linux-kernel - Re: How can a userspace program tell if the system supports the ACPI S4 state (Suspend-to-Disk)?

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Date:   Fri, 5 Feb 2021 14:05:36 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: How can a userspace program tell if the system supports the ACPI
 S4 state (Suspend-to-Disk)?

On Sat, Dec 12, 2020 at 2:22 AM Dexuan Cui <decui@...rosoft.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