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

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Date:   Thu, 10 Mar 2022 14:29:21 +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 Wed, 9 Mar 2022, Lukas Wunner wrote:

> On Mon, Mar 07, 2022 at 11:48:01AM +0200, Ilpo Järvinen wrote:
> > 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.
> 
> It's just that the above-quoted sentence in the commit message
> specifically mentions RS485.

That sentence is just to justify why addressing mode is needed,
not to take a stance on whether it is only used with RS485 or not.

> If you intend to use it with RS232
> as well, that should be made explicit, otherwise one wonders why
> it wasn't integrated into struct serial_rs485.
> 
> I have no idea how common 9th bit addressing mode is with RS232.
> Goggle turns up links saying it's mainly used with RS485, "but also
> RS232".  Since RS232 isn't a bus but a point-to-point link,
> 9th bit addressing doesn't seem to make as much sense.

While I don't know any better, I can image though that with an 
RS232-to-RS485 converter, it could make some sense.

If I put them back to serial_rs485 / rs485 config, it's basically just 
where I initially started from with this patchset (offlist).


-- 
 i.