What is the purpose of WFI and WFE instructions and the event signals ?
We have 2 instructions for entering low-power standby state where most clocks are gated: WFI and WFE. They differ slightly in their entry and wake-up conditions, with the main difference being that WFE makes use of the event register, the SEV instruction and EVENTI, EVENTO signals. WFI is targeted at entering either standby, dormant or shutdown mode, where an interrupt is required to wake-up the processor. A usage for WFE is to put it into a spinlock loop. Where a CPU wants to access a shared resource such as shared memory, we can use a semaphore flag location managed by exclusive load and store access. If multiple CPUs are trying to access the resource, one will get access and will start to use the resource while the other CPUs will be stuck in the spinlock loop. To save power, you can insert the WFE instruction into the loop so the CPUs instead of looping continuously will enter STANDBTWFE. Then the CPU who has been using the resource should execute SEV instruction after it has finished using the resource. This will wake up all other CPUs from STANDBYWFE and another CPU can then access the shared resource. The reason for having EVENTI and EVENTO is to export a pulse on EVENTO when an SEV instruction is executed by any of the CPUs. This signal would connect to EVENTI of a second Cortex-A5 MPCore cluster and would cause any CPUs in STANDBYWFE state to leave standby. So these signals just expand the usage of WFE mode across multiple clusters. If you have a single cluster, then you do not need to use them.
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