Hello!
I'm not sure that this is what is meant by "Clock source" in struct timex.h.
STA_CLK can take only two values ("A" and "B"), and is read-only.
It was added in commit ee9851b218b8bafa22942b5404505ff3d2d34324
(kernel 2.6.25) at the same time MOD_CLKA and MOD_CLKB were removed.
> tree 581f455
This is a patch to the adjtimex.2 file that add more
detail about clock source in man page which is asked to be fixed.
I got this information from standard document RFC5909 which is
available at https://tools.ietf.org/html/rfc5905#appendix-A.5.5.1
Signed-off-by: Puni
