Regarding hw irq to Linux irq mapping on ARM

[Shaju Abraham]

On Wed, Sep 22, 2010 at 3:32 AM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> On Tue, 2010-09-21 at 16:57 -0300, Grant Likely wrote:
>> 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.
>
> Note that the powerpc hw -> linux IRQ mapping (virtualized irq numbering
> scheme) is orthogonal to the device-tree. It can be used without the
> device-tree and the device-tree doesn't mandate such a scheme.
>
> However, when used together, it does provide some nice features such as
> making most cases of cascaded controllers totally trivial.
>
>> 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.
>
> I like keeping it somewhat orthogonal. See how I do that on powerpc.
> That way, you can still exploit it, map interrupts etc... even if your
> device-tree happens to be deficient or missing.
>
> The main grief one could have with my scheme is the naming of irq_host
> which has confused people in the past. It should probably be irq_domain
> to make clear what it is. It generally have a 1:1 relationship to the
> irq_chip but there are cases where that isn't the case (essentially
> where you have multiple irq_chip per domain) for various reasons so it's
> better to keep those separate.
>

How do we handle multiplexed interrupts ? For example a single interrupt line
representing multple irq sources.
We have ARM systems with  single interrupt line on a VIC  muxed to
192 different GPIO sources (each capable of generating interrupts). For such a
system the virq to hw irq mapping can get complex.





> Ben.
>
>
>
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