@burrbull
What am I doing wrong here?

Without this fix:

$ cargo c --features stm32h7b3,rt,defmt
[...]
error[E0599]: no method named `isr` found for struct `Periph<stm32h7::stm32h7b3::rtc::RegisterB
lock, 1476411392>` in the current scope
   --> src/rtc.rs:765:26
    |
765 |                 self.reg.isr().modify(|_, w| w.wutf().clear_bit());
    |                          ^^^
    |
help: there is a method `icsr` with a similar name
    |
765 -                 self.reg.isr().modify(|_, w| w.wutf().clear_bit());
765 +                 self.reg.icsr().modify(|_, w| w.wutf().clear_bit());
[...]

For reference I can see ICSR here

Or perhaps this should not even be done in the first place. There does seem to be quite a bit of differences between the RTC in RM0455 as compared to RM0433. In RM0433 we have the battery backed ram in RTC_BKP[0-31]R but I can not seem to find that in RM0455 even though it is mentioned as a feature from what I can tell.

You should not do this on PAC level. H7A/B have newer RTC.
ISR and ICSR work different.

Ok, thank you. Makes sense.

Do you know how one is supposed to access the battery backed sram from the rm0455 devices? Is that supposed to be accessable throught the pac as with the other devices or is the hal supposed to use some other method like raw pointers or arrays in linker sections?

How about this? Do you think this should be done? From what I can tell, the SSN in RM0455 4.9.4 seems to do the same thing as SNB from rm0433 4.9.4, however at a different bit position and width

rm0455

Bits 12:6 SSN1: Bank 1 sector erase selection number
These bits are used to select the target sector for an erase operation (they are unused
otherwise). SSN1 can be programmed only when LOCK1 is cleared to 0.
0x00: Sector 0 of user flash bank 1 selected
0x01: Sector 1 of user flash bank 1 selected
..

rm0433

Bits 10:8 SNB1: Bank 1 sector erase selection number
These bits are used to select the target sector for a sector erase operation (they are unused
otherwise). SNB1 can be programmed only when LOCK1 is cleared to 0.
000: sector 0 of bank 1
001: sector 1 of bank 1
...

Let me know if you want me to revert this too :)

Not sure. I'm fine with both variants.