MIPS: strnlen_user.S: Fix a CPU_DADDI_WORKAROUNDS regression

Correct a regression introduced with 8453eebd [MIPS: Fix strnlen_user()
return value in case of overlong strings.] causing assembler warnings
and broken code generated in __strnlen_kernel_nocheck_asm:

arch/mips/lib/strnlen_user.S: Assembler messages:
arch/mips/lib/strnlen_user.S:64: Warning: Macro instruction expanded into multiple instructions in a branch delay slot

with the CPU_DADDI_WORKAROUNDS option set, resulting in the function
looping indefinitely upon mounting NFS root.

Use conditional assembly to avoid a microMIPS code size regression.
Using $at unconditionally would cause such a regression as there are no
16-bit instruction encodings available for ALU operations using this
register.  Using $v1 unconditionally would produce short microMIPS
encodings, but would prevent this register from being used across calls
to this function.

The extra LI operation introduced is free, replacing a NOP originally
scheduled into the delay slot of the branch that follows.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10205/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/arch/mips/lib/strnlen_user.S b/arch/mips/lib/strnlen_user.S
index 7d12c0dded3d..77e64942f004 100644
--- a/arch/mips/lib/strnlen_user.S
+++ b/arch/mips/lib/strnlen_user.S
@@ -34,7 +34,12 @@ LEAF(__strnlen_\func\()_asm)
 FEXPORT(__strnlen_\func\()_nocheck_asm)
         move            v0, a0
         PTR_ADDU        a1, a0                  # stop pointer
-1:      beq             v0, a1, 1f              # limit reached?
+1:
+#ifdef CONFIG_CPU_DADDI_WORKAROUNDS
+        .set            noat
+        li              AT, 1
+#endif
+        beq             v0, a1, 1f              # limit reached?
 .ifeqs "\func", "kernel"
         EX(lb, t0, (v0), .Lfault\@)
 .else
@@ -42,7 +47,13 @@ FEXPORT(__strnlen_\func\()_nocheck_asm)
 .endif
         .set            noreorder
         bnez            t0, 1b
-1:       PTR_ADDIU      v0, 1
+1:
+#ifndef CONFIG_CPU_DADDI_WORKAROUNDS
+         PTR_ADDIU      v0, 1
+#else
+         PTR_ADDU       v0, AT
+        .set            at
+#endif
         .set            reorder
         PTR_SUBU        v0, a0
         jr              ra