[PATCH] efi_loader: stop watchdog devices in ExitBootServices()

[Heinrich Schuchardt]

Am 4. Juli 2026 18:17:07 MESZ schrieb Tom Rini <trini at konsulko.com>:
>On Sat, Jul 04, 2026 at 05:21:25PM +0200, Carlo Caione wrote:
>> 
>> 
>> > On 4 Jul 2026, at 16:32, Tom Rini <trini at konsulko.com> wrote:
>> > 
>> > On Sat, Jul 04, 2026 at 03:52:29PM +0200, Carlo Caione wrote:
>> > 
>> >> U-Boot can autonomously start a hardware watchdog
>> >> (CONFIG_WATCHDOG_AUTOSTART, default y) and service it from its main
>> >> loop, but the EFI boot path never stops it: efi_exit_boot_services()
>> >> tears the devices down without calling wdt_stop_all(), and a watchdog
>> >> driver without a .remove hook leaves the hardware ticking across the
>> >> firmware-to-OS handoff.
>> >> 
>> >> An EFI-booted OS that does not take over the SoC watchdog within the
>> >> remaining timeout is reset mid-boot at a wall-clock-dependent point.
>> >> 
>> >> The UEFI specification (v2.11, section 7.5 "Miscellaneous Boot
>> >> Services", EFI_BOOT_SERVICES.SetWatchdogTimer()) is explicit about the
>> >> one watchdog it allows across the handoff:
>> >> 
>> >>  "The watchdog timer is only used during boot services. On
>> >>   successful completion of EFI_BOOT_SERVICES.ExitBootServices() the
>> >>   watchdog timer is disabled."
>> >> 
>> >> U-Boot's UEFI watchdog (an EFI timer event) complies by construction. A
>> >> platform watchdog silently surviving the handoff defeats the purpose of
>> >> that rule: the OS has no generic way to know it is running, let alone to
>> >> service it.
>> >> 
>> >> Stop all watchdog devices in efi_exit_boot_services(), before the device
>> >> teardown.
>> >> 
>> >> Signed-off-by: Carlo Caione <ccaione at baylibre.com>
>> > 
>> > NAK. We have had this come up before, and in short, the UEFI
>> > specification needs to be fixed here, as generally speaking a watchdog
>> > should never be stopped, as that leaves a gap where the system can hang
>> > and thus defeat the point of having enabled a watchdog.
>> 
>> 
>> Fair enough.
>> 
>> For the record, the failure that motivated this: a pristine Ubuntu 26.04 arm64 image on a MediaTek Genio 700 EVK dies silently about 60 seconds after ExitBootServices(), 100% reproducible, at a wall-clock dependent point. The generic kernel ships the watchdog driver as a module, so nothing services the watchdog in time, and nothing tells the OS that it is running. From the user's side this is indistinguishable from broken firmware.
>> 
>> In the meantime, is the accepted pattern hooking up a wdt_stop_all() in the EFI path via board_quiesce_devices()? That keeps the watchdog armed for the whole firmware lifetime and makes the handoff behaviour the board maintainer's call.
>
>No, Ubuntu needs to fix their kernel image to have the watchdog
>available early. I hate to be difficult on this, but it's been a known
>issue for years at this point that the spec needs to be updated. The
>workaround here is for the watchdog to not be started until the OS is
>ready to maintain it.
>

Best open a Launchpad bug explaining why this watchdog driver needs to be built in and not a module.

Best regards

Heinrich