Very sparse and high interrupt map with SPARSE_IRQ

[Michael Bohan]

Hi,

I am working with a Qualcomm SPMI device that supports up to 32768 
interrupts. Now, in practice, not nearly all of these interrupts will be 
populated on a real device. Most likely on the order of 200-300 
interrupts will be specified in Device Tree. The problem is the set of 
active interrupts will change on future devices sharing the same 
architecture, and there's really no predicting which of this range will 
be active. Ideally, the supporting code should never have to change.

To support such a device, I am considering using SPARSE_IRQ and 
allocating the irq_desc at runtime as necessary while walking the Device 
Tree. To keep the mapping function simple and fast, I was thinking of 
using discontinuous, high system interrupt numbers that can be computed 
with a simple O(1) operation. Alternatively, I could use a radix tree or 
hash to map these to more traditional, lower and contiguous interrupt 
numbers, but I'm not aware of any significant benefit in doing so.

As far as I can tell, the only potential problems with using such high 
interrupt numbers (eg. 33102) are:

1. IRQ_BITMAP_BITS must be expanded to support the entire possible range 
(eg. ~0-33500). IRQ_BITMAP_BITS is defined as NR_IRQS. This will waste 
~3 KB on such a range. To me 3 KB can be justified if it speeds up the 
fast path interrupt handling.
2. NR_IRQS will increase beyond the HARDIRQ_BITS limitation, which 
governs the number of nested interrupts. But as mentioned, we won't 
actually have more real interrupts than the maximum setting (10 bit) -- 
it's just that our NR_IRQ definition will be so high to trip an older 
ARM check for NR_IRQS going beyond HARDIRQ_BITS.

So basically, I'm asking whether this analysis is correct, and what I'm 
doing seems reasonable. I'd also like to propose a couple changes as a 
consequence of what I mentioned above:

1. Add another macro to distinguish between the actual number of 
interrupts a system supports and the *highest* number it supports. Eg. 
NR_IRQS seems to imply a quantity - not a maximum value. But it's 
currently being used to cover both constraints.
2. Remove the check in arch/arm/include/asm/hardirq.h for HARDIRQ_BITS 
being too low. Actually, if 1) were really implemented, then most likely 
NR_IRQS would be below 1024, and this check would not be violated. But 
regardless, per the comment in kernel/irq/internals.h, we're probably 
bound by interrupt stack for such systems, anyways.

Thanks,
Mike

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum