README on the Vectored Interrupt Controller of the LH7A404
==========================================================

The 404 revision of the LH7A40X series comes with two vectored
interrupts controllers.  While the kernel does use some of the
features of these devices, it is far from the purpose for which they
were designed.

When this README was written, the implementation of the VICs was in
flux.  It is possible that some details, especially with priorities,
will change.

The VIC support code is inspired by routines written by Sharp.


Priority Control
----------------

The significant reason for using the VIC's vectoring is to control
interrupt priorities.  There are two tables in
arch/arm/mach-lh7a40x/irq-lh7a404.c that look something like this.

  static unsigned char irq_pri_vic1[] = { IRQ_GPIO3INTR, };
  static unsigned char irq_pri_vic2[] = {
	IRQ_T3UI, IRQ_GPIO7INTR,
	IRQ_UART1INTR, IRQ_UART2INTR, IRQ_UART3INTR, };

The initialization code reads these tables and inserts a vector
address and enable for each indicated IRQ.  Vectored interrupts have
higher priority than non-vectored interrupts.  So, on VIC1,
IRQ_GPIO3INTR will be served before any other non-FIQ interrupt.  Due
to the way that the vectoring works, IRQ_T3UI is the next highest
priority followed by the other vectored interrupts on VIC2.  After
that, the non-vectored interrupts are scanned in VIC1 then in VIC2.


ISR
---

The interrupt service routine macro get_irqnr() in
arch/arm/kernel/entry-armv.S scans the VICs for the next active
interrupt.  The vectoring makes this code somewhat larger than it was
before using vectoring (refer to the LH7A400 implementation).  In the
case where an interrupt is vectored, the implementation will tend to
be faster than the non-vectored version.  However, the worst-case path
is longer.

It is worth noting that at present, there is no need to read
VIC2_VECTADDR because the register appears to be shared between the
controllers.  The code is written such that if this changes, it ought
to still work properly.


Vector Addresses
----------------

The proper use of the vectoring hardware would jump to the ISR
specified by the vectoring address.  Linux isn't structured to take
advantage of this feature, though it might be possible to change
things to support it.

In this implementation, the vectoring address is used to speed the
search for the active IRQ.  The address is coded such that the lowest
6 bits store the IRQ number for vectored interrupts.  These numbers
correspond to the bits in the interrupt status registers.  IRQ zero is
the lowest interrupt bit in VIC1.  IRQ 32 is the lowest interrupt bit
in VIC2.  Because zero is a valid IRQ number and because we cannot
detect whether or not there is a valid vectoring address if that
address is zero, the eigth bit (0x100) is set for vectored interrupts.
The address for IRQ 0x18 (VIC2) is 0x118.  Only the ninth bit is set
for the default handler on VIC1 and only the tenth bit is set for the
default handler on VIC2.

In other words.

  0x000		- no active interrupt
  0x1ii		- vectored interrupt 0xii
  0x2xx		- unvectored interrupt on VIC1 (xx is don't care)
  0x4xx		- unvectored interrupt on VIC2 (xx is don't care)