[PATCH v2] xen-blkfront: allow discard-* nodes to be optional

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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: ed30bf317c5ce ('xen-blkfront: Handle discard requests.')
Reported-by: Arthur Borsboom <arthurborsboom@xxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: linux-block@xxxxxxxxxxxxxxx
Cc: Arthur Borsboom <arthurborsboom@xxxxxxxxx>
---
Changes since v2:
 - Allow all discard-* nodes to be optional.
---
 drivers/block/xen-blkfront.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 5265975b3fba..e1c6798889f4 100644
--- a/drivers/block/xen-blkfront.c
+++ b/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,
-               "discard-granularity", "%u", &discard_granularity,
-               "discard-alignment", "%u", &discard_alignment,
-               NULL);
-       if (!err) {
-               info->discard_granularity = discard_granularity;
-               info->discard_alignment = discard_alignment;
-       }
+       info->discard_granularity = xenbus_read_unsigned(info->xbdev->otherend,
+                                                        "discard-granularity",
+                                                        0);
+       info->discard_alignment = xenbus_read_unsigned(info->xbdev->otherend,
+                                                      "discard-alignment", 0);
        info->feature_secdiscard =
                !!xenbus_read_unsigned(info->xbdev->otherend, "discard-secure",
                                       0);
-- 
2.29.2