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
