Re: [PATCH] xen/rpi4: implement watchdog-based reset

[Julien Grall <julien@xxxxxxx>]

On 04/06/2020 12:59, Corey Minyard wrote:
> On Thu, Jun 04, 2020 at 09:15:33AM +0100, Julien Grall wrote:
> > Hi,
> >
> > On 04/06/2020 01:15, Corey Minyard wrote:
> > > On Wed, Jun 03, 2020 at 03:31:56PM -0700, Stefano Stabellini wrote:
> > > > Touching the watchdog is required to be able to reboot the board.
> > > >
> > > > The implementation is based on
> > > > drivers/watchdog/bcm2835_wdt.c:__bcm2835_restart in Linux.
> > > Ah, I was looking at this just today, as it had been annoying me
> > > greatly.  This works for me, so:
> > >
> > > Tested-by: Corey Minyard <cminyard@xxxxxxxxxx>
> > >
> > > However, I was wondering if it might be better to handle this by failing
> > > the operation in xen and passing it back to dom0 to do.  On the Pi you
> > > send a firmware message to reboot, and that seems like too much to do in
> > > Xen, but it would seem possible to send this back to dom0.
> > I don't think this is possible in the current setup. Xen will usually
> > restart the platform if Dom0 requested a clean reboot or crashed. So the
> > domain wouldn't be in state to service such call.
> Ok, I hadn't looked at Xen yet, I didn't know how much shutdown of dom0
> happens on a reset.
>
> > > Just a
> > > thought, as it might be a more general fix for other devices in the same
> > > situation.
> > What are the devices you have in mind?
> Nothing in particular, but other systems might have the same issue.  I
> guess you have ACPI implemented on x86 already.  It just seemed that
> Linux already has to be able to do this, and passing the buck back there
> might be a more general solution.
I don't really see how you can make a generic solution here. Each 
devices may have a different way to act.
For anything related to power control, the right solution is to 
implement PSCI/SCMI in your firmware so you don't need to rely on Dom0 
(which may not exist) for such thing.
Cheers,

--
Julien Grall