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

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Date:   Fri, 9 Apr 2021 07:08:02 +0000
From:   "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
To:     "keescook@...omium.org" <keescook@...omium.org>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>
CC:     "agross@...nel.org" <agross@...nel.org>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        linux-power <linux-power@...rohmeurope.com>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        "lgirdwood@...il.com" <lgirdwood@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>
Subject: Re: [PATCH v4 3/7] regulator: IRQ based event/error notification
 helpers

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@...rohmeurope.com> 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@...rohmeurope.com> wrote:
> > > > > On Wed, 2021-04-07 at 01:44 +0300, Andy Shevchenko wrote:
> > > > > > On Tuesday, April 6, 2021, Matti Vaittinen <
> > > > > > matti.vaittinen@...rohmeurope.com> 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?

Best Regards
	Matti Vaittinen