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

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Message-ID: <ab43569c-6488-12a6-823-3ef09f2849d@linux.intel.com>
Date:   Mon, 7 Mar 2022 11:48:01 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Lukas Wunner <lukas@...ner.de>
cc:     linux-serial <linux-serial@...r.kernel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <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 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.