> From: Rafael J. Wysocki <rafael@xxxxxxxxxx> > Sent: Friday, February 5, 2021 5:06 AM > To: Dexuan Cui <decui@xxxxxxxxxxxxx> > Cc: linux-acpi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > linux-hyperv@xxxxxxxxxxxxxxx; Michael Kelley <mikelley@xxxxxxxxxxxxx> > 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@xxxxxxxxxxxxx> 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 This works on x86. Thanks a lot! BTW, does this also work on ARM64? Thanks, -- Dexuan
