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

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Message-ID: <5450FAF3.2010208@neo-technologies.fr>
Date:	Wed, 29 Oct 2014 15:34:27 +0100
From:	NEO-Technologies / Julien CHAUVEAU 
	<julien.chauveau@...-technologies.fr>
To:	Max Schwarz <max.schwarz@...ine.de>,
	Karl Palsson <karlp@...ak.net.au>
CC:	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

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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