Re: Interrupt handling - Linux

[anish kumar]

On Wed, 2012-11-28 at 20:10 +0530, manty kuma wrote:
> In linux interrupt programming, we do request_irq(...) in this
> function, the first argument is irq number. If i am not wrong, this is
> the interrupt line that we are requesting from kernel. For one
Right.
>  particular hardware, is this IRQ line fixed or can it register on any
> line based on the availability? The concept is not clear. Kindly
In linux there are two concepts related to interrupt which is never
clearly mentioned anywhere(at least not that I know of) and that is why
let me clarify.
1. Hardware interrupt number.Given by your irq controller or the
hardware which is capable of generating the interrupts.
2. Software interrupt number assigned by linux interrupt handling core.

So the first question which arises in mind is: why does linux generates
the software interrupt number?Won't hardware interrupt number be enough
to keep everyone happy?

Simple reason is just for book keeping as the software interrupt numbers
generated would be linear as we are in control of what numbers to
allocate and if we start using the numbers generated by irq controller
which can generate random numbers then searching and indexing would
require expensive operations as compared to working in linear
domain(experts can add more here if I am not to the point).

However if your irq controller is capable of choosing the interrupt
numbers then linux irq number will be same as hardware interrupt number.

So let's come back to the question.So for a particular hardware the
hardware interrupt line would be always fixed as well as the software
interrupt numbers generated by the linux irq core.
 
>  explain. Also, when i do interrupt programming for AVR or ARM, all
> the peripherals are having fixed IRQ numbers. and they are having
>  handlers. There is no concept of interrupt lines as such. So my
> second question is how are IRQ lines and IRQ numbers related?
IRQ lines are connected to irq controller and you should have a look at
the driver of your irq controller as to how does it assign the hardware
irq numbers(probably by reading some registers).All the peripherals are
connected to the irq controller such as keyboard and mouse and they have
fixed irq lines.Once a signal is asserted the irq controller raises an
interrupt to arm core and arm core in turn raises a hardware
interrupt.This hardware interrupt will call into linux irq handling
code.Which interrupt handler to be called is already decided by the
individual drivers, remember they have called request_irq with an
interrupt number.
This interrupt number would be a software interrupt number as explained
before and this number to hardware interrupt number association is done
by the interrupt controller or the chip driver which is capable of
taking(handling) one interrupt and calling individual interrupt handlers
after reading the corresponding registers(read handle_nested_irq).
This conversion of hardware interrupt number to software interrupt
number is done in /kernel/irq/irqdomain.c file.
 
PS:I may be wrong but this description is from what I have read in the
code.Please do point out any mistakes.
> 
> 
> Thanks,
> Sandeep
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