interrupt handler in arm question

[Konstantin Zertsekel]

> 2) In case of FIQ mode to which routine it jumps to,basically in IRQ
> mode it jumps to    either __irq_svc or __irq_usr as you explained
> earlier.
>    I have looked at the source code, but I couldn't figure out in
> case of FIQ, where its    branching to.

That's for a good reason :-)
FIQ exception is not implemented in Linux Kernel.
Actually, when ARM CPU takes FIQ exception, it branches to 0xffff001c
address, which contains the below code from entry-armv.S:
vector_fiq:
        disable_fiq
        subs    pc, lr, #4

Now, disable_fiq not implemented for all the platforms.
But, anyway, 'subs pc, lr, #4' returns the CPU to the point where
FIQ exception has happened and the system continues to run as usual.


>>> I am trying to understand how the interrupt handler in arm working.By
>>> looking at the code,what I understood,when an interrupt
>>> happens arm disables the irq, saves the cpsr to spsr,save current pc
>>> to lr and switches to irq mode.
>>
>> Yes, that's right.
>> Important to understand that IRQ processor mode has its (mode-private)
>> own sp, lr and spsr registers.
>> Additionally, mode-private registers (sp, lr and spsr) are
>> inaccessible from other modes.

>>> So in case of interrupt it branches to the vector_irq and there it
>>> saves some registers and depending on which context
>>> its happened,it will call __irq_user or __irq_svc. But before that it
>>> switches to the supervisory mode.

>> That's right. In Linux Kernel the IRQ, Data Abort, Prefetch Abort, SWI
>> and Undefined exceptions
>> are handled in SVC processor mode. In Linux SVC processor mode of ARM
>> CPU is called "Kernel mode".
>> You have to switch to SVC processor mode from IRQ (and other processor
>> modes) to enabled reentrant
>> interrupts.
>> Simplistically, it works like this: (1) IRQ exception is entered, (2)
>> spsr_irq, r0 and lr_irq is saved
>> on the private IRQ stack (its size is only 12 bytes), (3)
>> 'vector_stub' macro check from what
>> processor mode we got here - kernel mode or user mode - and calculates
>> what to call __irq_user
>> or__irq_svc and (4) the last thing it does 'movs pc, lr' which loads
>> spsr_irq into cpsr and puts lr
>> into pc (lr now contains __irq_svc or __irq_user and spsr_irq[4:0]
>> contains SVC mode bits).
>> Mind, that r0 points to private IRQ stack that contains original r0,
>> lr_irq and spsr_irq.

> 1) Does the LINUX handles the interrupt like this in all other
> architectures.I mean handler will always be executed in SVC mode.

Sorry, I am too unfamiliar with other architectures... to my detriment actually.
Can anybody fill the knowledge gap here?!

--- Kosta