linux-kernel - Re: [PATCH v2] ARM: dts: rockchip: use internal pull-up resistors on I2C busses

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Message-ID: <20141029150536.GA3876@palmtree.beeroclock.net>
Date:	Wed, 29 Oct 2014 15:05:36 +0000
From:	Karl Palsson <karlp@...ak.net.au>
To:	NEO-Technologies / Julien CHAUVEAU 
	<julien.chauveau@...-technologies.fr>
Cc:	Max Schwarz <max.schwarz@...ine.de>,
	Heiko Stübner <heiko@...ech.de>,
	Addy Ke <addy.ke@...k-chips.com>, wsa@...-dreams.de,
	Mark Rutland <mark.rutland@....com>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	"moderated list:ARM/Rockchip SoC..." 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] ARM: dts: rockchip: use internal pull-up resistors on
 I2C busses


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?  

Cheers,
Karl P


On Wed, Oct 29, 2014 at 03:34:27PM +0100, NEO-Technologies / Julien CHAUVEAU wrote:
> Hi everyone,
> 
> Okay, I understand your opinion. So let's drop my patch in this case.
> 
> Thank you for your comments.
> 
> Julien
> 
> 
> Le 29/10/2014 15:02, Max Schwarz a écrit :
> >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 resistors by
> >either including pullup resistors on the board or by enabling the internal
> >ones.
> >Either way, you should think a moment about the consequences (frequency/trace
> >length limitations), which is why I'm also against the pullup-by-default
> >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 are seeing
> >without any pullups. With your i2cdetect results my first thought would 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 standard
> >>with what smaller micros do, but I've no experience with these bigger
> >>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 file, but
> >>for people deliberately selecting pullups for their timing and load
> >>expectations, being required to take an extra step to turn off something
> >>seems unexpected.
> >>
> >>If you _want_ to be able to probe an i2c bus for devices added aftermarket,
> >>on a board that didn't get i2c pull ups because no devices were planned,
> >>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 => off     -
> >>board2, no peripsh, pulls in case     => off     -
> >>board3, no periphs, forgot pulls, pray=> 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 turn off
> >>things that are not default anyway.
> >>
> >>Sincerely,
> >>Karl Palsson
> >>
> >>On Wed, Oct 29, 2014 at 02:17:23PM +0100, Heiko Stübner 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 collect
> >>>some more opinions of people knowing a lot more about i2c internals than
> >>>myself :-) .
> >>>
> >>>
> >>>Thanks
> >>>Heiko
> >>>
> >>>
> >>>[0] http://lists.infradead.org/pipermail/linux-rockchip/2014-October/000934.html
> 
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