Re: [PATCH 1/1] efi_loader: stop watchdogs in ExitBootServices()

From: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
To: Simon Glass <sjg@chromium.org>, Tom Rini <trini@konsulko.com>
Cc: Etienne Carriere <etienne.carriere@linaro.org>,
	Heinrich Schuchardt <heinrich.schuchardt@canonical.com>,
	u-boot@lists.denx.de, Andre Przywara <andre.przywara@arm.com>,
	Ilias Apalodimas <ilias.apalodimas@linaro.org>
Subject: Re: [PATCH 1/1] efi_loader: stop watchdogs in ExitBootServices()
Date: Fri, 3 Feb 2023 08:30:19 +0100	[thread overview]
Message-ID: <22478c7f-ffa0-0bf7-5473-0ba1ee7478c3@prevas.dk> (raw)
In-Reply-To: <CAPnjgZ0noUFUDnfQd6xV=WfbKjaW4UVziNUsQvU-FFD7LVtvQw@mail.gmail.com>

On 03/02/2023 03.15, Simon Glass wrote:
> Hi Tom,
> 
> On Thu, 2 Feb 2023 at 10:22, Tom Rini <trini@konsulko.com> wrote:
>>

>> Honestly, not really? Some good number of SoCs will start the watchdog
>> in ROM and these are also the ones that don't allow you to turn it off.
> 
> I hope not, that sounds really risky. How would you debug such a platform?

_Every single_ custom piece of industrial (as opposed to consumer-grade)
hardware I've worked on as a consultant has had an external,
always-running, gpio-petted watchdog. It's simply just something that
the hardware designers include, and in some cases that's even due to
certification requirements. So an always-running, cannot-be-turned-off,
watchdog is a real thing, in real hardware, and if specs don't account
for that, well, the spec is just paper, and we can ignore it.

As for debugging and bringup, I've seen various solutions (depending on
the actual watchdog chip). Usually there's some way to place a jumper
that will either feed the watchdog from some, say, 32kHz output from an
RTC or elsewhere, or place a jumper to pull up/pull down some
enable/disable pin to the watchdog chip. IOW, when you have physical
access to the PCB lying on your desk, you can disable the watchdog, but
there's no way to do that in the field or in production.

Rasmus