Re: SMP enablement on Cortex-R52 (using PSCI ?)

[Sudeep Holla]

Hi Ayan,

On Fri, Apr 14, 2023 at 12:24:38PM +0100, Ayan Kumar Halder wrote:
> Hi PSCI developers,
> 
> We have a SoC where there are 4 Cortex-R52 which is distributed in two
> clusters. So we have 2 Cortex-R52 in one cluster and 2 Cortex-R52 in another
> cluster.
> 
> We wish to enable SMP on the 2 R52 within a cluster with Xen hypervisor (EL2
> software) running on them.
> 
> We are trying to explore if we can use PSCI for booting the secondary cores.
> 
> Refer Cortex-R52 TRM
> (https://developer.arm.com/documentation/100026/0101/?lang=en ), it
> specifies the following :-
> 
> Page 24 - Section 1.4.1
> 
> "Support for Exception levels, EL0, EL1, and EL2."
> 
> Page 30 - Section 2.1.6
> 
> "The Cortex-R52 processor does not implement TrustZone® technology. It does
> not support the ability to distinguish between secure and non-secure
> physical memories."
> 
> Thus, there is no EL3 and secure world in Cortex-R52. It implements
> AArch32-V8R architecture.
>

KVM hypervisor use PSCI to bring up secondaries in the VMs. So I am sure we
must be able to use the interface on Cortex-R52 without EL3.

> 
> Refer PSCI design document,
> https://developer.arm.com/documentation/den0022/e/?lang=en
> 
> Page 18 -
> "The PSCI specification focuses on the interface between Security states for
> power management. It provides a method for issuing power management
> requests. To deal with the requests, the PPF must include a PSCI
> implementation. A PSCI implementation might require communication between
> the PPF and a Trusted OS or SP."
> 
> Page 17 - Privileged Platform Firmware (PPF)
> 
> "For Armv7 systems, or Armv8 systems using AArch32 at EL3, PPF executes in
> EL3."
> 
> From the above two statements, I infer that PSCI requires a PPF (running at
> EL3) and a Trusted OS (running at secure EL2). If this is correct, then R52 
> cannot support PSCI. Please correct me if I am mistaken.
> 
> I wish to know how do we wake up the secondary core if PSCI is not
> supported.
>

I will check with the authors if EL3 is a must for PSCI implementation, but
IMO it must not be though every aspects described in the spec may not apply
when used across EL2/EL1 boundaries especially when EL3 is not implemented
in the hardware.

-- 
Regards,
Sudeep