IPLs - One too many?

[Matt Thomas <matt%3am-software.com@localhost>]

I've been talking to Chris Gilbert for a while about eliminating nested
interrupts in one or more of the ARM ports.

Ignoring IPL_SOFT* IPLs for the moment, ARM currently has
IPL_NONE < IPL_VM < IPL_SCHED < IPL_HIGH

Since IPL_SCHED == IPL_CLOCK, once IPL_SCHED is reached you block
clock interrupts.  Since clock interrupts are the highest priority
interrupts, you have basically blocked all interrupts.  So there
is little difference between IPL_HIGH and IPL_SCHED.

So why have both?  Can I just make IPL_HIGH == IPL_SCHED?

This means I have three IPL value, IPL_NONE, IPL_VM, IPL_HIGH.

And now I can directly map those to the ARM CPSR bits IF32_bits
(IRQ, FIQ) as 00, 10, 11 so I can make IPL_NONE=0, IPL_VM=2, and
IPL_HIGH=3 and have no reason to store a s/w copy of the IPL since
the CPU status word will contain an encoding of it.

Much/All of the IRQ/FIQ enable/disable in the kernel can then just
become spl calls.