[PATCH 2/2] watchdog: add watchdog behavior configuration

Mark Kettenis
2020-09-24T10:22:44Z

> Content-Type: text/plain; charset=US-ASCII;
> format=flowed
> Date: Thu, 24 Sep 2020 10:20:30 +0200
> From: Michael Walle <michael@walle.cc>
> Cc: trini at konsulko.com, xypron.glpk at gmx.de, u-boot at lists.denx.de,
> agraf at csgraf.de, marex at denx.de, judge.packham at gmail.com, sr at denx.de,
> rayagonda.kokatanur at broadcom.com
>
> Am 2020-09-24 10:10, schrieb Mark Kettenis:
> >> Date: Thu, 24 Sep 2020 09:33:50 +0200
> >> From: Michael Walle <michael@walle.cc>
> >>
> >> Am 2020-09-23 19:35, schrieb Tom Rini:
> >> > On Wed, Sep 23, 2020 at 07:31:00PM +0200, Heinrich Schuchardt wrote:
> >> >> On 9/23/20 7:14 PM, Tom Rini wrote:
> >> >> > On Wed, Sep 23, 2020 at 07:01:54PM +0200, Mark Kettenis wrote:
> >> >> >>> From: Michael Walle <michael@walle.cc>
> >> >> >>> Date: Wed, 23 Sep 2020 18:45:27 +0200
> >> >> >>>
> >> >> >>> Let the user choose between three different behaviours of the watchdog:
> >> >> >>> (1) Keep the watchdog disabled
> >> >> >>> (2) Supervise u-boot
> >> >> >>> (3) Supervise u-boot and the operating systen (default)
> >> >> >>>
> >> >> >>> Option (2) will disable the watchdog right before handing control to the
> >> >> >>> operating system. This is useful when the OS is not aware of the
> >> >> >>> watchdog. Option (3) doesn't disable the watchdog and assumes the OS
> >> >> >>> will continue servicing.
> >> >> >>
> >> >> >> (3) can't be the default, at least for EFI
> >> >> >>
> >> >> >> The UEFI standard explicitly says that upon calling
> >> >> >> ExitBootServices(), the watchdog timer is disabled.
> >> >> >>
> >> >> >> In general, you can't expect an OS to have support for a particular
> >> >> >> watchdog timer. So (3) only makes sense in cases where U-Boot is
> >> >> >> bundled with an OS image.
> >> >> >
> >> >> > We need to be careful here then. The current and historical / generally
> >> >> > expected behavior is if we've enabled the watchdog we supervise it and
> >> >> > leave it enabled for the OS. Given what UEFI requires I'd like to see
> >> >> > that case handled with a print about disabling the watchdog so it's not
> >>
> >> I agree with "current and historical behavior" but not with "expected
> >> behavior".
> >>
> >> I was thinking about something like
> >>
> >> +choice
> >> + prompt "Watchdog behavior"
> >> + default WATCHDOG_SUPERVISE_U_BOOT if EFI_LOADER
> >> + default WATCHDOG_SUPERVISE_OS if !EFI_LOADER
> >> + depends on WDT
> >>
> >> Unfortunately, EFI_LOADER is default y for any architecture != ARM.
> >> Therefore, it is likely we are changing the behavior of some boards
> >> and I agree this isn't what we want.
> >
> > I think you are misreading that. The following stanza:
> >
> > depends on OF_LIBFDT && ( \
> > ARM && (SYS_CPU = arm1136 || \
> > SYS_CPU = arm1176 || \
> > SYS_CPU = armv7 || \
> > SYS_CPU = armv8) || \
> > X86 || RISCV || SANDBOX)
> >
> > means that EFI_LOADER is onlu ever defined on (newish) ARM, X86, RISCV
> > and SANDBOX. Which makes sense since there is no EFI calling
> > convention defined for other architectures like MIPS and PPC.
>
> I've missed that, but that will just limit the search space.
> Like how can we figure out what board has both EFI_LOADER=y and
> WDT=y set? If there is no such board, then we can use the
> defaults above. If there are such boards we will have to put
> CONFIG_SUPERVISE_OS=y in their defconfig.
>
> If EFI_LOADER would have had no default y, then a simple
> grep for EFI_LOADER=y (and WDT=y) would have been sufficient.

And this is where the conflict of interest surfaces.

From a "running a generic OS" standpoint of view we really want to
have EFI_LOADER enabled on as many ARM/X86/RISCV boards as possible
and want to discourage the sometimes heavy customization that some of
the board vendors have done historically. Such customizations often
break booting a generic OS or at least create inconsistencies tat are
confusing to users of such a generic OS. And WDT=y pretty is such a
customization.

At the same time there obviously is the desire from some vendors to
integrate U-Boot with an OS (which in practice probably always is a
customized Linux kernel). This scenario typically uses "bootm"
instead of the EFI loader and I believe that only in this context the
historic watchdog behaviour makes sense.

Therefore I think it makes sense to always disable any running
hardware watchdog timer when starting an EFI payload, and reenable it
if that payload returns to the bootloader. I don't think printing a
message when doing so makes sense as users of the EFI loader expect
any watchdog timer to be disabled, but printing a message that a
hardware watchdog is being disabled before starting the EFI payload
may be acceptable.

Please note that very few ARM board configurations actually set WDT=y,
so generic OS users may simply not have noticed issues with the
current "policy" of leaving a hardware watchdog running when booting
a generic OS.