Re: [PATCH 3/3] ARM: at91/tclib: mask interruptions at shutdown and probe

[Boris BREZILLON]

Hi Thierry,

On Wed, 20 Aug 2014 09:31:13 +0200
Thierry Reding <thierry.reding@xxxxxxxxx> wrote:

> On Wed, Aug 20, 2014 at 01:01:30AM +0200, Boris BREZILLON wrote:
> > Hi Jean-Christophe,
> > 
> > On Wed, 20 Aug 2014 06:11:17 +0800
> > Jean-Christophe PLAGNIOL-VILLARD <plagnioj@xxxxxxxxxxxx> wrote:
> > 
> > > Hi,
> > > 
> > > 	This is a bit weird as the clock of the TC should be off and the irq free
> > > 
> > > 	so this should never happened we need to investigate more why this append
> > 
> > I may have found the source of this bug.
> > 
> > As Gael stated, when you're kexec-ing a new kernel your previous kernel
> > could be using the tbc_clksrc driver (and especially the clkevent
> > device). Thus the kernel might have planned a timer event and then been
> > asked to shutdown the machine (requested by the kexec code).
> > In this case the AIC interrupt connected to the TC Block is disabled
> > but not the interrupts within the TCB IP (IDR registers), possibly
> > leaving a pending interrupt before booting the new kernel.
> > 
> > When the tcb_clksrc driver is loaded by the new kernel it enables the
> > interrupt line by calling setup_irq [1] while the clockevent device is
> > not registered yet [2]. Thus the event_handler is still NULL when the
> > AIC line connected to the TCB is unmasked. Remember that an interrupt
> > is still pending on this HW block, which will lead to an immediate call
> > to the ch2_irq handler, which tries to call the event_handler, which in
> > turns is NULL because clkevent device registration has not taken place
> > at this moment => Kernel panic.
> > ITOH, we can't register the clkevent device before the irq handler is
> > set up, because we should be ready to handle clkevent request at the
> > time clockevents_config_and_register is called.
> > 
> > This leaves two solution:
> >  1) disable the TCB irqs (using TCB IDR registers) before calling
> >  setup_irq in the tcb_clksrc driver
> >  2) disable the TCB irqs at the tclib level (as proposed by Gael)
> > 
> > I prefer solution #2 because it fixes the bug for all TCB users (not
> > just the tcb_clksrc driver).
> 
> Wouldn't a more proper fix be to only enable the IRQ (setup_irq()) once
> everything has properly been set up? That's certainly how all other
> drivers are doing this. Generally I think it's best to assume that an
> interrupt can fire at any point after it's been enabled, so everything
> should be set up prior to enabling it.

Sure. And, AFAIK, another common practice is to disable all interrupts
and acknowledge all pending interrupts before registering a new irq
handler to avoid inheriting peripheral dirty state from previous usage
(either the bootloader, or the previous kernel when using kexec).

This being said, I really think we should leave the HW in a clean state
when shutdown is called. And disabling interrupts at the tclib level
(in a shutdown callback) ensure that.

> 
> Also, does anyone know why this driver uses setup_irq() rather than the
> more idiomatic request_irq()?

Because nobody has sanitized this driver yet ;-).

Best Regards,

Boris



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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