[PARISC] Add PREEMPT support

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

arch/parisc/Kconfig

@@ -177,14 +177,10 @@ config ARCH_DISCONTIGMEM_DEFAULT
 	def_bool y
 	depends on ARCH_DISCONTIGMEM_ENABLE
 
+source "kernel/Kconfig.preempt"
 source "kernel/Kconfig.hz"
 source "mm/Kconfig"
 
-config PREEMPT
-	bool
-#	bool "Preemptible Kernel"
-	default n
-
 config COMPAT
 	def_bool y
 	depends on 64BIT

arch/parisc/kernel/entry.S

@@ -1014,14 +1014,21 @@ intr_restore:
 	nop
 	nop
 
+#ifndef CONFIG_PREEMPT
+# define intr_do_preempt	intr_restore
+#endif /* !CONFIG_PREEMPT */
+
 	.import schedule,code
 intr_do_resched:
-	/* Only do reschedule if we are returning to user space */
+	/* Only call schedule on return to userspace. If we're returning
+	 * to kernel space, we may schedule if CONFIG_PREEMPT, otherwise
+	 * we jump back to intr_restore.
+	 */
 	LDREG	PT_IASQ0(%r16), %r20
-	CMPIB= 0,%r20,intr_restore /* backward */
+	CMPIB=	0, %r20, intr_do_preempt
 	nop
 	LDREG	PT_IASQ1(%r16), %r20
-	CMPIB= 0,%r20,intr_restore /* backward */
+	CMPIB=	0, %r20, intr_do_preempt
 	nop
 
 #ifdef CONFIG_64BIT
@@ -1037,6 +1044,32 @@ intr_do_resched:
 #endif
 	ldo	R%intr_check_sig(%r2), %r2
 
+	/* preempt the current task on returning to kernel
+	 * mode from an interrupt, iff need_resched is set,
+	 * and preempt_count is 0. otherwise, we continue on
+	 * our merry way back to the current running task.
+	 */
+#ifdef CONFIG_PREEMPT
+	.import preempt_schedule_irq,code
+intr_do_preempt:
+	rsm	PSW_SM_I, %r0		/* disable interrupts */
+
+	/* current_thread_info()->preempt_count */
+	mfctl	%cr30, %r1
+	LDREG	TI_PRE_COUNT(%r1), %r19
+	CMPIB<>	0, %r19, intr_restore	/* if preempt_count > 0 */
+	nop				/* prev insn branched backwards */
+
+	/* check if we interrupted a critical path */
+	LDREG	PT_PSW(%r16), %r20
+	bb,<,n	%r20, 31 - PSW_SM_I, intr_restore
+	nop
+
+	BL	preempt_schedule_irq, %r2
+	nop
+
+	b	intr_restore		/* ssm PSW_SM_I done by intr_restore */
+#endif /* CONFIG_PREEMPT */
 
 	.import do_signal,code
 intr_do_signal:

include/asm-parisc/thread_info.h

@@ -49,7 +49,8 @@ struct thread_info {
 
 #endif /* !__ASSEMBLY */
 
-#define PREEMPT_ACTIVE          0x10000000
+#define PREEMPT_ACTIVE_BIT	28
+#define PREEMPT_ACTIVE		(1 << PREEMPT_ACTIVE_BIT)
 
 /*
  * thread information flags