parisc: qemu idle sleep support

Add qemu idle sleep support when running under qemu with SeaBIOS PDC
firmware.

Like the power architecture we use the "or" assembler instructions,
which translate to nops on real hardware, to indicate that qemu shall
idle sleep.
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: Richard Henderson <rth@twiddle.net>
CC: stable@vger.kernel.org # v4.9+

arch/parisc/kernel/process.c

@@ -39,6 +39,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/fs.h>
+#include <linux/cpu.h>
 #include <linux/module.h>
 #include <linux/personality.h>
 #include <linux/ptrace.h>
@@ -183,6 +184,44 @@ int dump_task_fpu (struct task_struct *tsk, elf_fpregset_t *r)
 	return 1;
 }
 
+/*
+ * Idle thread support
+ *
+ * Detect when running on QEMU with SeaBIOS PDC Firmware and let
+ * QEMU idle the host too.
+ */
+
+int running_on_qemu __read_mostly;
+
+void __cpuidle arch_cpu_idle_dead(void)
+{
+	/* nop on real hardware, qemu will offline CPU. */
+	asm volatile("or %%r31,%%r31,%%r31\n":::);
+}
+
+void __cpuidle arch_cpu_idle(void)
+{
+	local_irq_enable();
+
+	/* nop on real hardware, qemu will idle sleep. */
+	asm volatile("or %%r10,%%r10,%%r10\n":::);
+}
+
+static int __init parisc_idle_init(void)
+{
+	const char *marker;
+
+	/* check QEMU/SeaBIOS marker in PAGE0 */
+	marker = (char *) &PAGE0->pad0;
+	running_on_qemu = (memcmp(marker, "SeaBIOS", 8) == 0);
+
+	if (!running_on_qemu)
+		cpu_idle_poll_ctrl(1);
+
+	return 0;
+}
+arch_initcall(parisc_idle_init);
+
 /*
  * Copy architecture-specific thread state
  */