit (write-unit) to stay
+ * consistent with the ->writesize field.
+ *
+ * The @wunit argument can be extracted from an absolute offset using
+ * mtd_offset_to_wunit(). @info is filled with the pairing information attached
+ * to @wunit.
+ *
+ * From the pairing info the MTD user can find all the wunits paired with
+ * @wunit using the following loop:
+ *
+ * for (i = 0; i < mtd_pairing_groups(mtd); i++) {
+ *	info.pair = i;
+ *	mtd_pairing_info_to_wunit(mtd, &info);
+ *	...
+ * }
+ */
+int mtd_wunit_to_pairing_info(struct mtd_info *mtd, int wunit,
+			      struct mtd_pairing_info *info)
+{
+	int npairs = mtd_wunit_per_eb(mtd) / mtd_pairing_groups(mtd);
+
+	if (wunit < 0 || wunit >= npairs)
+		return -EINVAL;
+
+	if (mtd->pairing && mtd->pairing->get_info)
+		return mtd->pairing->get_info(mtd, wunit, info);
+
+	info->group = 0;
+	info->pair = wunit;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mtd_wunit_to_pairing_info);
+
+/**
+ * mtd_wunit_to_pairing_info - get wunit from pairing information
+ * @mtd: pointer to new MTD device info structure
+ * @info: pairing information struct
+ *
+ * Returns a positive number representing the wunit associated to the info
+ * struct, or a negative error code.
+ *
+ * This is the reverse of mtd_wunit_to_pairing_info(), and can help one to
+ * iterate over all wunits of a given pair (see mtd_wunit_to_pairing_info()
+ * doc).
+ *
+ * It can also be used to only program the first page of each pair (i.e.
+ * page attached to group 0), which allows one to use an MLC NAND in
+ * software-emulated SLC mode:
+ *
+ * info.group = 0;
+ * npairs = mtd_wunit_per_eb(mtd) / mtd_pairing_groups(mtd);
+ * for (info.pair = 0; info.pair < npairs; info.pair++) {
+ *	wunit = mtd_pairing_info_to_wunit(mtd, &info);
+ *	mtd_write(mtd, mtd_wunit_to_offset(mtd, blkoffs, wunit),
+ *		  mtd->writesize, &retlen, buf + (i * mtd->writesize));
+ * }
+ */
+int mtd_pairing_info_to_wunit(struct mtd_info *mtd,
+			      const struct mtd_pairing_info *info)
+{
+	int ngroups = mtd_pairing_groups(mtd);
+	int npairs = mtd_wunit_per_eb(mtd) / ngroups;
+
+	if (!info || info->pair < 0 || info->pair >= npairs ||
+	    info->group < 0 || info->group >= ngroups)
+		return -EINVAL;
+
+	if (mtd->pairing && mtd->pairing->get_wunit)
+		return mtd->pairing->get_wunit(mtd, info);
+
+	return info->pair;
+}
+EXPORT_SYMBOL_GPL(mtd_pairing_info_to_wunit);
+
+/**
+ * mtd_pairing_groups - get the number of pairing groups
+ * @mtd: pointer to new MTD device info structure
+ *
+ * Returns the number of pairing groups.
+ *
+ * This number is usually equal to the number of bits exposed by a single
+ * cell, and can be used in conjunction with mtd_pairing_info_to_wunit()
+ * to iterate over all pages of a given pair.
+ */
+int mtd_pairing_groups(struct mtd_info *mtd)
+{
+	if (!mtd->pairing || !mtd->pairing->ngroups)
+		return 1;
+
+	return mtd->pairing->ngroups;
+}
+EXPORT_SYMBOL_GPL(mtd_pairing_groups);
+
+/**
  *	add_mtd_device - register an MTD device
  *	@mtd: pointer to new MTD device info structure
  *
diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
index 1f13e32556f8..e32a0ac2298f 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -397,6 +397,7 @@ static struct mtd_part *allocate_partition(struct mtd_info *master,
 	slave->mtd.oobsize = master->oobsize;
 	slave->mtd.oobavail = master->oobavail;
 	slave->mtd.subpage_sft = master->subpage_sft;
+	slave->mtd.pairing = master->pairing;
 
 	slave->mtd.name = name;
 	slave->mtd.owner = master->owner;