Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <MWHPR21MB0863BA3D689DDEC3CA6BC262BFC91@MWHPR21MB0863.namprd21.prod.outlook.com>
In-Reply-To: <MWHPR21MB0863BA3D689DDEC3CA6BC262BFC91@MWHPR21MB0863.namprd21.prod.outlook.com>
From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 5 Feb 2021 14:05:36 +0100
Message-ID: <CAJZ5v0jRgeAsyZXpm-XdL6GCKWk5=yVh1s4fZ3m0++NJK-gYBg@mail.gmail.com>
Subject: Re: How can a userspace program tell if the system supports the ACPI
 S4 state (Suspend-to-Disk)?
To:     Dexuan Cui <decui@microsoft.com>
Cc:     "linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        Michael Kelley <mikelley@microsoft.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Sat, Dec 12, 2020 at 2:22 AM Dexuan Cui <decui@microsoft.com> wrote:
>
> Hi all,
> It looks like Linux can hibernate even if the system does not support the ACPI
> S4 state, as long as the system can shut down, so "cat /sys/power/state"
> always contains "disk", unless we specify the kernel parameter "nohibernate"
> or we use LOCKDOWN_HIBERNATION.
>
> In some scenarios IMO it can still be useful if the userspace is able to detect
> if the ACPI S4 state is supported or not, e.g. when a Linux guest runs on
> Hyper-V, Hyper-V uses the virtual ACPI S4 state as an indicator of the proper
> support of the tool stack on the host, i.e. the guest is discouraged from
> trying hibernation if the state is not supported.
>
> I know we can check the S4 state by 'dmesg':
>
> # dmesg |grep ACPI: | grep support
> [    3.034134] ACPI: (supports S0 S4 S5)
>
> But this method is unreliable because the kernel msg buffer can be filled
> and overwritten. Is there any better method? If not, do you think if the
> below patch is appropriate? Thanks!

Sorry for the delay.

If ACPI S4 is supported, /sys/power/disk will list "platform" as one
of the options (and it will be the default one then).  Otherwise,
"platform" is not present in /sys/power/disk, because ACPI is the only
user of hibernation_ops.

HTH