2.5.1 PChannel protocol

Communicates power state changes between a power controller and a device.

The behaviors of the protocol are:

pactive (uint32_t pstate) : void

This master behavior is implemented by a power controller. A device calls this method to give a hint to the power controller that it can change to a particular power state. A power controller can then take appropriate action, typically communicating with the device by calling device.prequest(new_power_state).

The power state is type uint32_t because it is the responsibility of the system using PChannels to enumerate the power states that it supports. For example, ARMv8-A cores use the following enumeration for power states:

enum { OFF = 0, OFF_EMU, MEM_RET, MEM_RET_EMU, LOGIC_RET, FULL_RET, MEM_OFF, FUNC_RET, ON, WARM_RST, DBG_RECOV }
prequest (uint32_t pstate) : sg::PChannel::presp_t

This slave behavior is implemented by a device, for instance a core. A power controller typically calls this method and checks for the response from the device, which can either be ACCEPT or DENY.

sg::PChannel::presp_t
This enumeration provides two values, ACCEPT and DENY. It is returned by the prequest() method, depending on the state requested and the current state of the core.

Usage

You can use PChannels to replace STANDBYWFI and STANDBYWFE signaling.

For example, using STANDBYWFI or STANDBYWFE:

  • Core drives STANDBYWFI signal HIGH.
  • Power controller performs logic x.

Equivalent behavior using PChannels:

  • Core calls pactive(OFF).
  • Power controller calls prequest(OFF) to change the core to OFF.
  • Power controller performs logic x.
  • To wake up the core, the power controller calls prequest(ON).

Note:

  • For a LISA+ example that demonstrates how to use PChannel, see $PVLIB_HOME/examples/LISA/VP_PChannel/.
  • For SystemC test code that demonstrates how to use PChannel, see $PVLIB_HOME/Platforms/SystemCExport/EVS_Components/EVS_PChannel/.
Non-ConfidentialPDF file icon PDF version100964_1161_00_en
Copyright © 2014–2019 Arm Limited or its affiliates. All rights reserved.