sh: speculative execution support for SH7780.

SH7780 has a speculative execution mode where it can speculatively
perform an instruction fetch for subroutine returns, this allows it
to be enabled. There are some various pitfalls associated with this
mode, so it's left as depending on CONFIG_EXPERIMENTAL and not
enabled by default.

Signed-off-by: Paul Mundt <[e-mail]>

arch/sh/Kconfig

···
 	  Selecting this option will enable an in-kernel API for manipulating
 	  the store queues integrated in the SH-4 processors.
 
+config SPECULATIVE_EXECUTION
+	bool "Speculative subroutine return"
+	depends on CPU_SUBTYPE_SH7780 && EXPERIMENTAL
+	help
+	  This enables support for a speculative instruction fetch for
+	  subroutine return. There are various pitfalls associated with
+	  this, as outlined in the SH7780 hardware manual.
+
+	  If unsure, say N.
+
 config CPU_HAS_INTEVT
 	bool
 

arch/sh/kernel/cpu/init.c

···
 onchip_setup(fpu);
 onchip_setup(dsp);
 
+#ifdef CONFIG_SPECULATIVE_EXECUTION
+#define CPUOPM		0xff2f0000
+#define CPUOPM_RABD	(1 << 5)
+
+static void __init speculative_execution_init(void)
+{
+	/* Clear RABD */
+	ctrl_outl(ctrl_inl(CPUOPM) & ~CPUOPM_RABD, CPUOPM);
+
+	/* Flush the update */
+	(void)ctrl_inl(CPUOPM);
+	ctrl_barrier();
+}
+#else
+#define speculative_execution_init()	do { } while (0)
+#endif
+
 /*
  * Generic first-level cache init
  */
···
 	 */
 	ubc_wakeup();
 #endif
+
+	speculative_execution_init();
 }