ARM: let CPUs not being able to run in ARM mode enter in THUMB mode

Some ARM cores are not capable to run in ARM mode (e.g. Cortex-M3). So
obviously these cannot enter the kernel in ARM mode. Make an exception
for them and let them enter in THUMB mode.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Message-Id: 1358162123-30113-1-git-send-email-u.kleine-koenig@pengutronix.de
Acked-by: Nicolas Pitre <nico@linaro.org>

arch/arm/Kconfig

@@ -1656,8 +1656,9 @@ config HZ
 	default 100
 
 config THUMB2_KERNEL
-	bool "Compile the kernel in Thumb-2 mode"
+	bool "Compile the kernel in Thumb-2 mode" if !CPU_THUMBONLY
 	depends on CPU_V7 && !CPU_V6 && !CPU_V6K
+	default y if CPU_THUMBONLY
 	select AEABI
 	select ARM_ASM_UNIFIED
 	select ARM_UNWIND

arch/arm/kernel/head-nommu.S

@@ -32,15 +32,21 @@
  * numbers for r1.
  *
  */
-	.arm
 
 	__HEAD
+
+#ifdef CONFIG_CPU_THUMBONLY
+	.thumb
+ENTRY(stext)
+#else
+	.arm
 ENTRY(stext)
 
  THUMB(	adr	r9, BSYM(1f)	)	@ Kernel is always entered in ARM.
  THUMB(	bx	r9		)	@ If this is a Thumb-2 kernel,
  THUMB(	.thumb			)	@ switch to Thumb now.
  THUMB(1:			)
+#endif
 
 	setmode	PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
 						@ and irqs disabled

arch/arm/mm/Kconfig

@@ -397,6 +397,13 @@ config CPU_V7
 	select CPU_PABRT_V7
 	select CPU_TLB_V7 if MMU
 
+config CPU_THUMBONLY
+	bool
+	# There are no CPUs available with MMU that don't implement an ARM ISA:
+	depends on !MMU
+	help
+	  Select this if your CPU doesn't support the 32 bit ARM instructions.
+
 # Figure out what processor architecture version we should be using.
 # This defines the compiler instruction set which depends on the machine type.
 config CPU_32v3
@@ -608,7 +615,7 @@ config ARCH_DMA_ADDR_T_64BIT
 	bool
 
 config ARM_THUMB
-	bool "Support Thumb user binaries"
+	bool "Support Thumb user binaries" if !CPU_THUMBONLY
 	depends on CPU_ARM720T || CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || CPU_ARM1020 || CPU_ARM1020E || CPU_ARM1022 || CPU_ARM1026 || CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_V6 || CPU_V6K || CPU_V7 || CPU_FEROCEON
 	default y
 	help