[PATCH] mmc: mkfs takes hours on some combinations of eMMC device and host controller

[Al Cooper <alcooperx@gmail.com>]

mkfs.ext4 will erase the entire partition on the eMMC device before
writing the actual filesystem. The number of blocks erased on each
erase eMMC command is determined at run time based on the max erase
or trim time specified by the EXT_CSD in the eMMC device and the max eMMC
command timeout supported by the host controller. The routine in the
kernel that calculates the max number of blocks specified per command
returns 1 with some combinations of host controllers with a short max
command timeout and eMMC devices with long max erase or trim time.
This will end up requiring over 8 million erase sequences on a 4GB
eMMC partition and will take many hours.

For example, on a host controller with a 50MHz timeout clock
specified in the Host CAPS register and an eMMC device
with a TRIM Multiplier of 6 specified in the EXT_CSD we get
2^27/50000000=2.68 secs for a max command timeout and 6*.300=1.8 secs
for a trim operation which only allows 1 per trim command. The problem
seems to be in mmc_do_calc_max_discard() which does it's calculations
based on erase blocks but converts to and returns write blocks
(2MB blocks to 512 bytes blocks for a typical eMMC device) unless
the value is 1 in which case it just returns the 1. The routine also
subtracts 1 from the max calculation before converting from erase to
write blocks which should not be needed.

This change will convert all non-zero max calculations from erase
to write blocks and will no longer subtract 1 from the erase block
max before converting to write blocks. This allow mkfs.ext4 to run
in 30 secs instead of >10 hours.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
---
 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 23f10f7..1b61ac0 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -2231,16 +2231,13 @@ 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;
 }
-- 
1.9.0.138.g2de3478