= GIET-VM / Interrupt Handler =


The [source:soft/giet_vm/giet_kernel/irq_handler.c irq_handler.c] and [source:soft/giet_vm/giet_kernel/irq_handler.h irq_handler.h] files define the kernel functions that are used to handle interrupts. They are prefixed by "_" to remind that they can only be executed by a processor in kernel mode.

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The GIET_VM interrupt handler supports only the SOCLIB XCU interrupt controler. In a multi-cluster architectures, it must exist one XCU controller in ech cluster containing processors. 

A multi-channel XCU component in a given cluster must contain (NB_PROCS_MAX * IRQ_PER_PROCESSOR) channels  (one channel = one XCU output IRQ).

There is three interrupt vectors per processor (stored in each processor's scheduler) for the three  interrupts types: '''HWI''' (Hardware Interrupt), '''PTI''' (Programmable Timer Interrupt), and '''WTI''' (Write Triggered Interrupt). Each interrupt vector entry contains two fields:
||isr_id         ||bits[15:0]    || defines the type of ISR to be executed ||
||channel_id ||bits[31:16]  || defines the channel for multi-channels ISR ||

 == __Interrupt routing__ ==

Regarding the allocation of interrupts to processors (IRQ routing using the XCU_MASK registers), the GIET-VM implement the following policy ('''lpid''' is the local processor index, between 0 and 7):
 1. The GIET-VM uses only one XCU output IRQ per processor  (with index = lpid * IRQ_PER_PROCESSOR), even if the hardware platform contains more than one IRQ_PER_PROCESSOR.

 2. In each cluster the local '''HWI'''  generated by the local peripherals are ''statically'' allocated and distributed to local processors, to share the load between all processors).

 3. In each cluster, one '''PTI''' is statically allocated to each processor for context switch (pti_id = lpid). The TICK period is defined by the GIET_TICK_VALUE parameter in the giet_config.h file. The associated '''_isr_tick()''' interrupt service routine forces a context switch on the target processor.

 4. In each cluster, 4 '''WTI''' mailbox (called WAKE_UP, EXT_IRQ_ONE, EXT_IRQ_TWO, EXT_IRQ_TER) are statically allocated to each processor. The first one is used by the GIET_VM boot-loader for processor wakup, and is used by the kernel for inter-processor interrupts.  

 5. The '''isr_wakup()''' interrupt service routine is associated to the WAKE_UP interrupt. When the destination processor is not in wait state (low-power mode), the ISR is executed. If the processor executing the idle_task, or if the value written in the WTI mailbox is non zero, this ISR force a context switch on the target processor.

 6. The three EXT_IRQ_ONE, EXT_IRQ_TWO and EXT_IRQ_TER interrupts are dynamically allocated to external IRQS generated by the external peripherals (through the IOPIC component), in order to route the external IRQ to the processor that launched the I/O operation. 

The array below define the static routing of WTIs to the local processors, implemented by the XCU masks: 
|| '''WTI name'''   || ''' WTI index''' ||
|| WAKE_UP       || lpid           ||
|| EXT_IRQ_ONE || NB_PROCS_MAX + lpid ||
|| EXT_IRQ_TWO || 2*NB_PROCS_MAX + lpid ||
|| EXT_IRQ_TER  || 3*NB_PROCS_MAX + lpid ||

 == __Functions used for all HWI / PTI / WTI interrupts__ ==

 === void '''_irq_demux'''() ===
This function access the XCU component to get the interrupt vector entry:  
It uses the [source:soft/giet_vm/giet_drivers/xcu_driver.c _xcu_get_index()]  functions to get the IRQ type ( HWI / PTI / WTI ), and the index in the corresponding interrupt vector.
Any index value larger than 31 means "no active interrupt", and the default ISR is executed.

All ISRs (but the default ISR) must have the
same three arguments :
 * unsigned int irq_type,  
 * unsigned int irq_id,  
 * unsigned int channel 

As most ISRs (Interrupt Service Routine) are associated to a specific peripheral, these ISR are defined in the
[wiki:periphal_drivers drivers]. But the following ISRs are defined in the [source:soft/giet_vm/giet_kernel/irq_handler.c irq_handler.c] file:


 === void '''_isr_wakup'''( unsigned int irq_type,  unsigned int irq_id,  unsigned int channel ) ===
This ISR can only be executed after a WTI (IPI) awake an idle processor, or to force a context switch
on a remote processor. The context switch is only executed if the current task is the IDLE_TASK, or if the value written in the mailbox is non zero.

 === void '''_isr_tick'''( unsigned int irq_type,  unsigned int irq_id,  unsigned int channel ) ===
This ISR is in charge of context switch, and handles the IRQs generated by
// the "system" timers. It can be PTI in case of XCU, or it can be HWI generated
// by an external timer in case of ICU.
// The ISR acknowledges the IRQ, and calls the _ctx_switch() function.

 === void '''_isr_default'''() ===
This default ISR is called  when the interrupt handler is called, and there is no active IRQ. It simply displays a warning message on the kernel TTY[0].

 == __Functions used for dynamic allocation of external IRQs__ ==

 === void '''_ext_irq_init'''() ===
This function is only used when the architecture contains an external IOPIC component. It initializes the _ext_irq_index[isr][channel] array,
defining the IRQ index associated to an (isr_type/isr_channel) couple, as specified in the mapping.
This array is used by the kernel for dynamic routing of external IRQs.

 === void '''_ext_irq_alloc( unsigned int isr_type , unsigned int isr_channel , unsigned int* wti_index ) ===
This function is used when the architecture contains an external IOPIC component. It dynamically allocates an external IRQ signaling completion of an I/O operation to the processor P[x,y,p] running the calling task. The two (isr_type, isr_channel) arguments define actually the external IRQ to be routed.
 * '''isr_type''' : type of ISR 
 * '''isr_channel''' : ISR channel (for multi-channels peripherals)
 * '''wti_index''' : return value defining the index of the WTI mailbox allocated to P[x,y,p]
This function does three things: 
 1. it allocates a WTI mailbox in the XCU of cluster[x,y] to the requesting processor ( index is in [4*p+1, 4*p+2, 4*p+3] ) ; 
 2. it initialises the IOPIC entry associated to the (isr_type/isr_channel) IRQ.
 3. it initializes the proper entry in the WTI interrupt vector associated to processor P[x,y,p].

 === void '''_ext_irq_release( unsigned int isr_type , unsigned int isr_channel , unsigned int wti_index ) ===
This function is used when the architecture contains an external IOPIC component. It desallocates the ressources allocated by the  previous _ext_irq_alloc() function to the calling processor. The two (isr_type, isr_channel) arguments define actually the external IRQ to be released.
 * '''isr_type''' : type of ISR 
 * '''isr_channel''' : ISR channel (for multi-channels peripherals)
 * '''wti_index''' :  index of the WTI mailbox allocated to P[x,y,p] to be released.
This function does only two things: 
 1.  it desactivates the PIC entry associated to the (isr_type/isr_channel) IRQ.
 2. it releases the WTI mailbox allocated to P[x,y,p].
 3. The WTI interrupt vector is NOT modified