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Message-ID: <9efb39597fa7b36b6c4202ab73fae6610194e45e.camel@infradead.org>
Subject: Re: [RFC PATCH v3 5/5] arm64: Use SYSTEM_OFF2 PSCI call to power
 off for hibernate
From: David Woodhouse <dwmw2@infradead.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, Paolo Bonzini
 <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, Oliver Upton
 <oliver.upton@linux.dev>, James Morse <james.morse@arm.com>, Suzuki K
 Poulose <suzuki.poulose@arm.com>, Zenghui Yu <yuzenghui@huawei.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Mark Rutland <mark.rutland@arm.com>, Lorenzo Pieralisi
 <lpieralisi@kernel.org>, "Rafael J. Wysocki" <rafael@kernel.org>, Len Brown
 <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>, Mostafa Saleh
 <smostafa@google.com>, Jean-Philippe Brucker <jean-philippe@linaro.org>, 
 linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
 kvmarm@lists.linux.dev,  linux-pm@vger.kernel.org
Date: Fri, 22 Mar 2024 16:12:44 +0000
In-Reply-To: <86jzluz24b.wl-maz@kernel.org>
References: <20240319130957.1050637-1-dwmw2@infradead.org>
	 <20240319130957.1050637-6-dwmw2@infradead.org>
	 <86jzluz24b.wl-maz@kernel.org>
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On Fri, 2024-03-22 at 16:02 +0000, Marc Zyngier wrote:
> On Tue, 19 Mar 2024 12:59:06 +0000,
> David Woodhouse <dwmw2@infradead.org> wrote:
>=20
> [...]
>=20
> > +static void __init psci_init_system_off2(void)
> > +{
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0int ret;
> > +
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ret =3D psci_features(PSCI_F=
N_NATIVE(1_3, SYSTEM_OFF2));
> > +
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (ret !=3D PSCI_RET_NOT_SU=
PPORTED)
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0psci_system_off2_supported =3D true;
>=20
> It'd be worth considering the (slightly broken) case where SYSTEM_OFF2
> is supported, but HIBERNATE_OFF is not set in the response, as the
> spec doesn't say that this bit is mandatory (it seems legal to
> implement SYSTEM_OFF2 without any hibernate type, making it similar to
> SYSTEM_OFF).

Such is not my understanding. If SYSTEM_OFF2 is supported, then
HIBERNATE_OFF *is* mandatory.

The next update to the spec is turning the PSCI_FEATURES response into
a *bitmap* of the available features, and I believe it will mandate
that bit zero is set.

And if for whatever reason that SYSTEM_OFF2/HIBERNATE_OFF call
*doesn't* work, Linux will end up doing a 'real' poweroff, first
through EFI and then finally as a last resort with a PSCI SYSTEM_OFF.
So it would be OK to have false positives in the detection.

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