[FRC PATCH v2] mmc: core: Optimize case for exactly one erase-group budget TRIM

[David Jander <david@protonic.nl>]

In the (not so unlikely) case that the mmc controller timeout budget is
enough for exactly one erase-group, the simplification of allowing one
sector has an enormous performance penalty. We optimize this special case
by introducing a flag that prohibits erase-group boundary crossing, so
that we can allow trimming more than one sector at a time.

Signed-off-by: David Jander <david@protonic.nl>
---

Changes since v1:
 - Added more comment

 drivers/mmc/core/core.c  | 38 ++++++++++++++++++++++++++++++++++----
 include/linux/mmc/card.h |  1 +
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 92e7671..2771391 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -2089,6 +2089,7 @@ int mmc_erase(struct mmc_card *card, unsigned int from, unsigned int nr,
 	      unsigned int arg)
 {
 	unsigned int rem, to = from + nr;
+	int err;
 
 	if (!(card->host->caps & MMC_CAP_ERASE) ||
 	    !(card->csd.cmdclass & CCC_ERASE))
@@ -2139,6 +2140,23 @@ int mmc_erase(struct mmc_card *card, unsigned int from, unsigned int nr,
 	/* 'from' and 'to' are inclusive */
 	to -= 1;
 
+	/*
+	 * Special case where only one erase-group fits in the timeout budget:
+	 * If the region crosses an erase-group boundary on this particular
+	 * case, we will be trimming more than one erase-group which, does not
+	 * fit in the timeout budget of the controller, so we need to split it
+	 * and call mmc_do_erase() twice if necessary. This special case is
+	 * identified by the card->eg_boundary flag.
+	 */
+	if ((arg & MMC_TRIM_ARGS) && (card->eg_boundary) &&
+	    (from % card->erase_size)) {
+		rem = card->erase_size - (from % card->erase_size);
+		err = mmc_do_erase(card, from, from + rem - 1, arg);
+		from += rem;
+		if ((err) || (to <= from))
+			return err;
+	}
+
 	return mmc_do_erase(card, from, to, arg);
 }
 EXPORT_SYMBOL(mmc_erase);
@@ -2234,16 +2252,28 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
 	if (!qty)
 		return 0;
 
+	/*
+	 * When specifying a sector range to trim, chances are we might cross
+	 * an erase-group boundary even if the amount of sectors is less than
+	 * one erase-group.
+	 * If we can only fit one erase-group in the controller timeout budget,
+	 * we have to care that erase-group boundaries are not crossed by a
+	 * single trim operation. We flag that special case with "eg_boundary".
+	 * In all other cases we can just decrement qty and pretend that we
+	 * always touch (qty + 1) erase-groups as a simple optimization.
+	 */
 	if (qty == 1)
-		return 1;
+		card->eg_boundary = 1;
+	else
+		qty--;
 
 	/* Convert qty to sectors */
 	if (card->erase_shift)
-		max_discard = --qty << card->erase_shift;
+		max_discard = qty << card->erase_shift;
 	else if (mmc_card_sd(card))
-		max_discard = qty;
+		max_discard = qty + 1;
 	else
-		max_discard = --qty * card->erase_size;
+		max_discard = qty * card->erase_size;
 
 	return max_discard;
 }
diff --git a/include/linux/mmc/card.h b/include/linux/mmc/card.h
index 19f0175..704b60d 100644
--- a/include/linux/mmc/card.h
+++ b/include/linux/mmc/card.h
@@ -282,6 +282,7 @@ struct mmc_card {
 	unsigned int		erase_size;	/* erase size in sectors */
  	unsigned int		erase_shift;	/* if erase unit is power 2 */
  	unsigned int		pref_erase;	/* in sectors */
+	unsigned int		eg_boundary;	/* don't cross erase-group boundaries */
  	u8			erased_byte;	/* value of erased bytes */
 
 	u32			raw_cid[4];	/* raw card CID */
-- 
2.1.4