[ PATCH ] PowerPC cascade UIC IRQ handler fix.

[Benjamin Herrenschmidt]

On Thu, 2007-08-02 at 13:48 +1000, David Gibson wrote:
> On Mon, Jul 30, 2007 at 08:35:17PM +0400, Valentine Barshak wrote:
> > PPC44x cascade UIC irq handler fix.
> > 
> > According to PPC44x UM, if an interrupt is configured as level-sensitive,
> > and a clear is attempted on the UIC_SR, the UIC_SR field is not
> > cleared if the incoming interrupt signal is at the asserted polarity.
> > This causes us to enter a cascade handler twice, since we first ack
> > parent UIC interrupt and ack child UIC one after that.
> > The patch checks child UIC msr value and returns IRQ_HANDLED
> > if there're no pending interrupts. Otherwise we get a kernel panic
> > with a "Fatal exception in interrupt" (illegal vector).
> > The patch also fixes status flags.
> > 
> > Signed-off-by: Valentine Barshak <vbarshak at ru.mvista.com>
> 
> Hrm... This doesn't seem like the right fix to me.  Instead, I think
> the cascaded IRQ handler should ack the interrupt on the child first.
> I'm a little surprised it doesn't at the moment.

Well, we certainly do also need to make the code more solid vs.
spurrious interrupts.

The main thing is, if the cascade is a level interrupt, it should
probably use a smarter cascade handler that masks it, handle the child
interrupts, then unmasks it.

Ben.