From owner-freebsd-arm@freebsd.org Mon Jul 23 15:24:44 2018 Return-Path: Delivered-To: freebsd-arm@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 8F8EE104DD97 for ; Mon, 23 Jul 2018 15:24:44 +0000 (UTC) (envelope-from ian@freebsd.org) Received: from outbound1a.eu.mailhop.org (outbound1a.eu.mailhop.org [52.58.109.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 18E528140E for ; Mon, 23 Jul 2018 15:24:43 +0000 (UTC) (envelope-from ian@freebsd.org) X-MHO-RoutePath: aGlwcGll X-MHO-User: 83408b58-8e8c-11e8-aff6-0b9b8210da61 X-Report-Abuse-To: https://support.duocircle.com/support/solutions/articles/5000540958-duocircle-standard-smtp-abuse-information X-Originating-IP: 67.177.211.60 X-Mail-Handler: DuoCircle Outbound SMTP Received: from ilsoft.org (unknown [67.177.211.60]) by outbound1.eu.mailhop.org (Halon) with ESMTPSA id 83408b58-8e8c-11e8-aff6-0b9b8210da61; Mon, 23 Jul 2018 15:24:39 +0000 (UTC) Received: from rev (rev [172.22.42.240]) by ilsoft.org (8.15.2/8.15.2) with ESMTP id w6NFOcGK013224; Mon, 23 Jul 2018 09:24:38 -0600 (MDT) (envelope-from ian@freebsd.org) Message-ID: <1532359478.1344.142.camel@freebsd.org> Subject: Re: rpi3 and Adafruit GPS Hat From: Ian Lepore To: Per olof Ljungmark , David Cornejo Cc: freebsd-arm Date: Mon, 23 Jul 2018 09:24:38 -0600 In-Reply-To: References: <47f49a55-66b0-1c02-4530-4701a3bd0c43@nethead.se> <20180718170157.GA40221@night.db.net> <7a14173c-cc28-6dc7-3787-a5b77a396b30@nethead.se> <1532357176.1344.130.camel@freebsd.org> Content-Type: text/plain; charset="ISO-8859-1" X-Mailer: Evolution 3.18.5.1 FreeBSD GNOME Team Port Mime-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: freebsd-arm@freebsd.org X-Mailman-Version: 2.1.27 Precedence: list List-Id: "Porting FreeBSD to ARM processors." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 23 Jul 2018 15:24:44 -0000 On Mon, 2018-07-23 at 17:20 +0200, Per olof Ljungmark wrote: > On 07/23/18 16:46, Ian Lepore wrote: > > > > On Mon, 2018-07-23 at 11:40 +0200, Per olof Ljungmark wrote: > > > > > > On 07/23/18 10:46, David Cornejo wrote: > > > > > > > > > > > > this might be a little blasphemous, but for grins I tried an Oncore with > > > > PPS to a GPIO and running the serial through a TTL-USB serial cable and > > > > that seems to work ok. > > > > > > > > there's probably some good reason that this is a bad idea. > > > Depends on what precision you are after, but for lowest possible jitter > > > you need to use the uart, the difference is in magnitudes. > > > > > Technically that may be correct, but it's meaningless. On a usb 1.x > > adapter there may be ~500us of jitter from one measurement to the next. > > On a usb 2.x adapter the jitter drops to typically ~60us. Those values > > are pretty much in the noise for ntpd, which uses a median filter to > > smooth any serious jitter out of the measurements. > > > > Here are some real-world measurements. The pps source for all 3 inputs > > is the same gps-disciplined rubidium oscillator, so all the jitter is > > within the uart, usb hardware, and freebsd drivers. The usb adapters > > are both FTDI chips, which have a fixed latency on reporting a change > > on the DCD pin (pin-change status messages are only delivered once a > > millisecond on ftdi chips). > > > >      remote      refid   st t when poll reach   delay   offset  jitter > > ====================================================================== > > xPPS(0)          .uart.   0 l    6   16  377    0.000    1.097   0.001 > > xPPS(1)          .usb1.   0 l    4   16  377    0.000   -0.051   0.773 > > oPPS(2)          .usb2.   0 l    4   16  377    0.000   -0.001   0.035 > > *dvb.hippie.lan  .GPS.    1 u   12   64  377    1.234    1.296   2.707 > > +utcnist2.colora .NIST.   1 u    1   64  377   13.605    3.940   2.729 > > > > You can see in this case ntpd actually chose the usb2 pps input as the > > system peer. It did so because at startup the clock offset was closer > > than the uart, and the difference in jitter between the two wasn't > > significant, so the ntpd code that prevents clock-hopping chose to > > stick with the peer with the smaller offset. > > > Yes, I was technically correct but of course you are right too - > however, the main problem is not the jitter but rather that I am unable > to switch off the serial console and stop the u-boot loader from > receiving NMEA data. The Adafruit GPS Hat is made to sit right on the Pi > 40-pin header, as you probably know. > > To quote one of the posters in the thread I linked to: > "... I am seriously baffled by how difficult (nearly impossible) it had > been to get rid of the serial console..." > > Unfortunately I am not fluid enough to figure out where to make the > changes, the advices I've seen so far is not applicable to 12-CURRENT in > an easy way. > > And that is also why I wrote bugreport 229976. It may be that all the > folks involved with FreeBSD/ARM are serious developers so they do not > see it as a problem but for me it is, being more on the > application/administration side of things. > > Thanks, > > //per > Unfortunately, I can't help with the rpi part of this, since it's related to the firmware and uboot, and that part of the rpi world has changed drastically since I was involved with it years ago. I know you can build a custom uboot that disables serial console support completely, but I don't know if there's a way to achieve that with the stock uboot. -- Ian