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

[Marek Vasut]

On Tuesday, July 15, 2014 at 11:41:25 AM, Daniel Thompson wrote:
> On 14/07/14 14:51, Harro Haan wrote:
> > 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.
> 
> Makes sense.
> 
> Avoiding this problem on GICv2 is easy (thanks to the aliased intack
> register) but on iMX.6 we have only a GICv1.

Yep.

> > 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;
> 
> I've made a more complete attempt to fix this. Could you test the
> following? (and be prepared to fuzz the line numbers)

There's also another workaround, look at [1], but it's really a perverse hack 
thus far (blush). What I did there is I got hinted that an L1 page table can 
have this NS bit set. If this bit is set for a mapping, all accesses to memory 
area via that mapping will be non-secure. And then, in turn, by doing a non-
secure read of the INTACK register, it will not ever happen that the FIQ number 
will pop up in the INTACK. I only do a non-secure read of the INTACK register,
all other registers of the GICv1 are read via regular secure-mode accesses.

[1] http://git.denx.de/?p=linux-denx/linux-denx-
marex.git;a=shortlog;h=refs/heads/topic/socfpga/fiq-2014-07-10_01
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