UM0851 datasheet(37/245 Pages) STMICROELECTRONICS

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UM0851

Platform section

Doc ID 16604 Rev 2

37/245

kernel stack of the process which was interrupted), reads LR and SPSR from a temporary

IRQ stack and saves them on the SVC mode stack. It increments the preempt count and

calls get_irqnr_and_base() to find out the IRQ line number.

The SPEAr architecture provides the get_irqnr_and_base (arch/arm/mach-

spearxxx/include/mach/entry-macro.S) function to query the interrupt controller to find out

which IRQ line raised this interrupt.

After this, __irq_svc() calls asm_do_IRQ() which in turn calls the IRQ handler (the interrupt

handler that you registered through request_irq ( )) or a default IRQ handler registered by

default during boot time.

After the completion of these actions, the IRQ line for which the interrupt was raised is

unmasked and do_level_irq() returns. After this interrupt handling is complete __irq_svc ()

restores the state of the interrupted process.

So now the IRQ infrastructure is in place, and various modules can register their IRQ

handlers through request_irq(). When you call request_irq( ), the kernel appends your IRQ

handler to the list of all the IRQ handlers registered for that particular IRQ line. It does not

change the exception vector table.

As described above, the Linux implementation for interrupt handling is not a vectored

approach, hence it does not utilize the vector interrupt capabilities provided by the VIC

hardware.

3.2.5

VIC usage

The current implementation in Linux does not allow the exploitation of the complete

capabilities of the VIC. Within the current capabilities that have been added in the Linux,

you can use the VIC in the following ways.

If a driver needs to register a specific interrupt handler, the following call is provided by

Linux:

/*The following functions declared in <linux/interrupt.h> implement the interrupt

registration interface and is use to register the interrupt handler.*/

int request_irq(unsigned int irq, irqreturn_t (*handler)( ), unsigned long flags,

const char

*dev_name, void *dev_id);

/* The details of the above fields are as follow:

irq: interrupt number being requested

(*handler)(): pointer to the handler being installed

in driver.

flags: Options related to interrupt management.

*dev_name: string passed here is used in /proc/interrupts to show the owner of

interrupts

*dev_id: Pointer used for shared interrupt lines

/* The function below is use to unregister the interrupt handler. */

void free_irq(unsigned int irq, void *dev_id);

To check for the interrupt sources being used in the system, type in the following command.

$cat

/proc/interrupts

CPU0

16:

3903

spear-vic

gp_timer

18:

spear-vic

<NULL>

20:

spear-vic

spear-jpeg

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