linux-kernel - [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max

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Date:	Mon,  1 Jun 2015 11:20:35 +0200
From:	David Jander <david@...tonic.nl>
To:	Pierre Ossman <pierre@...man.eu>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	Johan Rudholm <johan.rudholm@...s.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Jander <david@...tonic.nl>
Subject: [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max_discard()

qty is the maximum number of discard that _do_ fit in the timeout, not
the first amount that does _not_ fit anymore.
This seemingly harmless error has a very severe performance impact when
the timeout value is enough for only 1 erase group.

Signed-off-by: David Jander <david@...tonic.nl>
---
 drivers/mmc/core/core.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 92e7671..1f9573b 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -2234,16 +2234,13 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
 	if (!qty)
 		return 0;
 
-	if (qty == 1)
-		return 1;
-
 	/* 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;
 	else
-		max_discard = --qty * card->erase_size;
+		max_discard = qty * card->erase_size;
 
 	return max_discard;
 }
-- 
2.1.4

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