xen-blkfront: allow discard-* nodes to be optional

This is inline with the specification described in blkif.h:

 * discard-granularity: should be set to the physical block size if
   node is not present.
 * discard-alignment, discard-secure: should be set to 0 if node not
   present.

This was detected as QEMU would only create the discard-granularity
node but not discard-alignment, and thus the setup done in
blkfront_setup_discard would fail.

Fix blkfront_setup_discard to not fail on missing nodes, and also fix
blkif_set_queue_limits to set the discard granularity to the physical
block size if none is specified in xenbus.

Fixes: ed30bf317c ('xen-blkfront: Handle discard requests.')
Reported-by: Arthur Borsboom <arthurborsboom@gmail.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-By: Arthur Borsboom <arthurborsboom@gmail.com>
Link: https://lore.kernel.org/r/20210119105727.95173-1-roger.pau@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/block/xen-blkfront.c

@@ -945,7 +945,8 @@ static void blkif_set_queue_limits(struct blkfront_info *info)
 	if (info->feature_discard) {
 		blk_queue_flag_set(QUEUE_FLAG_DISCARD, rq);
 		blk_queue_max_discard_sectors(rq, get_capacity(gd));
-		rq->limits.discard_granularity = info->discard_granularity;
+		rq->limits.discard_granularity = info->discard_granularity ?:
+						 info->physical_sector_size;
 		rq->limits.discard_alignment = info->discard_alignment;
 		if (info->feature_secdiscard)
 			blk_queue_flag_set(QUEUE_FLAG_SECERASE, rq);
@@ -2179,19 +2180,12 @@ static void blkfront_closing(struct blkfront_info *info)
 
 static void blkfront_setup_discard(struct blkfront_info *info)
 {
-	int err;
-	unsigned int discard_granularity;
-	unsigned int discard_alignment;
-
 	info->feature_discard = 1;
-	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+	info->discard_granularity = xenbus_read_unsigned(info->xbdev->otherend,
-		"discard-granularity", "%u", &discard_granularity,
+							 "discard-granularity",
-		"discard-alignment", "%u", &discard_alignment,
+							 0);
-		NULL);
+	info->discard_alignment = xenbus_read_unsigned(info->xbdev->otherend,
-	if (!err) {
+						       "discard-alignment", 0);
-		info->discard_granularity = discard_granularity;
-		info->discard_alignment = discard_alignment;
-	}
 	info->feature_secdiscard =
 		!!xenbus_read_unsigned(info->xbdev->otherend, "discard-secure",
 				       0);