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

From: Lukas Wunner <[email-masked]>
Date: Sun, 6 Mar 2022 20:40:01 +0100

On Wed, Mar 02, 2022 at 11:56:05AM +0200, Ilpo Järvinen wrote:
> This patch adds generic support for serial multipoint
> addressing. Two new ioctls are added. TIOCSADDR is used to

Nit:  "This patch adds..." is superfluous.  Just write "Add ..."
in imperative mood.

> 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.

> [*] 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.

Thanks,

Lukas