kernel: add per-thread errno support

Saves an errno per-thread, retrieved via _get_errno(), instead of
changing the value of a global variable during context switches to avoid
a hit to the context switch performance.

Per-arch asm implementations are provided for maximum performance.

Enabled by default, but can be disabled via the CONFIG_ERRNO option.

Change-Id: I81d57a2e318c94c68eee913ae0d4ca3a3609c7a4
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>

arch/arm/core/cortex_m/Makefile

@@ -6,3 +6,5 @@ asflags-y = $(ccflags-y)
 obj-y = vector_table.o reset.o \
 	prep_c.o scs.o scb.o nmi.o \
 	sw_isr_table.o
+
+obj-$(CONFIG_ERRNO) += errno.o

arch/arm/core/cortex_m/errno.S

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2015 Wind River Systems, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/** @file
+ *
+ * @brief Per-thread errno accessor function
+ *
+ * Allow accessing the errno for the current thread without involving the
+ * context switching.
+ */
+
+#define _ASMLANGUAGE
+
+#include <nano_private.h>
+#include <offsets.h>	/* nanokernel structure offset definitions */
+
+_ASM_FILE_PROLOGUE
+
+GTEXT(_nanokernel)
+GTEXT(_get_errno)
+
+SECTION_FUNC(TEXT, _get_errno)
+
+	ldr r0, =_nanokernel
+	ldr r0, [r0, #__tNANO_current_OFFSET]
+	adds.n r0, #__tTCS_errno_var_OFFSET
+	bx lr