[Buildroot] [PATCH v3 1/2] MIPS: add support for XBurst cores

[Vicente Olivert Riera]

The Ingenic XBurst is a MIPS32R2 microprocessor.

It has a bug in the FPU that can generate incorrect results in certain
cases. The problem shows up when you have several fused madd
instructions in sequence with dependant operands.

Using the -mno-fused-madd option prevents gcc from emitting these
instructions. This patch adds changes to the toolchain wrapper to use
that option.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera at imgtec.com>
---
Changes v2 -> v3:
 - Improve commit log and add help section. (Suggested by Arnout)
Changes v1 -> v2:
 - Do not add -mmxu to the wrapper.
---
 arch/Config.in.mips            | 14 ++++++++++++++
 toolchain/toolchain-wrapper.c  |  3 +++
 toolchain/toolchain-wrapper.mk |  5 +++++
 3 files changed, 22 insertions(+)

diff --git a/arch/Config.in.mips b/arch/Config.in.mips
index 3662fed..7998cea 100644
--- a/arch/Config.in.mips
+++ b/arch/Config.in.mips
@@ -63,6 +63,19 @@ config BR2_mips_p5600
 	bool "P5600"
 	depends on !BR2_ARCH_IS_64
 	select BR2_MIPS_CPU_MIPS32R5
+config BR2_mips_xburst
+	bool "XBurst"
+	depends on !BR2_ARCH_IS_64
+	select BR2_MIPS_CPU_MIPS32R2
+	help
+	  The Ingenic XBurst is a MIPS32R2 microprocessor. It has a bug
+	  in the FPU that can generate incorrect results in certain
+	  cases. The problem shows up when you have several fused madd
+	  instructions in sequence with dependant operands. This
+	  requires the -mno-fused-madd compiler option to be used in
+	  order to prevent emitting these instructions.
+
+	  See http://www.ingenic.com/en/?xburst.html
 config BR2_mips_64
 	bool "Generic MIPS64"
 	depends on BR2_ARCH_IS_64
@@ -137,6 +150,7 @@ config BR2_GCC_TARGET_ARCH
 	default "m5101"		if BR2_mips_m5101
 	default "m6201"		if BR2_mips_m6201
 	default "p5600"		if BR2_mips_p5600
+	default "mips32r2"	if BR2_mips_xburst
 	default "mips64"	if BR2_mips_64
 	default "mips64r2"	if BR2_mips_64r2
 	default "mips64r5"	if BR2_mips_64r5
diff --git a/toolchain/toolchain-wrapper.c b/toolchain/toolchain-wrapper.c
index 925d013..d59629b 100644
--- a/toolchain/toolchain-wrapper.c
+++ b/toolchain/toolchain-wrapper.c
@@ -66,6 +66,9 @@ static char *predef_args[] = {
 #ifdef BR_OMIT_LOCK_PREFIX
 	"-Wa,-momit-lock-prefix=yes",
 #endif
+#ifdef BR_NO_FUSED_MADD
+	"-mno-fused-madd",
+#endif
 #ifdef BR_BINFMT_FLAT
 	"-Wl,-elf2flt",
 #endif
diff --git a/toolchain/toolchain-wrapper.mk b/toolchain/toolchain-wrapper.mk
index e7aa5fb..c7b5019 100644
--- a/toolchain/toolchain-wrapper.mk
+++ b/toolchain/toolchain-wrapper.mk
@@ -26,6 +26,11 @@ ifeq ($(BR2_x86_x1000),y)
 TOOLCHAIN_WRAPPER_ARGS += -DBR_OMIT_LOCK_PREFIX
 endif
 
+# Avoid FPU bug on XBurst CPUs
+ifeq ($(BR2_mips_xburst),y)
+TOOLCHAIN_WRAPPER_ARGS += -DBR_NO_FUSED_MADD
+endif
+
 ifeq ($(BR2_CCACHE_USE_BASEDIR),y)
 TOOLCHAIN_WRAPPER_ARGS += -DBR_CCACHE_BASEDIR='"$(BASE_DIR)"'
 endif
-- 
2.10.1