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

From: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>
To: Lukas Wunner <lukas@wunner.de>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>,
	linux-sh@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	Max Filippov <jcmvbkbc@gmail.com>, Rich Felker <dalias@libc.org>,
	Paul Mackerras <paulus@samba.org>,
	sparclinux@vger.kernel.org, linux-api@vger.kernel.org,
	Jiri Slaby <jirislaby@kernel.org>,
	linux-arch@vger.kernel.org,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Helge Deller <deller@gmx.de>,
	linux-doc@vger.kernel.org,
	linux-serial <linux-serial@vger.kernel.org>,
	Matt Turner <mattst88@gmail.com>,
	linux-xtensa@linux-xtensa.org, Arnd Bergmann <arnd@arndb.de>,
	Johan Hovold <johan@kernel.org>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Richard Henderson <rth@twiddle.net>,
	Chris Zankel <chris@zankel.net>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	linux-parisc@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-mips@vger.kernel.org, linux-alpha@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [RFC PATCH 6/7] serial: General support for multipoint addresses
Date: Mon, 7 Mar 2022 11:48:01 +0200 (EET)	[thread overview]
Message-ID: <ab43569c-6488-12a6-823-3ef09f2849d@linux.intel.com> (raw)
In-Reply-To: <20220306194001.GD19394@wunner.de>

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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.