From mboxrd@z Thu Jan 1 00:00:00 1970 From: Karl Palsson Subject: Re: [PATCH v2] ARM: dts: rockchip: use internal pull-up resistors on I2C busses Date: Wed, 29 Oct 2014 15:05:36 +0000 Message-ID: <20141029150536.GA3876@palmtree.beeroclock.net> References: <1414492597-13566-1-git-send-email-julien.chauveau@neo-technologies.fr> <2234299.BSD6RSf1dG@diego> <20141029134415.GA3499@palmtree.beeroclock.net> <32904940.RFLKbTnBmv@xq-nb> <5450FAF3.2010208@neo-technologies.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <5450FAF3.2010208@neo-technologies.fr> Sender: linux-kernel-owner@vger.kernel.org To: NEO-Technologies / Julien CHAUVEAU Cc: Max Schwarz , Heiko =?iso-8859-1?Q?St=FCbner?= , Addy Ke , wsa@the-dreams.de, Mark Rutland , "open list:OPEN FIRMWARE AND..." , Russell King , Pawel Moll , Ian Campbell , open list , "open list:ARM/Rockchip SoC..." , Rob Herring , Kumar Gala , "moderated list:ARM/Rockchip SoC..." List-Id: devicetree@vger.kernel.org However, and possibly out of line, but should we consider submitting a = patch to remove the pullups by default for exynos that Doug hinted at? =20 Cheers, Karl P On Wed, Oct 29, 2014 at 03:34:27PM +0100, NEO-Technologies / Julien CHA= UVEAU wrote: > Hi everyone, >=20 > Okay, I understand your opinion. So let's drop my patch in this case. >=20 > Thank you for your comments. >=20 > Julien >=20 >=20 > Le 29/10/2014 15:02, Max Schwarz a =E9crit : > >Hi, > > > >I'll agree with Karl and Doug. If you (as a board vendor/maintainer/= etc) want > >to use I2C, it's *your* responsibility to provide the pullup resisto= rs by > >either including pullup resistors on the board or by enabling the in= ternal > >ones. > >Either way, you should think a moment about the consequences (freque= ncy/trace > >length limitations), which is why I'm also against the pullup-by-def= ault > >behavior. > > > >Also, it's much harder to diagnose effects like Doug is describing (= slightly > >out-of-spec due to internal + external pulls) than the effects you a= re seeing > >without any pullups. With your i2cdetect results my first thought wo= uld have > >been "are there pullups on the bus?". > > > >Cheers, > > Max > > > >Am Mittwoch, 29. Oktober 2014, 13:44:15 schrieb Karl Palsson: > >>I'd be more inclined to have pulls disabled by default, it's more s= tandard > >>with what smaller micros do, but I've no experience with these bigg= er > >>cortex-a parts. It's also the "least surprise" path. If you want = to try > >>and use the onboard pullups, you can specify that in your board fil= e, but > >>for people deliberately selecting pullups for their timing and load > >>expectations, being required to take an extra step to turn off some= thing > >>seems unexpected. > >> > >>If you _want_ to be able to probe an i2c bus for devices added afte= rmarket, > >>on a board that didn't get i2c pull ups because no devices were pla= nned, > >>and you want to turn on the internal pullups for that, I think that= 's > >>something you need to do yourself, not making it a hard default in = the SoC > >>dtsi file. > >> > >>so, if it's off by default, you get this > >> dtsi dts > >>Board1, i2c periphs, designed pullups =3D> off - > >>board2, no peripsh, pulls in case =3D> off - > >>board3, no periphs, forgot pulls, pray=3D> off on > >> > >>If you turn it on by default, sure, it causes no harm in most cases= , but > >>you're no longer getting the values you expect, without having to t= urn off > >>things that are not default anyway. > >> > >>Sincerely, > >>Karl Palsson > >> > >>On Wed, Oct 29, 2014 at 02:17:23PM +0100, Heiko St=FCbner wrote: > >>>Hi Addy, Max, Wolfram, > >>> > >>>after Doug's explanation of disfavour [0] and Julien's subsequent = response > >>>I'm not sure which direction to go. So if possible I'd like to col= lect > >>>some more opinions of people knowing a lot more about i2c internal= s than > >>>myself :-) . > >>> > >>> > >>>Thanks > >>>Heiko > >>> > >>> > >>>[0] http://lists.infradead.org/pipermail/linux-rockchip/2014-Octob= er/000934.html >=20