Re: [PATCH v4 3/7] regulator: IRQ based event/error notification helpers

[Matti Vaittinen]

On Fri, 2021-04-09 at 10:08 +0300, Matti Vaittinen wrote:
> On Thu, 2021-04-08 at 20:20 -0700, Kees Cook wrote:
> > On Wed, Apr 07, 2021 at 03:50:15PM +0300, Andy Shevchenko wrote:
> > > On Wed, Apr 7, 2021 at 12:49 PM Vaittinen, Matti
> > > <Matti.Vaittinen@xxxxxxxxxxxxxxxxx> wrote:
> > > > On Wed, 2021-04-07 at 12:10 +0300, Andy Shevchenko wrote:
> > > > > On Wed, Apr 7, 2021 at 8:02 AM Matti Vaittinen
> > > > > <matti.vaittinen@xxxxxxxxxxxxxxxxx> wrote:
> > > > > > On Wed, 2021-04-07 at 01:44 +0300, Andy Shevchenko wrote:
> > > > > > > On Tuesday, April 6, 2021, Matti Vaittinen <
> > > > > > > matti.vaittinen@xxxxxxxxxxxxxxxxx> wrote:
> > > > > > > > +       BUG();
> > > > > > > > +}
> > 
> > This, though, are you sure you want to use BUG()? Linus gets upset
> > about
> > such things:
> > https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on
> > 
> 
> I see. I am unsure of what would be the best action in the regulator
> case we are handling here. To give the context, we assume here a
> situation where power has gone out of regulation and the hardware is
> probably failing. First countermeasure to protect what is left of HW
> is
> to shut-down the failing regulator. BUG() was called here as a last
> resort if shutting the power via regulator interface was not
> implemented or working.
> 
> Eg, we try to take what ever last measure we can to minimize the HW
> damage - and BUG() was used for this in the qcom driver where I stole
> the idea. Judging the comment related to BUG() in asm-generic/bug.h
> 
> /*
>  * Don't use BUG() or BUG_ON() unless there's really no way out; one
>  
> * example might be detecting data structure corruption in the middle
>  *
> of an operation that can't be backed out of.  If the (sub)system
>  * can
> somehow continue operating, perhaps with reduced functionality,
>  * it's
> probably not BUG-worthy.
>  *
>  * If you're tempted to BUG(), think
> again:  is completely giving up
>  * really the *only* solution?  There
> are usually better options, where
>  * users don't need to reboot ASAP and
> can mostly shut down cleanly.
>  */
> https://elixir.bootlin.com/linux/v5.12-rc6/source/include/asm-generic/bug.h#L55
> 
> this really might be valid use-case.
> 
> To me the real question is what happens after the BUG() - and if
> there
> is any generic handling or if it is platform/board specific? Does it
> actually have any chance to save the HW?
> 
> Mark already pointed that we might need to figure a way to punt a
> "failing event" to the user-space to initiate better "safety
> shutdown".
> Such event does not currently exist so I think the main use-case here
> is to do logging and potentially prevent enabling any further actions
> in the failing HW.
> 
> So - any better suggestions?
> 

Maybe we should take same approach as is taken in thermal_core? Quoting
the thermal documentation:

"On an event of critical trip temperature crossing. Thermal
framework             
allows the system to shutdown gracefully by calling
orderly_poweroff().          
In the event of a failure of orderly_poweroff() to shut down the
system          
we are in danger of keeping the system alive at undesirably
high                 
temperatures. To mitigate this high risk scenario we program a
work              
queue to fire after a pre-determined number of seconds to
start                  
an emergency shutdown of the device using the
kernel_power_off()                 
function. In case kernel_power_off() fails then
finally                          
emergency_restart() is called in the worst case."

Maybe this 'hardware protection, in-kernel, emergency HW saving
shutdown' - logic, should be pulled out of thermal_core.c (or at least
exported) for (other parts like) the regulators to use?

I don't like the idea relying in the user-space to be in shape it can
handle the situation. I may be mistaken but I think a quick action
might be required. Hence the in-kernel handling does not sound so bad
to me.

I am open to all education and suggestions. Meanwhile I am planning to
just convert the BUG() to WARN(). I don't claim I know how BUG() is
implemented on each platform - but my understanding is that it does not
guarantee any power to be cut but just halts the calling process(?). I
guess this does not guarantee what happens next - maybe it even keeps
the power enabled and end up just deadlocking the system by reserved
locks? I think thermal guys have been pondering this scenario for
severe temperature protection shutdown so I would like to hear your
opinions.


Best Regards
Matti Vaittinen