On Tue, Sep 24, 2019 at 09:54:41AM +0200, Oleksij Rempel wrote: > Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx> > --- > Documentation/user/user-manual.rst | 1 + > Documentation/user/watchdog.rst | 116 +++++++++++++++++++++++++++++ > 2 files changed, 117 insertions(+) > create mode 100644 Documentation/user/watchdog.rst > > diff --git a/Documentation/user/user-manual.rst b/Documentation/user/user-manual.rst > index f04981c3f0..41fdb8805c 100644 > --- a/Documentation/user/user-manual.rst > +++ b/Documentation/user/user-manual.rst > @@ -34,6 +34,7 @@ Contents: > state > random > debugging > + watchdog > > * :ref:`search` > * :ref:`genindex` > diff --git a/Documentation/user/watchdog.rst b/Documentation/user/watchdog.rst > new file mode 100644 > index 0000000000..87c63aa078 > --- /dev/null > +++ b/Documentation/user/watchdog.rst > @@ -0,0 +1,116 @@ > +Watchdog Support > +================ > + > +Warnings and Design Consideration > +--------------------------------- > + > +A watchdog is the last line of defense on misbehaving systems. Thus, proper > +hardware and watchdog design considerations should be made to be able to reduce > +the impact of failing systems in the field. In the best case, the bootloader > +should not touch it at all. No watchdog feeding should be done until > +application-critical software (or a userspace service manager such as > +'systemd') was started. > + > +In case the bootloader is responsible for watchdog activation, the system can > +be considered as failed by design. The following threats can affect the system > +which are mostly addressable by properly designed watchdog and watchdog > +strategy: > + > +- software-based misconfigurations or bugs prevent the system from starting. > +- glitches caused by under-voltage, inappropriate power-on sequence or noisy > + power supply. > +- physical damages caused by humidity, vibration or temperature. > +- temperature-based misbehavior of the system, e.g. clock is not running or > + running with wrong frequency. > +- chemical reactions, e.g. some clock crystals will stop to work in contact > + with Helium, see for example: > + https://ifixit.org/blog/11986/iphones-are-allergic-to-helium/ > +- failed storage prevents booting. NAND, SD, SSD, HDD, SPI-flash all of this > + some day stop to work because their read/write cycles are exceeded. > + > +In all these cases, the bootloader won't be able to start and a properly > +designed watchdog may take some action. For example: recover the system by > +resetting it, or power it off to reduce the damage. I haven't seen any watchdogs powering off the system. In the list above only in the case of glitches caused by under-voltage a watchdog makes a difference. In all the other cases a watchdog won't help either. Given that I don't agree to the claim that systems where the bootloader has to enable the watchdog are a design failure. Also I bet there are SoCs on which the watchdog can't be enabled by default before the bootloader. I wouldn't call boards designed around such a SoC a failure by design. Sascha -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
