handling spurious interrupts correctly

Author: jtsiomb

src/intr.c

@@ -60,6 +60,8 @@ static void gate_desc(desc_t *desc, uint16_t sel, uint32_t addr, int dpl, int ty
 /* defined in intr_asm.S */
 void set_idt(uint32_t addr, uint16_t limit);
 void intr_entry_default(void);
+void irq7_entry_check_spurious(void);
+void irq15_entry_check_spurious(void);
 
 /* the IDT (interrupt descriptor table) */
 static desc_t idt[256] __attribute__((aligned(8)));
@@ -90,6 +92,12 @@ void init_intr(void)
 	 */
 #include "intrtab.h"
 
+	/* change irq7 and irq15 to special entry points which first
+	 * make sure we didn't get a spurious interrupt before proceeding
+	 */
+	set_intr_entry(IRQ_TO_INTR(7), irq7_entry_check_spurious);
+	set_intr_entry(IRQ_TO_INTR(15), irq15_entry_check_spurious);
+
 	/* initialize the programmable interrupt controller
 	 * setting up the maping of IRQs [0, 15] to interrupts [32, 47]
 	 */

src/intr_asm.S

@@ -104,7 +104,42 @@ intr_entry_fast_timer:
 	out %al, $0x20
 	pop %eax
 	iret
-	
+
+/* special case for IRQ 7 and IRQ 15, to catch spurious interrupts */
+	.set PIC1_CMD, 0x20
+	.set PIC2_CMD, 0xa0
+	.set OCW2_EOI, 0x20
+	.set OCW3_ISR, 0x0b
+
+	.extern intr_entry_irq7
+	.global irq7_entry_check_spurious
+irq7_entry_check_spurious:
+	push %eax
+	mov $OCW3_ISR, %al
+	out %al, $PIC1_CMD
+	in $PIC1_CMD, %al
+	and $0x80, %al
+	pop %eax
+	jnz intr_entry_irq7
+	iret
+
+	.extern intr_entry_irq15
+	.global irq15_entry_check_spurious
+irq15_entry_check_spurious:
+	push %eax
+	mov $OCW3_ISR, %al
+	out %al, $PIC2_CMD
+	in $PIC2_CMD, %al
+	and $0x80, %al
+	jnz 0f
+	# it was spurious, send EOI to master PIC and iret
+	mov $OCW2_EOI, %al
+	out %al, $PIC1_CMD
+	pop %eax
+	iret
+0:	pop %eax
+	jmp intr_entry_irq15
+
 
 /* XXX not necessary for now, just leaving it in in case it's useful
  * down the road.