Re: [PATCH v8 0/4] arm: KGDB NMI/FIQ support

[Harro Haan]

On 10 July 2014 10:03, Daniel Thompson <daniel.thompson@xxxxxxxxxx> wrote:
>
> This patchset makes it possible to use kgdb's NMI infrastructure on ARM
> platforms.
>
> The patches have been previously circulated as part of a large patchset
> mixing together ARM architecture code and driver changes
> (http://thread.gmane.org/gmane.linux.ports.arm.kernel/333901 ). This
> patchset is dramatically cut down to include only the arch/arm code. The
> driver code (irqchip and tty/serial) will follow when/if the arch code
> is accepted.
>
> The first two patches modify the FIQ infrastructure to allow interrupt
> controller drivers to register callbacks (the fiq_chip structure) to
> manage FIQ routings and to ACK and EOI the FIQ. This makes it possible
> to use FIQ in multi-platform kernels and with recent ARM interrupt
> controllers.
>

Daniel,

Thanks for the patches. I have tested the fiq and irq-gic patches on
an i.MX6 (SabreSD board) for a different purpose:
A FIQ timer interrupt at 1 kHz. The TWD watchdog block is used in
timer mode for this (interrupt ID 30). The GIC affinity is set to CPU
core 0 only for this interrupt ID.

I was surprised by the following behavior:
$ cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
 29:       5459       3381       3175       3029       GIC  29  twd
 30:         11          0          0          0       GIC  30  fake-fiq

Once every few seconds is interrupt ID 30 handled by the regular GIC
handler instead of the FIQ exception path (which causes for example a
bit more latencies). The other thousands of FIQ's are handled by the
FIQ exception path. The problem is also confirmed by the following
stackframe of the Lauterbach tooling:
fake_fiq_handler(irq = 30, ...)
handle_percpu_devid_irq(irq = 30, ...)
generic_handle_irq(irq = 30)
handle_IRQ(irq = 30, ...)
gic_handle_irq
__irq_svc
-->exception

Notes:
- The problem will occur more often by executing the following command:
  $ while true; do hackbench 20; done &
- Reading the GIC_CPU_INTACK register returns the interrupt ID of the
highest priority pending interrupt.
- The GIC_CPU_INTACK register (used by fiq_ack) will return spurious
interrupt ID 0x3FF when read in fake_fiq_handler, because
GIC_CPU_INTACK is already read by gic_handle_irq.
- The FIQ exception will not occur anymore after gic_handle_irq
read/acknowledges the FIQ by reading the GIC_CPU_INTACK register

>From the behavior above I conclude that the GIC_CPU_INTACK register is
first updated before the FIQ exception is generated, so there is a
small period of time that gic_handle_irq can read/acknowledge the FIQ.

I can reduce the number of occurrences (not prevent it) by adding the
following hack to irq-gic.c
@@ -297,10 +309,12 @@ static asmlinkage void __exception_irq_entry
gic_handle_irq(struct pt_regs *regs
  u32 irqstat, irqnr;
  struct gic_chip_data *gic = &gic_data[0];
  void __iomem *cpu_base = gic_data_cpu_base(gic);

  do {
+ while(readl_relaxed(gic_data_dist_base(gic) + GIC_DIST_PENDING_SET)
& (1 << 30))
+   printk(KERN_ERR "TEMP: gic_handle_irq: wait for FIQ exception\n");
  irqstat = readl_relaxed(cpu_base + GIC_CPU_INTACK);
  irqnr = irqstat & ~0x1c00;

Regards,

Harro Haan
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