2. Arm SiP Services

This document enumerates and describes the Arm SiP (Silicon Provider) services.

SiP services are non-standard, platform-specific services offered by the silicon implementer or platform provider. They are accessed via SMC (“SMC calls”) instruction executed from Exception Levels below EL3. SMC calls for SiP services:

  • Follow SMC Calling Convention;

  • Use SMC function IDs that fall in the SiP range, which are 0xc2000000 - 0xc200ffff for 64-bit calls, and 0x82000000 - 0x8200ffff for 32-bit calls.

The Arm SiP implementation offers the following services:

  • Performance Measurement Framework (PMF)

  • Execution State Switching service

Source definitions for Arm SiP service are located in the arm_sip_svc.h header file.

2.1. Performance Measurement Framework (PMF)

The Performance Measurement Framework allows callers to retrieve timestamps captured at various paths in TF-A execution.

2.2. Execution State Switching service

Execution State Switching service provides a mechanism for a non-secure lower Exception Level (either EL2, or NS EL1 if EL2 isn’t implemented) to request to switch its execution state (a.k.a. Register Width), either from AArch64 to AArch32, or from AArch32 to AArch64, for the calling CPU. This service is only available when Trusted Firmware-A (TF-A) is built for AArch64 (i.e. when build option ARCH is set to aarch64).

2.2.1. ARM_SIP_SVC_EXE_STATE_SWITCH

Arguments:
    uint32_t Function ID
    uint32_t PC hi
    uint32_t PC lo
    uint32_t Cookie hi
    uint32_t Cookie lo

Return:
    uint32_t

The function ID parameter must be 0x82000020. It uniquely identifies the Execution State Switching service being requested.

The parameters PC hi and PC lo defines upper and lower words, respectively, of the entry point (physical address) at which execution should start, after Execution State has been switched. When calling from AArch64, PC hi must be 0.

When execution starts at the supplied entry point after Execution State has been switched, the parameters Cookie hi and Cookie lo are passed in CPU registers 0 and 1, respectively. When calling from AArch64, Cookie hi must be 0.

This call can only be made on the primary CPU, before any secondaries were brought up with CPU_ON PSCI call. Otherwise, the call will always fail.

The effect of switching execution state is as if the Exception Level were entered for the first time, following power on. This means CPU registers that have a defined reset value by the Architecture will assume that value. Other registers should not be expected to hold their values before the call was made. CPU endianness, however, is preserved from the previous execution state. Note that this switches the execution state of the calling CPU only. This is not a substitute for PSCI SYSTEM_RESET.

The service may return the following error codes:

  • STATE_SW_E_PARAM: If any of the parameters were deemed invalid for a specific request.

  • STATE_SW_E_DENIED: If the call is not successful, or when TF-A is built for AArch32.

If the call is successful, the caller wouldn’t observe the SMC returning. Instead, execution starts at the supplied entry point, with the CPU registers 0 and 1 populated with the supplied Cookie hi and Cookie lo values, respectively.

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