[PATCH] scsi: fc: use get/put_unaligned64 for wwn access

[Arnd Bergmann]

A bug in the gcc-6.0 prerelease version caused at least one
driver (lpfc) to have excessive stack usage when dealing with
wwn data, on the ARM architecture.

lpfc_scsi.c: In function 'lpfc_find_next_oas_lun':
lpfc_scsi.c:117:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]

I have reported this as a gcc regression in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70232

However, using a better implementation of wwn_to_u64() not only
helps with the particular gcc problem but also leads to better
object code for any version or architecture.

The kernel already provides get_unaligned_be64() and
put_unaligned_be64() helper functions that provide an
optimized implementation with the desired semantics.

The lpfc_find_next_oas_lun() function in the example that
grew from 1146 bytes to 5144 bytes when moving from gcc-5.3
to gcc-6.0 is now 804 bytes, as the optimized
get_unaligned_be64() load can be done in three instructions.
The stack usage is now down to 28 bytes from 128 bytes with
gcc-5.3 before.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 include/scsi/scsi_transport_fc.h | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/include/scsi/scsi_transport_fc.h b/include/scsi/scsi_transport_fc.h
index 784bc2c0929f..bf66ea6bed2b 100644
--- a/include/scsi/scsi_transport_fc.h
+++ b/include/scsi/scsi_transport_fc.h
@@ -28,6 +28,7 @@
 #define SCSI_TRANSPORT_FC_H
 
 #include <linux/sched.h>
+#include <asm/unaligned.h>
 #include <scsi/scsi.h>
 #include <scsi/scsi_netlink.h>
 
@@ -797,22 +798,12 @@ fc_remote_port_chkready(struct fc_rport *rport)
 
 static inline u64 wwn_to_u64(u8 *wwn)
 {
-	return (u64)wwn[0] << 56 | (u64)wwn[1] << 48 |
-	    (u64)wwn[2] << 40 | (u64)wwn[3] << 32 |
-	    (u64)wwn[4] << 24 | (u64)wwn[5] << 16 |
-	    (u64)wwn[6] <<  8 | (u64)wwn[7];
+	return get_unaligned_be64(wwn);
 }
 
 static inline void u64_to_wwn(u64 inm, u8 *wwn)
 {
-	wwn[0] = (inm >> 56) & 0xff;
-	wwn[1] = (inm >> 48) & 0xff;
-	wwn[2] = (inm >> 40) & 0xff;
-	wwn[3] = (inm >> 32) & 0xff;
-	wwn[4] = (inm >> 24) & 0xff;
-	wwn[5] = (inm >> 16) & 0xff;
-	wwn[6] = (inm >> 8) & 0xff;
-	wwn[7] = inm & 0xff;
+	put_unaligned_be64(inm, wwn);
 }
 
 /**
-- 
2.7.0