linux-kernel - [PATCH 3.16 295/328] MIPS: memset: Fix CPU_DADDI_WORKAROUNDS `small

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Message-ID: <lsq.1544392233.100726603@decadent.org.uk>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Ralf Baechle" <ralf@...ux-mips.org>,
        "Paul Burton" <paul.burton@...s.com>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: [PATCH 3.16 295/328] MIPS: memset: Fix CPU_DADDI_WORKAROUNDS
 `small_fixup' regression

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Maciej W. Rozycki" <macro@...ux-mips.org>

commit 148b9aba99e0bbadf361747d21456e1589015f74 upstream.

Fix a commit 8a8158c85e1e ("MIPS: memset.S: EVA & fault support for
small_memset") regression and remove assembly warnings:

arch/mips/lib/memset.S: Assembler messages:
arch/mips/lib/memset.S:243: Warning: Macro instruction expanded into multiple instructions in a branch delay slot

triggering with the CPU_DADDI_WORKAROUNDS option set and this code:

	PTR_SUBU	a2, t1, a0
	jr		ra
	 PTR_ADDIU	a2, 1

This is because with that option in place the DADDIU instruction, which
the PTR_ADDIU CPP macro expands to, becomes a GAS macro, which in turn
expands to an LI/DADDU (or actually ADDIU/DADDU) sequence:

 13c:	01a4302f 	dsubu	a2,t1,a0
 140:	03e00008 	jr	ra
 144:	24010001 	li	at,1
 148:	00c1302d 	daddu	a2,a2,at
	...

Correct this by switching off the `noreorder' assembly mode and letting
GAS schedule this jump's delay slot, as there is nothing special about
it that would require manual scheduling.  With this change in place
correct code is produced:

 13c:	01a4302f 	dsubu	a2,t1,a0
 140:	24010001 	li	at,1
 144:	03e00008 	jr	ra
 148:	00c1302d 	daddu	a2,a2,at
	...

Signed-off-by: Maciej W. Rozycki <macro@...ux-mips.org>
Signed-off-by: Paul Burton <paul.burton@...s.com>
Fixes: 8a8158c85e1e ("MIPS: memset.S: EVA & fault support for small_memset")
Patchwork: https://patchwork.linux-mips.org/patch/20833/
Cc: Ralf Baechle <ralf@...ux-mips.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/mips/lib/memset.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/mips/lib/memset.S
+++ b/arch/mips/lib/memset.S
@@ -213,9 +213,11 @@
 	 nop
 
 .Lsmall_fixup\@:
+	.set		reorder
 	PTR_SUBU	a2, t1, a0
+	PTR_ADDIU	a2, 1
 	jr		ra
-	 PTR_ADDIU	a2, 1
+	.set		noreorder
 
 	.endm