linux-kernel - Re: [RFC PATCH 0/3] Remove unused EFI runtime APIs

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Message-ID: <CAMj1kXEXpBF8hPaVMU0sDgNysYT66MDRmr3JHO4Lg1sJB_Yteg@mail.gmail.com>
Date: Tue, 15 Jul 2025 13:29:15 +1000
From: Ard Biesheuvel <ardb@...nel.org>
To: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	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, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] Remove unused EFI runtime APIs

On Mon, 14 Jul 2025 at 18:11, Heinrich Schuchardt
<heinrich.schuchardt@...onical.com> wrote:
>
> On 7/14/25 08:08, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Using EFI runtime services to program the RTC to wake up the system is
> > supported in theory, but rarely works in practice. Fortunately, this
> > functionality is rarely [if ever] used to begin with so we can just drop
> > it. (Note that the EFI rtc driver is not used by x86, which programs the
> > CMOS rtc directly)
>
> The main problem I see with firmware offering access to the RTC via UEFI
> services is that two different drivers, the firmware one and the Linux
> one might be trying to access the same busses or registers which might
> lead to unexpected results.
>
> Recently there was a discussion in the RISC-V technical group for the
> server platform specification where the same issue was discussed
> concerning SetTime().
>
> As a UEFI firmware should not care which operating system is booted, it
> should be up to the OS to disable EFI access to the RTC if it has native
> access.
>
> Could we disable all the EFI services for the RTC in Linux dynamically
> when an RTC driver is successfully probed?
>

I don't think this would be the right way to do it.

It also depends on whether ACPI or DT is being used to describe the
platform to the OS.

ACPI does not support describing the RTC device, so it should provide
the EFI services.

DT can describe the RTC device directly, so I think it is acceptable
for such firmware to mark all RTC routines unsupported in the RT_PROP
table, and just expose the RTC device directly.

The OS shouldn't have to reason about these things: it is up to the
platform to describe itself unambiguously.