Hi Varun,

Could you elaborate on the use case for denying a specific PE from being turned off using CPU_OFF call? The idea of introducing a new platform hook and invoking it early sounds like a safe approach to me.

Thanks,

Madhukar

From: Varun Wadekar via TF-A <tf-a@lists.trustedfirmware.org>
Sent: Friday, April 21, 2023 11:26 AM
To: Okash Khawaja <okash@google.com>
Cc: tf-a@lists.trustedfirmware.org; Bo Yan <byan@nvidia.com>
Subject: [TF-A] Re: Allow platforms to deny CPU power off request

Thanks, Okash. Yes, that is doable.

From: Okash Khawaja <okash@google.com>
Sent: Friday, April 21, 2023 3:25 PM
To: Varun Wadekar <vwadekar@nvidia.com>
Cc: tf-a@lists.trustedfirmware.org; Bo Yan <byan@nvidia.com>
Subject: Re: [TF-A] Allow platforms to deny CPU power off request

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Hi,

If CPU power off info is already known when the PSCI SMC is received in EL3, adding a platform hook early in `psci_do_cpu_off()` [1] may save rolling back any activity. It can return PSCI_E_DENIED.

[1] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmware-a/+/refs/heads/master/lib/psci/psci_off.c#43

On Fri, Apr 21, 2023 at 1:43 PM Varun Wadekar via TF-A <tf-a@lists.trustedfirmware.org> wrote:

Hello,

We have a use case where we want to deny the CPU power off from a certain PE. The current lib/psci does not provide a way for a platform to do this. We have support to return an error from svc_off, but this does not cover the scenario where svc_off is NULL.

There are multiple ways of handling this scenario.

Introduce a new platform function to check if CPU_OFF is allowed on the core
Return an error code from the pwr_domain_off() handler and unwind the CPU_OFF request
Something else.

Any suggestions?

-Varun

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