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Date:   Tue, 10 Oct 2023 18:19:59 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Oleksij Rempel <o.rempel@pengutronix.de>
Cc:     Liam Girdwood <lgirdwood@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>, kernel@pengutronix.de,
        linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
        Naresh Solanki <naresh.solanki@9elements.com>,
        zev@bewilderbeest.net, Sebastian Reichel <sre@kernel.org>,
        linux-pm@vger.kernel.org
Subject: Re: [PATCH v1 3/3] regulator: fixed: forward under-voltage events
Message-ID: <c2ee404d-d07f-42c6-b5ba-41659773e8eb@sirena.org.uk>
References: <20231010085906.3440452-1-o.rempel@pengutronix.de>
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On Tue, Oct 10, 2023 at 02:55:31PM +0200, Oleksij Rempel wrote:

> The hardware I am working with has an under-voltage sensor on the 24V
> supply regulator and some backup capacitors to run SoC for 100ms. I want
> to forward under-voltage events across a chain of different regulators
> to a designated consumer. For instance, to the mmc driver, enabling it
> to initiate shutdown before power loss occurs.  Additionally, a bit can
> be set in the volatile memory of a scratch pad in an RTC clock to record
> sudden power loss, which can be checked on the next system start.

So it sounds like the underlying need is to flag the notifications from
one of the regulators as being system wide and then take action based on
those notifications somewhere basically disconnected?  That does seem
like a good use case.

The MMC doesn't specifically care that it is handling a regulator
notification, it more wants to know that the system is dying and doesn't
really care how we figured that out so if we can hook it into a system
level notificaiton it'd be happy and would also be able to handle other
critical faults.  I would have thought that we should have some
mechanisms for this already for RAS type stuff but I'm drawing a blank
on what it actually is if there is an existing abstraction.  It could
potentially go through userspace though there's latency concerns there
which might not be ideal, there should at least be some policy for
userspace.

For the regulator itself we probably want a way to identify regulators
as being system critical so they start notifying.  It would be tempting
to just do that by default but that would likely cause some issues for
example with regulators for things like SD cards which are more likely
to get hardware problems that don't comprimise the entire system.  We
could do that with DT, either a property or some sort of runtime
consumer, but it might be better to have a control in sysfs that
userspace can turn on?  OTOH the ability do something about this depends
on specific hardware design...

I've copied in Sebastian since this sounds like the sort of thing that
power supplies might have some kind of handling for, or at least if we
need to add something we should make it so that the power supplies can
be joined up to it.  I do see temperature and capacity alerts in the
sysfs ABI for power supplies, but nothing for voltage.

I've also coped in Naresh and Zev who've been discussing something
vaugely similar with userspace notifications for the userspace consumer
- it's not the same thing given that you don't specifically need
userspace to be involved here but it feels like it might have something
of a similar shape, or at least there might be some shared interest.

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