Re: Question regarding informing MMC subsystem about upcoming power loss.

[Christian Loehle <christian.loehle@arm.com>]

On 05/03/2024 15:22, Patryk wrote:
> Hi,
> I'm wondering - is there a way/interface that I could use to notify
> the Linux kernel that the power is about to go down? I'm thinking
> about SD cards in particular and how to make them more immune to
> unexpected power loss, and I've found that some SD cards support
> something like "mmc_poweroff_notify", which is called in the
> mmc_suspend function.

Immune to what exactly?
- (User) data loss of data that is still in the cache of the sd card?
(That would be saved by a timely cache flush)
- (User) data loss of data that should've been written to flash already?
- SD cards breaking?
- Explosions?

(Anything but the first IMO should be fixed by procurement and not by the
kernel, but I'm not sure if that's consensus).

> Assuming that I have the information about upcoming power loss
> (provided by e.g. external interrupt, PSU voltage monitoring etc) how
> should I pass this information to the Kernel so that it will try to
> clean up resources - in particular MMC subsystem?
>
> Let me show some background - we have a SoM which has a lot of
> components. On the SoM we have a PSU which operates on 24V voltage. We
> would like to monitor the PSU's voltage and when we detect that the
> voltage has dropped from 24V to 12V this is highly likely an
> unexpected power loss/cut, therefore we would like to inform the
> Kernel (in particular mmc subsystem) that the power loss is coming. I
> can imagine something like this - we have a driver that does voltage
> measurements or waits for an external interrupt (triggered when PSU's
> voltage drops from 24V to 12V) and then calls some functions that will
> allow the Kernel to do some cleanups, but the question is what are
> these functions? Are there any?

There was a discussion, currently there aren't really, but it depends
on the scenario you're afraid of. Just issuing a cache flush might be fine.
https://www.phoronix.com/news/Linux-Priority-Based-Shutdown
https://lore.kernel.org/lkml/2023112403-laxative-lustiness-6a7f@gregkh/T/

One of the fundamental questions IMO remains: How much time do we actually
have between being notified?

Best Regards.
Christian