linux-kernel - Re: [PATCH/RFT 1/6] i2c: designware: use open drain for recovery GPIO

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Message-ID: <20180724072637.smhlrx4kpyh6hvwa@ninjato>
Date:   Tue, 24 Jul 2018 09:26:37 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Phil Reid <preid@...ctromag.com.au>
Cc:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        linux-i2c@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        kernel@...gutronix.de,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFT 1/6] i2c: designware: use open drain for recovery GPIO

Hi Phil,

> > So, it is not possible to read SCL status then? Hmm, currently a working
> > get_scl is required...
...
> > Well, I don't know much about this IP core and how/where it is used. I
> > just wonder what happens if another user comes along using an
> > open-drain GPIO. Is that possible?
> > 
> > I assume it is the same with SDA? Non open-drain? Output only?
> > 
> 
> Just had a closer look at how it's setup here.
> Maybe the following helps.

Thanks for the detailed explanation. I am just afraid it is a litle too
detailed for me. I am not sure if I can read it correctly:

When you read the SCL/SDA GPIO, does it return the true state of the
SCL/SDA line or does it just reflect the value it was set to output?

> There's no concept of HiZ internally in the FPGA.

Which probably means SDA is to be treated the same as SCL -> push/pull.

> If there was some kinda of OpenDrain gpio driver that modelled a FET
> driven by a push pull GPIO I guess it could be made to work.

Still, that sounds quite unlikely to me, so we can for now assume that
all designware users will have push/pull?

Disclaimer: I have zero experience with this core, I don't know how hard
it is to modify or which versions are out there.

Thanks for your help,

   Wolfram

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