On Tue, 15 Jun 2021, Greg Kroah-Hartman wrote:

quoted

Add a hook for `uart_report_port' to let serial ports report extra 
properties beyond `irq' and `base_baud'.  Use it with the 8250 backend 
to report extra baud rates supported above the base rate for ports with 
the UPF_MAGIC_MULTIPLIER property, so that people have a way to find out 
that they are supported with their system, e.g.:

[...]

Ick, really?  What relies on this print message?  Why do we need a whole
new uart port hook for this?

 As I noted, perhaps too briefly, in the commit description (see the last 
sentence above) people need to be made aware of the extra baud rates above 
`base_baud' supported, or otherwise they'll have no way to figure out they 
can use them.

 Reporting tweaked `base_baud' would I think cause confusion from the 
inconsistency with the TIOCGSERIAL/TIOCSSERIAL ioctls (e.g. `setserial'), 
and the sysfs flags:

$ cat /sys/class/tty/ttyS[0-2]/flags
0x10010040
0x10010040
0x90000040
$ 

(here from a Malta board) are IMO too obscure for anyone to figure this 
out (bit 16 means the two extra baud rates are supported).

 As explained with 1/5 we could set `base_baud' to 460800 instead and 
hardcode bit 15 of the divisor to 1, relying on undocumented Super I/O IC 
behaviour, but that would require more exhaustive verification than I am 
able to do with just a single board and IC type and revision.

Isn't there some other way for your specific variant to print out
another message if you really want to do something "odd" like this?

 There's always another way. :)  How about?

serial8250.0: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
serial8250.0: ttyS0 extra baud rates supported: 230400, 460800

And you did not document what your new change did anywhere in the tree,
so people are going to be confused.

 We've been somewhat terse about things, but you're probably right here.  
Sorry about that.

I've taken the other patches here, but not this one.

 Thanks.  I've posted an alternative printing a message like above, with 
some commentary.  Let me know if that makes you feel more convinced.

  Maciej