I have updated the patch to a new v4 in order to remove the race in
sysfs file creation and add sysfs file description to a Documentation
directory. But still the patch creates the sysfs file separately for
each serial TTY device by assigning attribute_groups to the struct
device and not for the whole driver at once as Greg advised because I
was unable to figure out how to do that (even though I tried pretty
hard). Does it make sense like this? Or do you have any hint for a
better way to do it, please?

I'm fine with it - dunno about GregKH. Given the way tty 'drivers' and
tty devices fit together I'm not 100% sure it makes sense to do it per
driver. Plus once we have the basis we can fix a lot of the detail
afterwards.

Alan