mmc: core: don't set limits.discard_granularity as 0

In mmc_queue_setup_discard() the mmc driver queue's discard_granularity
might be set as 0 (when card->pref_erase > max_discard) while the mmc
device still declares to support discard operation. This is buggy and
triggered the following kernel warning message,

WARNING: CPU: 0 PID: 135 at __blkdev_issue_discard+0x200/0x294
CPU: 0 PID: 135 Comm: f2fs_discard-17 Not tainted 5.9.0-rc6 #1
Hardware name: Google Kevin (DT)
pstate: 00000005 (nzcv daif -PAN -UAO BTYPE=--)
pc : __blkdev_issue_discard+0x200/0x294
lr : __blkdev_issue_discard+0x54/0x294
sp : ffff800011dd3b10
x29: ffff800011dd3b10 x28: 0000000000000000 x27: ffff800011dd3cc4 x26: ffff800011dd3e18 x25: 000000000004e69b x24: 0000000000000c40 x23: ffff0000f1deaaf0 x22: ffff0000f2849200 x21: 00000000002734d8 x20: 0000000000000008 x19: 0000000000000000 x18: 0000000000000000 x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14: 0000000000000394 x13: 0000000000000000 x12: 0000000000000000 x11: 0000000000000000 x10: 00000000000008b0 x9 : ffff800011dd3cb0 x8 : 000000000004e69b x7 : 0000000000000000 x6 : ffff0000f1926400 x5 : ffff0000f1940800 x4 : 0000000000000000 x3 : 0000000000000c40 x2 : 0000000000000008 x1 : 00000000002734d8 x0 : 0000000000000000 Call trace:
__blkdev_issue_discard+0x200/0x294
__submit_discard_cmd+0x128/0x374
__issue_discard_cmd_orderly+0x188/0x244
__issue_discard_cmd+0x2e8/0x33c
issue_discard_thread+0xe8/0x2f0
kthread+0x11c/0x120
ret_from_fork+0x10/0x1c
---[ end trace e4c8023d33dfe77a ]---

This patch fixes the issue by setting discard_granularity as SECTOR_SIZE
instead of 0 when (card->pref_erase > max_discard) is true. Now no more
complain from __blkdev_issue_discard() for the improper value of discard
granularity.

This issue is exposed after commit b35fd742 ("block: check queue's
limits.discard_granularity in __blkdev_issue_discard()"), a "Fixes:" tag
is also added for the commit to make sure people won't miss this patch
after applying the change of __blkdev_issue_discard().

Fixes: e056a1b5 ("mmc: queue: let host controllers specify maximum discard timeout")
Fixes: b35fd742 ("block: check queue's limits.discard_granularity in __blkdev_issue_discard()").
Reported-and-tested-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20201002013852.51968-1-colyli@suse.deSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/queue.c

@@ -190,7 +190,7 @@ static void mmc_queue_setup_discard(struct request_queue *q,
 	q->limits.discard_granularity = card->pref_erase << 9;
 	/* granularity must not be greater than max. discard */
 	if (card->pref_erase > max_discard)
-		q->limits.discard_granularity = 0;
+		q->limits.discard_granularity = SECTOR_SIZE;
 	if (mmc_can_secure_erase_trim(card))
 		blk_queue_flag_set(QUEUE_FLAG_SECERASE, q);
 }