[RFC PATCH 6/7] serial: General support for multipoint addresses

[Ilpo Järvinen]

On Sun, 6 Mar 2022, Lukas Wunner wrote:

> On Wed, Mar 02, 2022 at 11:56:05AM +0200, Ilpo Järvinen wrote:
> 
> > This change is necessary for supporting devices with RS485
> > multipoint addressing [*].
> 
> If this is only used with RS485, why can't we just store the
> addresses in struct serial_rs485 and use the existing TIOCSRS485
> and TIOCGRS485 ioctls?  There's 20 bytes of padding left in
> struct serial_rs485 which you could use.  No need to add more
> user-space ABI.

It could if it is agreed that serial multipoint addressing is just
a thing in RS-485 and nowhere else? In that case, there is no point
in adding more generic support for it.

> > [*] Technically, RS485 is just an electronic spec and does not
> > itself specify the 9th bit addressing mode but 9th bit seems
> > at least "semi-standard" way to do addressing with RS485.
> 
> Is 9th bit addressing actually used by an Intel customer or was
> it implemented just for feature completeness? I think this mode
> isn't used often (I've never seen a use case myself), primarily
> because it requires disabling parity.

On what basis? ...The datasheet I'm looking at has a timing diagram 
with both D8 (9th bit) and parity so I think your information must be
incorrect. I don't have direct contacts with customers but I'm told
it's important for other org's customers.


-- 
 i.