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

[NEO-Technologies / Julien CHAUVEAU]

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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