scsi: target: Add callout to configure UNMAP settings

Add a callout to configure a backend's UNMAP settings. This will be used to
allow userspace to configure UNMAP after the initial device setup, similar
to how we can set up the other attributes post device configuration.

Link: https://lore.kernel.org/r/20220628200230.15052-3-michael.christie@oracle.comReviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_device.c

@@ -960,6 +960,12 @@ int target_configure_device(struct se_device *dev)
 	ret = dev->transport->configure_device(dev);
 	if (ret)
 		goto out_free_index;
+
+	if (dev->transport->configure_unmap &&
+	    dev->transport->configure_unmap(dev)) {
+		pr_debug("Discard support available, but disabled by default.\n");
+	}
+
 	/*
 	 * XXX: there is not much point to have two different values here..
 	 */

include/target/target_core_backend.h

@@ -37,6 +37,7 @@ struct target_backend_ops {
 	struct se_dev_plug *(*plug_device)(struct se_device *se_dev);
 	void (*unplug_device)(struct se_dev_plug *se_plug);
 
+	bool (*configure_unmap)(struct se_device *se_dev);
 	ssize_t (*set_configfs_dev_params)(struct se_device *,
 					   const char *, ssize_t);
 	ssize_t (*show_configfs_dev_params)(struct se_device *, char *);