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

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Date:   Tue, 17 Jul 2018 11:09:20 +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,

> > -	gpio = devm_gpiod_get(dev->dev, "scl", GPIOD_OUT_HIGH);
> > +	gpio = devm_gpiod_get(dev->dev, "scl", GPIOD_OUT_HIGH_OPEN_DRAIN);
> >   	if (IS_ERR(gpio)) {
> >   		r = PTR_ERR(gpio);
> >   		if (r == -ENOENT || r == -ENOSYS)
> > 
>
> This was intentional. The gpio we use to drive the i2c line is implemented in an
> FPGA and signals a buffer attached to the GPIO to drive scl OPEN drain. The GPIO is output
> only.

So, it is not possible to read SCL status then? Hmm, currently a working
get_scl is required...

> The gpio setup can still specify the the GPIO be allocated OPEN drain if someone wishes
> to use a "smarter" gpio.
> 
> So while the scl is open drain, there may be hardware in between that isn't.
> What would the correct way be to deal with that now?

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?

Regards,

   Wolfram


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