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

[Ard Biesheuvel <ardb@xxxxxxxxxx>]

On Wed, 16 Jul 2025 at 00:58, Sunil V L <sunilvl@xxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> wrote:
> > >
> > > On 7/14/25 08:08, Ard Biesheuvel wrote:
> > > > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > >
> > > > 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 for the pointer. This device did not exist yet when I last
looked at this stuff.

So it seems like TAD is a suitable way for exposing a RTC to the OS
without the need for a hardware specific driver. However, the existing
Linux driver does not appear to support get/set time, and is not
hooked up to the RTC subsystem [yet].