linux-kernel - Re: [RFC PATCH 1/3] efi-rtc: Remove wakeup functionality

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Message-ID: <CAMj1kXGG0Mi+AT7ZeoHLTDhVbbahP-PLRgY1=aSZMVQEgWz5sQ@mail.gmail.com>
Date: Mon, 14 Jul 2025 16:34:08 +1000
From: Ard Biesheuvel <ardb@...nel.org>
To: Demi Marie Obenour <demiobenour@...il.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, 
	Heinrich Schuchardt <heinrich.schuchardt@...onical.com>, Feng Tang <feng.tang@...ux.alibaba.com>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Juergen Gross <jgross@...e.com>, 
	Stefano Stabellini <sstabellini@...nel.org>, 
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>, Sunil V L <sunilvl@...tanamicro.com>, 
	Bibo Mao <maobibo@...ngson.cn>, linux-rtc@...r.kernel.org, linux-efi@...r.kernel.org, 
	xen-devel@...ts.xenproject.org, x86@...nel.org, 
	linux-riscv@...ts.infradead.org, loongarch@...ts.linux.dev
Subject: Re: [RFC PATCH 1/3] efi-rtc: Remove wakeup functionality

On Mon, 14 Jul 2025 at 16:22, Demi Marie Obenour <demiobenour@...il.com> wrote:
>
> On 7/14/25 02:19, Ard Biesheuvel wrote:
> > On Mon, 14 Jul 2025 at 16:13, Demi Marie Obenour <demiobenour@...il.com> wrote:
> >>
> >> On 7/14/25 02:08, Ard Biesheuvel wrote:
> >>> From: Ard Biesheuvel <ardb@...nel.org>
> >>>
> >>> The EFI rtc driver is used by non-x86 architectures only, and exposes
> >>> the get/set wakeup time functionality provided by the underlying
> >>> platform. This is usually broken on most platforms, and not widely used
> >>> to begin with [if at all], so let's just remove it.
> >> systemd uses the underlying functionality: a timer can wake the system up.
> >> I have no idea if that is implemented in terms of this function, though.
> >
> > To be clear, you are referring to wake from poweroff at some date/time
> > in the future, right?
>
> Yes.
>
> > This change does not remove this functionality from the RTC subsystem,
> > it just ceases to expose it on non-x86 EFI platforms that claim to
> > support it.
>
> Do these platforms generally expose the functionality in a different way?

On x86, the CMOS rtc is manipulated directly (and this is officially
condoned by the EFI spec).

On non-x86, this functionality rarely works, which is really the point
of this series.

> If not, systemd should probably document that the functionality is
> non-portable if it doesn't do that already.

Not sure what you mean by non-portable. This functionality should be
exposed in a generic manner (using the RTC subsystem interfaces), but
only if it can be relied upon. On x86, the RTC subsystem will use the
rtc-cmos driver, which implements the wakeup routines in terms of port
I/O.

If removing this functionality altogether from the EFI rtc driver is a
problem, perhaps it would be better to implement an allowlist based
solution that does not attempt to access the runtime services by
default.