On 05/05/2020 22.38, Bruno Thomsen wrote: > Hi Rasmus > > Den tir. 5. maj 2020 kl. 22.07 skrev Alexandre Belloni > <alexandre.bell...@bootlin.com>: >> >> On 05/05/2020 21:54:47+0200, Rasmus Villemoes wrote: >>> Hi Bruno >>> >>> I just noticed your "rtc: pcf2127: add tamper detection support" >>> (03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use >>> case of ours: >>> >>> We rely on the battery switch-over detection to distinguish a powerfail >>> during boot from a PORESET by the external watchdog (in the latter case, >>> the RTC is still powered throughout, meaning there is no battery >>> switch-over event). OTOH, we do not use the tamper detection - in fact, >>> the TS signal is unconnected on our board. >>> >>> We're currently still on 4.19, but we will eventually upgrade to a >>> kernel containing the above commit. So I was wondering if we could >>> figure out a way that would work for both of us - either some CONFIG >>> knob, or perhaps something in the device-tree. Any ideas? >>> >> >> Yes, I was working on a patch series last week allowing to read BF. I'm >> not sure clearing BTSE is your issue but clearing BF is. >> >> I'm going to send it tonight, I'll copy you, let me now if that works >> for you. You can then read BF using the RTC_VL_READ ioctl. The >> RTC_VL_BACKUP_SWITCH flag will be set if a switchover happened. >> The RTC_VL_CLR ioctl can be used to clear the flag. >> >> I think clearing BTSE is still the right thing to do. > > I think your use case is valid and it sounds like Alexandre solution will > solve it as you just need to know if a battery switch-over has happened > not when exactly it happened. > > I can help test the patches too.
Thanks for the quick replies, both. Unfortunately, being able to read BF from linux is not relevant to us - all the handling happens early in the bootloader (including clearing BF, so that we can detect that the previous boot failed only because of power fail - hence whether the linux driver clears BF or not is not relevant). We really just want linux to not touch the bits in CTRL3 at all. Hm, wait. Re-reading the above suggests that BF can get set even if BTSE is not, and a quick experiment shows that is true - I must have misread the data sheet. While I think that's fine for now (currently I only print the time of last switch-over as a diagnostic), I did have some use case in mind for comparing that timestamp to the current time and make decisions based on that. But until I figure out exactly what I want to use it for, and until we actually upgrade to 5.4+, there's no rush. Thanks, Rasmus