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Hi All, It’s about Interrupt Handling in Linux Kernel. I have following doubts regarding Interrupt Handling: -
What does the cli() and sti() functions
do, I know that “cli” and “sti” instructions in assembly
language clear and set the interrupt reporting for a processor which is
executing it. But on SMP machines, does cli() and sti() clear and set the
interrupt reporting on single processor (processor which is executing it), or
it works for all processors (I mean if one processor calls cli(), will it block
the interrupt reporting on all the processors). In assembly language I think “cli”
and “sti” instructions are for only that processor which executes
it and does not affect the interrupt reporting on other processor. Am I right ? -
In handle_IRQ_event() function (http://lxr.linux.no/source/arch/i386/kernel/irq.c?v=2.4.21#L437),
before calling the actual interrupt handler why are we checking the flag of only
first handler in list. If in case there are multiple drivers sharing a
particular handler (that means, for that interrupt line there are more than one
interrupt handler to be called), we should check the flag of each handler
before calling it, if that flag says that handler need is a fast handler, we
need to put the interrupt handling “off” by calling cli() for that
handler. In case if the flag of that handler says that it’s a slow
handler, we need to set the interrupt handling “on” by calling
sti(). But if you check the code of handle_IRQ_event() function, you will find
that only the flag of first handler is checked and then all other subsequent
handlers in list are called without checking a flag. -
Can someone tell me what’s the
use of checking the “global_irq_lock” variable, before invoking the
interrupt handler in handle_IRQ_event() function (http://lxr.linux.no/source/arch/i386/kernel/irq.c?v=2.4.21#L437).
LDD says that if this variable (global lock) is equal to zero then only the
interrupt handler will be invoked. My question is why so, what the benefit we
are getting out of it. Also let me know where in kernel the value of this
global lock is increased or decreased, it might help in understanding the use
of it. -
In hanle_IRQ_event() function (http://lxr.linux.no/source/arch/i386/kernel/irq.c?v=2.4.21#L437),
we are calling irq_enter() and irq_exit() function, which actually increase and
decreases the cpu specific irq count (local_irq_count variable). Can someone
tell me the use of this action, or why this cpu specific interrupt count is
maintained. -
Last question, in hanle_IRQ_event()
function, why are we calling _cli() function (for disabling the interrupt
reporting) once all the interrupt handlers have been invoked. You can find it
at the end of hanle_IRQ_event() function (http://lxr.linux.no/source/arch/i386/kernel/irq.c?v=2.4.21#L437). Regards, Gaurav |
