Documentation: efi: provide (U)EFI watchdog documentation

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 34 insertions, 1 deletions

diff --git a/Documentation/boards/efi.rst b/Documentation/boards/efi.rst
index f59bb1d5ba..3da2daac99 100644
--- a/Documentation/boards/efi.rst
+++ b/Documentation/boards/efi.rst
@@ -216,7 +216,6 @@ has a device parameter ``devpath`` which contains its device path:
   barebox:/ echo ${handle-00000000d0012198.devpath}
   pci_root(0)/Pci(0x1d,0x0)/Usb(0x1,0x0)/Usb(0x2,0x0)
 
-
 EFI variables
 -------------
 
@@ -327,3 +326,37 @@ compile EDK2.
                    mov     %fs, %rax
                    pushq   %rax
 
+(U)EFI Watchdog
+---------------
+
+(U)EFI provides basic watchdog support. Depending on the system implementation
+it can be a software or hardware watchdog. Within the (U)EFI specification it
+is described as follows:
+
+.. epigraph::
+
+   If the watchdog timer expires, the event is logged by the firmware. The system
+   may then either reset with the Runtime Service ResetSystem(), or perform a
+   platform specific action that must eventually cause the platform to be reset.
+   The watchdog timer is armed before the firmware's boot manager invokes an EFI
+   boot option.  The watchdog must be set to a period of 5 minutes. The EFI Image
+   may reset or disable the watchdog timer as needed. If control is returned to
+   the firmware's boot manager, the watchdog timer must be disabled.  The watchdog
+   timer is only used during boot services. On successful completion of
+   ExitBootServices() the watchdog timer is disabled.
+
+See page 186:
+https://uefi.org/sites/default/files/resources/UEFI_Spec_2_1_D.pdf
+
+Current linux kernel (v5.0) will execute ExitBootServices() during the early
+boot stage and thus will automatically disable the (U)EFI watchdog. Since it is
+a proper behavior according to the (U)EFI specification, it is impossible to
+protect full boot chain by using this watchdog only. It is recommended to use
+an alternative hardware watchdog, preferably started before the bootloader. If (U)EFI
+firmware lacks this feature, the bootloader should be able to start an alternative
+hardware watchdog on its own. Before implementing this kind of workaround
+please make sure (U)EFI watchdog is not using the same hardware as the alternative
+watchdog.
+
+Nevertheless, barebox provides access to the (U)EFI SetWatchdogTimer()
+interface over its internal watchdog framework.