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

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Message-ID: <aHZsohPoxLYML___@sunil-laptop>
Date: Tue, 15 Jul 2025 20:28:42 +0530
From: Sunil V L <sunilvl@...tanamicro.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
	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>,
	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 Tue, Jul 15, 2025 at 01:29:15PM +1000, Ard Biesheuvel wrote:
> 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.
> 
Hi Ard,
IIUC, TAD is defined for this purpose, right? 
https://uefi.org/specs/ACPI/6.6/09_ACPI_Defined_Devices_and_Device_Specific_Objects.html#time-and-alarm-device

> 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.

I agree. But I think even with ACPI, EFI GetTime/SetTime can return
unsupported if there is a TAD exposed with proper _GRT/_SRT and _GCP.

Thanks,
Sunil