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Date: Thu, 10 Mar 2022 16:09:30 +0200 From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com> To: Lukas Wunner <lukas@...ner.de> Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 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>, 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 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
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