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

[Heinrich Schuchardt <heinrich.schuchardt@canonical.com>]

On 7/14/25 08:08, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.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?

Best regards

Heinrich

> 
> The same applies to GetNextHighMonoCount(), which, if implemented,
> usually relies on SetVariable() under the hood *, which is often not
> supported at runtime by non-x86 platforms. But it has no known users
> either so let's drop support for it as well.
> 
> This means we need to drop the slightly pointless tests for it too.
> 
> * EDK2 based EFI implementations usually have a MTC variable carrying
>    the monotonic counter variable, which is therefore not truly
>    monotonic, given that SetVariable() will happily overwrite it.
> 
> Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Cc: Feng Tang <feng.tang@linux.alibaba.com>
> Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> Cc: Sunil V L <sunilvl@ventanamicro.com>
> Cc: Bibo Mao <maobibo@loongson.cn>
> Cc: linux-rtc@vger.kernel.org
> Cc: linux-efi@vger.kernel.org
> Cc: xen-devel@lists.xenproject.org
> Cc: x86@kernel.org
> Cc: linux-riscv@lists.infradead.org
> Cc: loongarch@lists.linux.dev
> 
> Ard Biesheuvel (3):
>    efi-rtc: Remove wakeup functionality
>    efi/test: Don't bother pseudo-testing unused EFI services
>    efi: Remove support for pointless, unused EFI services
> 
>   arch/x86/platform/efi/efi_64.c          |  22 ----
>   drivers/firmware/efi/runtime-wrappers.c |  68 ------------
>   drivers/firmware/efi/test/efi_test.c    | 108 +-------------------
>   drivers/rtc/rtc-efi.c                   |  76 +-------------
>   drivers/xen/efi.c                       |  56 ----------
>   include/linux/efi.h                     |   6 --
>   6 files changed, 4 insertions(+), 332 deletions(-)
>