Warn when DISABLE_BACKUP is enabled

F/2257

Author: danielinux

docs/compile.md

@@ -193,7 +193,9 @@ To circumvent the compile-time checks on the maximum allowed stack size, use `WO
 
 Optionally, it is possible to disable the backup copy of the current running firmware upon the installation of the
 update. This implies that no fall-back mechanism is protecting the target from a faulty firmware installation, but may be useful
-in some cases where it is not possible to write on the update partition from the bootloader.
+in some cases where it is not possible to write on the update partition from the bootloader. This also removes the
+power-fail-safe swap behavior: if power is lost while the update is being copied into the BOOT partition, the original
+firmware may already be partially overwritten and the device can be left unrecoverable.
 The associated compile-time option is
 
 `DISABLE_BACKUP=1`

options.mk

@@ -724,6 +724,7 @@ ifeq ($(NVM_FLASH_WRITEONCE),1)
 endif
 
 ifeq ($(DISABLE_BACKUP),1)
+  $(warning DISABLE_BACKUP=1 disables power-fail-safe updates; losing power during an update can leave BOOT partially written and unrecoverable)
   CFLAGS+= -D"DISABLE_BACKUP"
 endif