btrfs: fix fast csum implementation detection

The BTRFS_FS_CSUM_IMPL_FAST flag is currently set whenever a non-generic
crc32c is detected, which is the incorrect check if the file system uses
a different checksumming algorithm.  Refactor the code to only check
this if crc32c is actually used.  Note that in an ideal world the
information if an algorithm is hardware accelerated or not should be
provided by the crypto API instead, but that's left for another day.

CC: stable@vger.kernel.org # 5.4.x: c8a5f8ca: btrfs: print checksum type and implementation at mount time
CC: stable@vger.kernel.org # 5.4.x
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2250,6 +2250,20 @@ static int btrfs_init_csum_hash(struct btrfs_fs_info *fs_info, u16 csum_type)
 
 	fs_info->csum_shash = csum_shash;
 
+	/*
+	 * Check if the checksum implementation is a fast accelerated one.
+	 * As-is this is a bit of a hack and should be replaced once the csum
+	 * implementations provide that information themselves.
+	 */
+	switch (csum_type) {
+	case BTRFS_CSUM_TYPE_CRC32:
+		if (!strstr(crypto_shash_driver_name(csum_shash), "generic"))
+			set_bit(BTRFS_FS_CSUM_IMPL_FAST, &fs_info->flags);
+		break;
+	default:
+		break;
+	}
+
 	btrfs_info(fs_info, "using %s (%s) checksum algorithm",
 			btrfs_super_csum_name(csum_type),
 			crypto_shash_driver_name(csum_shash));

fs/btrfs/super.c

@@ -1516,8 +1516,6 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 		shrinker_debugfs_rename(&s->s_shrink, "sb-%s:%s", fs_type->name,
 					s->s_id);
 		btrfs_sb(s)->bdev_holder = fs_type;
-		if (!strstr(crc32c_impl(), "generic"))
-			set_bit(BTRFS_FS_CSUM_IMPL_FAST, &fs_info->flags);
 		error = btrfs_fill_super(s, fs_devices, data);
 	}
 	if (!error)