The bug scenario is rather simple: /dev/rtc existed with that config in
prior kernels and now, after "make oldconfig" it does not exist, it's a
plain regression and must be fixed.
that's wrong - 'make oldconfig' must work smoothly and in an expected
way.
If you argue that the new kernel should come up with a non-existent
/dev/rtc, then you are wrong and you go against established regression
handling policies of the kernel. Smooth migration via "make oldconfig"
is a must, otherwise we'd lose testers and users.
The old /dev/rtc might indeed have been unbelievably "bad" in various
ways and you dont even want to think about that code now, but it's your
code now that is used so you might as well think about the other 98% of
users who used the old /dev/rtc with old kernels. (not because they
wanted to use bad code, but because simply that was the default)
Ingo
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