linux-kernel - [PATCH] ARM: use assembly mnemonics for VFP register access

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Message-Id: <8bb16ac4b15a7e28a8e819ef9aae20bfc3f75fbc.1582266841.git.stefan@agner.ch>
Date:   Fri, 21 Feb 2020 07:34:21 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     linux@...linux.org.uk
Cc:     arnd@...db.de, manojgupta@...gle.com, jiancai@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, Stefan Agner <stefan@...er.ch>
Subject: [PATCH] ARM: use assembly mnemonics for VFP register access

Clang's integrated assembler does not allow to to use the mcr
instruction to access floating point co-processor registers:
arch/arm/vfp/vfpmodule.c:342:2: error: invalid operand for instruction
        fmxr(FPEXC, fpexc & ~(FPEXC_EX|FPEXC_DEX|FPEXC_FP2V|FPEXC_VV|FPEXC_TRAP_MASK));
        ^
arch/arm/vfp/vfpinstr.h:79:6: note: expanded from macro 'fmxr'
        asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr   " #_vfp_ ", %0" \
            ^
<inline asm>:1:6: note: instantiated into assembly here
        mcr p10, 7, r0, cr8, cr0, 0 @ fmxr      FPEXC, r0
            ^

The GNU assembler supports the .fpu directive at least since 2.17 (when
documentation has been added). Since Linux requires binutils 2.21 it is
safe to use .fpu directive. Use the .fpu directive and mnemonics for VFP
register access.

This allows to build vfpmodule.c with Clang and its integrated assembler.

Link: https://github.com/ClangBuiltLinux/linux/issues/905
Signed-off-by: Stefan Agner <stefan@...er.ch>
---
 arch/arm/vfp/vfpinstr.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
index 38dc154e39ff..799ccf065406 100644
--- a/arch/arm/vfp/vfpinstr.h
+++ b/arch/arm/vfp/vfpinstr.h
@@ -62,21 +62,17 @@
 #define FPSCR_C (1 << 29)
 #define FPSCR_V	(1 << 28)
 
-/*
- * Since we aren't building with -mfpu=vfp, we need to code
- * these instructions using their MRC/MCR equivalents.
- */
-#define vfpreg(_vfp_) #_vfp_
-
 #define fmrx(_vfp_) ({			\
 	u32 __v;			\
-	asm("mrc p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmrx	%0, " #_vfp_	\
+	asm(".fpu	vfpv2\n"	\
+	    "vmrs	%0, " #_vfp_	\
 	    : "=r" (__v) : : "cc");	\
 	__v;				\
  })
 
 #define fmxr(_vfp_,_var_)		\
-	asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr	" #_vfp_ ", %0"	\
+	asm(".fpu	vfpv2\n"	\
+	    "vmsr	" #_vfp_ ", %0"	\
 	   : : "r" (_var_) : "cc")
 
 u32 vfp_single_cpdo(u32 inst, u32 fpscr);
-- 
2.25.1