[PATCH 2/3] s390/dasd: Restrict discard support to virtual DASD devices

[Volkan Unal <vunal@linux.ibm.com>]

Enable discard operations only for virtual FBA DASD devices with
control unit type 0x6310. This includes:
 - Virtual disks (vdisk): Discard triggers page release in z/VM,
   allowing the hypervisor to reclaim memory pages
 - EDEV devices (SCSI/NVMe): Discard is accepted and results in
   zero-byte writes to the specified data range

Physical FBA DASD devices (CU type 0x3880) do not support discard.

Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Volkan Unal <vunal@linux.ibm.com>
---
 drivers/s390/block/dasd_fba.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/block/dasd_fba.c b/drivers/s390/block/dasd_fba.c
index 45a488db897f..2869f93ac0ed 100644
--- a/drivers/s390/block/dasd_fba.c
+++ b/drivers/s390/block/dasd_fba.c
@@ -115,6 +115,11 @@ locate_record(struct ccw1 * ccw, struct LO_fba_data *data, int rw,
 	data->blk_ct = block_ct;
 }
 
+static bool cdev_is_virtual_or_edev(struct ccw_device *cdev)
+{
+	return cdev->id.cu_type == DASD_FBA_CU_TYPE_VIRTUAL;
+}
+
 static int
 dasd_fba_check_characteristics(struct dasd_device *device)
 {
@@ -167,8 +172,9 @@ dasd_fba_check_characteristics(struct dasd_device *device)
 	if (readonly)
 		set_bit(DASD_FLAG_DEVICE_RO, &device->flags);
 
-	/* FBA supports discard, set the according feature bit */
-	dasd_set_feature(cdev, DASD_FEATURE_DISCARD, 1);
+	/* Virtual and EDEV FBA supports discard, set the according feature bit */
+	if (cdev_is_virtual_or_edev(cdev))
+		dasd_set_feature(cdev, DASD_FEATURE_DISCARD, 1);
 
 	dev_info(&device->cdev->dev,
 		 "New FBA DASD %04X/%02X (CU %04X/%02X) with %d MB "
-- 
2.55.0