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

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Lukas Wunner <lukas@wunner.de>
Cc: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>,
	linux-serial <linux-serial@vger.kernel.org>,
	"Jiri Slaby" <jirislaby@kernel.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Johan Hovold" <johan@kernel.org>,
	"Heikki Krogerus" <heikki.krogerus@linux.intel.com>,
	linux-api@vger.kernel.org, "Richard Henderson" <rth@twiddle.net>,
	"Ivan Kokshaysky" <ink@jurassic.park.msu.ru>,
	"Matt Turner" <mattst88@gmail.com>,
	linux-alpha@vger.kernel.org,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
	linux-mips@vger.kernel.org,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	"Helge Deller" <deller@gmx.de>,
	linux-parisc@vger.kernel.org,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	linuxppc-dev@lists.ozlabs.org,
	"Yoshinori Sato" <ysato@users.sourceforge.jp>,
	"Rich Felker" <dalias@libc.org>,
	linux-sh@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	sparclinux@vger.kernel.org, "Chris Zankel" <chris@zankel.net>,
	"Max Filippov" <jcmvbkbc@gmail.com>,
	linux-xtensa@linux-xtensa.org, "Arnd Bergmann" <arnd@arndb.de>,
	linux-arch@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [RFC PATCH 6/7] serial: General support for multipoint addresses
Date: Thu, 10 Mar 2022 16:09:30 +0200	[thread overview]
Message-ID: <YioGmu+KC9WT0KoG@smile.fi.intel.com> (raw)
In-Reply-To: <20220309190521.GA9832@wunner.de>

On Wed, Mar 09, 2022 at 08:05:21PM +0100, 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.  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.

In my student years I have an exercise to use 9-bit addressing mode on RS232.
Obviously I forgot all of the details, but I remember that that has a practical
application.

-- 
With Best Regards,
Andy Shevchenko