Regarding hw irq to Linux irq mapping on ARM

[Grant Likely]

On Tue, Sep 21, 2010 at 7:25 AM, Shaju Abraham <shaju.abraham at linaro.org> wrote:
> Hi Grant
>
> Since there does not exist a mechanism to map the hw irq to linux irq
> on ARM (device tree), I would like to discuss  with you the plans or
> ideas to implement the same.

I don't have any immediate plans, but this topic has come up a lot in
the last two weeks, so I guess I need to focus on it.  :-)  [cc'ing
devicetree-discuss and linux-arm-kernel as well as Lorenzo and Eric
since this is a conversation that should be had publically]

> Can you share with me your thoughts on it?
> I have browsed through the power pc code for the same. But not sure
> the same approach is usable on ARM as well.

I haven't thought deeply about the powerpc implementation of virqs to
determine if it is suitable for other architectures or not, but the
concept behind it is sound.  We need a method of mapping controller
specific IRQ (or hw irq) numbers into the global Linux irq space
(referred to a virqs from this point on).  First it requires a
per-controller reference which can be a pointer to a per-controller
data structure, or any other unique identifier.  It could even be the
interrupt controller device tree node pointer.  Just so long as there
is a reliable method to derive the virq from the controller reference
+ hw irq number.

There also needs to be a method for each interrupt controller to
register itself and allocate a portion of the virq range.  This
shouldn't be too hard.  PowerPC handles this with the irq_map[] flat
table.  This approach is limited to whatever NR_IRQs is set to, and
could potentially be limited by that, but on the other hand the number
of discrete IRQ sources in a system is limited so a flat table
(instead of a dynamic hash table) is probably sufficient.  It is
certainly simpler to implement.

I think the first step is to simply try generalizing the code in
arch/powerpc/kernel/irq.c.  It isn't very complex and it would give a
better impression of what needs to be done.  The ARM interrupt
controller drivers would need to be modified to register with the virq
infrastructure.  None of this is either ARM or OF specific; it would
be useful for any system than need to dynamically allocate IRQ
numbers.  I could see some x86 use cases (Xilinx FPGAs) where this
would be useful.

g.