Intel SMP Architecture


SMP architecture is used in the environment, where multiple processors are connected to a single shared memory, have access to all input and output interfaces, and are controlled by a single operating system. In our case, we have multiple Xtensa DSP cores, which use the same Firmware binary loaded to the shared L2 SRAM, and are controlled by the same instance of the XTOS.

Using SMP architecture

Sound Open Firmware implementation of SMP architecture involves separate and modified XTOS, which can be chosen by selecting appropriate arch flag during configuration step of building FW binary.

./configure --with-arch=xtensa-smp --with-platform=<platform> --with-dsp-core=<core> --with-root-dir=<root-dir> --host=<host>

Implementation details

The data structures critical to core execution need to be instantiated per core, instead of being accessed using static pointers. SMP implementation creates struct core_context to meet those demands. This structure contains pointers to the XTOS data along with struct irq_task, struct schedule_data, struct work_queue etc.

struct core_context {
   struct thread_data td;
   struct irq_task *irq_low_task;
   struct irq_task *irq_med_task;
   struct irq_task *irq_high_task;
   struct schedule_data *sch;
   struct work_queue *queue;
   struct idc *idc;

struct core_context is allocated by primary core for secondary cores before secondary core boot. Address of the struct core_context is written into THREADPTR processor register, which can later be retrieved by the secondary core after boot. Every core has its own instance of THREADPTR, so struct core_context address can be read anytime at any place of the code.

Communication between cores

Primary core can communicate with secondary cores by sending messages using the IDC mechanism. This mechanism is pretty much the same as IPC. Important data can be sent in two 32-bit IDC registers. Cores use interrupts to register for the incoming messages.

participant core0
participant core1
participant idc
participant platform

core0 -> idc : idc_init()
   activate idc

   idc -> platform : interrupt_register(irq, auto_unmask, irq_handler, idc)
      activate platform
   idc <-- platform
   deactivate platform

core0 <-- idc
deactivate idc

core0 -> idc : idc_send_msg(idc_msg, mode)
   activate idc

   idc -> core1 : irq_handler()
      activate core1
   idc <-- core1
   deactivate core1
core0 <-- idc
deactivate idc

core1 -> idc : idc_do_cmd(data)
   activate idc
idc <-- core1
deactivate idc