GIET-VM / Context Switch Handler

The ctx_handler.c​ and ctx_handler.h​ files define the data structure and functions used to handle context switching. They are prefixed by "_" to remind that they can only be executed by a processor in kernel mode.

A GIET_VM task is similar to a POSIX thread. A task is statically allocated to a physical processor, as defined in the mapping, and the GIET-VM does not support task migration. There is one scheduler per processor, and the max number of task on one single processor cannot be larger than 13. A task can be in three different states:

• RUNNING the task is currently executed.
• RUNABLE the task is not running, but can be selected for execution at the next TICK.
• BLOCKED the task is blocked, waiting on a condition defined in the NORUN context slot.

A task context is an array of 64 words = 256 bytes. It contains copies of processor registers (when the task is preempted):

• GPR[i] : generally stored in slot (i). $0, $26 & $27 are not saved.
• HI & LO registers : used by multiply/divide instructions.
• CP0 registers : EPC, SR, CR, BVAR
• CP2 registers : PTPR

It contains some general informations associated to the task:

• TTY : TTY channel allocated to the task
• NIC_RX : NIC_RX channel allocated to the task
• NIC_TX : NIC_TX channel allocated to the task
• CMA_RX : CMA channel for NIC_RX allocated to the task
• CMA_TX : CMA channel for NIC_TX allocated to the task
• CBA_FB : CMA channel for FBF display allocated to the task
• HBA : HBA channel local index
• TIM : TIM channel local index
• PTAB : page table virtual base address
• LTID : Task local index (in scheduler)
• VSID : Virtual space index
• N0RUN : bit-vector defining the task state ( blocked if non zero )
• TRDID : Thread index (in vspace)
• SIGNAL : bit-vector (inter-tasks signalisation)
• ENTRY : Virtual address of task entry point
• COPROC : cluster_xy : coordinates of coprocessor allocated to the task

[0] *	[8] $8	[16] $16	[24] $24	[32] EPC	[40] TTY	[48] TRDID
[1] $1	[9] $9	[17] $17	[25] $25	[33] CR	[41] CMA_FB	[49] GTID
[2] $2	[10] $10	[18] $18	[26] LO	[34] SR	[42] CMA_RX	[50] NORUN
[3] $3	[11] $11	[19] $19	[27] HI	[35] BVAR	[43] CMA_TX	[51] COPROC
[4] $4	[12] $12	[20] $20	[28] $28	[36] PTAB	[44] NIC_RX	[52] ENTRY
[5] $5	[13] $13	[21] $21	[29] SP	[37] LTID	[45] NIC_TX	[53] SIGNAL
[6] $6	[14] $14	[22] $22	[30] $30	[38] VSID	[46] TIM	[54] *
[7] $7	[15] $15	[23] $23	[31] RA	[39] PTPR	[47] HBA	[55] *

void _ctx_switch()

This function performs a context switch between the running (calling) task and another runable task, using a round-robin sheduling policy between all tasks allocated to a given processor (static allocation). It selects the next runable task to resume execution. If the only runable task is the current task, return without context switch. If there is no runable task, the scheduler switch to the default "idle" task. The return address contained in $31 is saved in the current task context (in the ctx[31] slot), and the function actually returns to the address contained in the ctx[31] slot of the next task context.

void _ctx_exec_task( unsigned int ltid )

void _ctx_kill_task( unsigned int ltid )

void _ctx_eret()

The address of this function is used to initialise the return address in the "idle" task context.

void _idle_task()

This function is executed task when no other task can be executed.