block: Add a disk flag to block integrity profile

So far we have relied on the app tag size to determine whether a disk
has been formatted with T10 protection information or not. However, not
all target devices provide application tag storage.

Add a flag to the block integrity profile that indicates whether the
disk has been formatted with protection information.

Signed-off-by: Martin K. Petersen <[e-mail]>
Reviewed-by: Sagi Grimberg <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/ABI/testing/sysfs-block

···
 		512 bytes of data.
 
 
+What:		/sys/block/<disk>/integrity/device_is_integrity_capable
+Date:		July 2014
+Contact:	Martin K. Petersen <[e-mail]>
+Description:
+		Indicates whether a storage device is capable of storing
+		integrity metadata. Set if the device is T10 PI-capable.
+
+
 What:		/sys/block/<disk>/integrity/write_generate
 Date:		June 2008
 Contact:	Martin K. Petersen <[e-mail]>

block/blk-integrity.c

···
 	return sprintf(page, "%d\n", (bi->flags & BLK_INTEGRITY_GENERATE) != 0);
 }
 
+static ssize_t integrity_device_show(struct blk_integrity *bi, char *page)
+{
+	return sprintf(page, "%u\n",
+		       (bi->flags & BLK_INTEGRITY_DEVICE_CAPABLE) != 0);
+}
+
 static struct integrity_sysfs_entry integrity_format_entry = {
 	.attr = { .name = "format", .mode = S_IRUGO },
 	.show = integrity_format_show,
···
 	.store = integrity_generate_store,
 };
 
+static struct integrity_sysfs_entry integrity_device_entry = {
+	.attr = { .name = "device_is_integrity_capable", .mode = S_IRUGO },
+	.show = integrity_device_show,
+};
+
 static struct attribute *integrity_attrs[] = {
 	&integrity_format_entry.attr,
 	&integrity_tag_size_entry.attr,
 	&integrity_verify_entry.attr,
 	&integrity_generate_entry.attr,
+	&integrity_device_entry.attr,
 	NULL,
 };
 

drivers/scsi/sd_dif.c

···
 		  "Enabling DIX %s protection\n", disk->integrity->name);
 
 	/* Signal to block layer that we support sector tagging */
-	if (dif && type && sdkp->ATO) {
+	if (dif && type) {
+
+		disk->integrity->flags |= BLK_INTEGRITY_DEVICE_CAPABLE;
+
+		if (!sdkp)
+			return;
+
 		if (type == SD_DIF_TYPE3_PROTECTION)
 			disk->integrity->tag_size = sizeof(u16) + sizeof(u32);
 		else

include/linux/blkdev.h

···
 enum blk_integrity_flags {
 	BLK_INTEGRITY_VERIFY		= 1 << 0,
 	BLK_INTEGRITY_GENERATE		= 1 << 1,
+	BLK_INTEGRITY_DEVICE_CAPABLE	= 1 << 2,
 };
 
 struct blk_integrity_iter {