linux-kernel - Re: [RFC PATCH 6/7] serial: General support for multipoint addresses

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Date:   Sun, 6 Mar 2022 20:40:01 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-serial@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-api@...r.kernel.org, Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Arnd Bergmann <arnd@...db.de>,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH 6/7] serial: General support for multipoint addresses

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