Re: spinlock recursion when running q800 emulation in qemu

[Finn Thain <fthain@xxxxxxxxxxxxxx>]

On Wed, 6 Mar 2024, Brad Boyer wrote:

> It looks like the Q800 supports switching VIA1 to IRQ6 through the
> alternate IRQ mapping. 

Yes, and that's what happens in QEMU. That was easily confirmed with the 
patch below. As a result, you get less clocksource drift during I/O but 
you also get a lot of interrupt nesting.


# cat /proc/interrupts
           CPU0       
  3:          2      auto      sonic
  4:         45      auto      ttyS0
  7:          0      auto      NMI
 10:       2089       mac      ADB
 14:        988       mac      timer
 19:        792       mac      ESP
 56:          0       mac      sonic
ERR:          0


diff --git a/arch/m68k/mac/via.c b/arch/m68k/mac/via.c
index 01e6b0e37f8d..aa3bdbaf2ff4 100644
--- a/arch/m68k/mac/via.c
+++ b/arch/m68k/mac/via.c
@@ -386,6 +386,9 @@ void via1_irq(struct irq_desc *desc)
 	int irq_num;
 	unsigned char irq_bit, events;
 
+	if (irq_desc_get_irq(desc) == 6)
+		printk_once(KERN_INFO "%s: using remapped irq\n", __func__);
+
 	events = via1[vIFR] & via1[vIER] & 0x7F;
 	if (!events)
 		return;