From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rob Herring Subject: Re: Serial ports with non connected RTS/CTS Date: Mon, 4 Apr 2016 00:16:33 -0500 Message-ID: <20160404051633.GP17806@rob-hp-laptop> References: <56A89C48.1040604@parkeon.com> <57009DD6.2050006@hurleysoftware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <57009DD6.2050006-WaGBZJeGNqdsbIuE7sb01tBPR1lH4CV8@public.gmane.org> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Peter Hurley Cc: Martin Fuzzey , "linux-serial-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" List-Id: devicetree@vger.kernel.org On Sat, Apr 02, 2016 at 09:36:38PM -0700, Peter Hurley wrote: > On 01/27/2016 02:30 AM, Martin Fuzzey wrote: > > Hi all, > > > > currently, at least for 8250 based serial ports using of_serial, there is no way of indicating that the RTS / CTS lines, although supported by the UART, are not actually connected on the board. > > > > This means that userspace needs to know about this and not use the CRTSCTS termios flag (otherwise no data is sent). > > > > This is currently the case for Debian stattach for example (nettools 1.60) which unconditionally uses CRTSCTS (but the busybox version of slattach has a "-F" option that can be used to disable CRTSCTS). > > > > Would it not be better to allow this situation to be described in the device tree? > > > > Such an option already exists for the imx UART (""fsl,uart-has-rtscts") > > > > of_serial already has the "auto-flow-control" DT property but, as stated in the documentation: > > "The driver is allowed to detect support for the capability even without this property." > > > > In the past there was a (since reverted) property "has-hw-flow-control" > > See > > 06aa82e "serial: uart: add hw flow control support configuration" > > a6eec92 " Revert "serial: uart: add hw flow control support configuration" > > > > However that just provided another (redundant) way of *activating* hardware flow control and did not allow it to be disactivated. > > > > Would a new DT property "no-rtscts" to do this be acceptable? Yes. It's really a "the h/w designers left CTS tied/floating inactive by mistake" flag. What is done with RTS doesn't really matter. So just "no-cts" perhaps. > > I think it would have to be a negative property to avoid breaking old device trees. Agreed. Rob -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html