linux-kernel - RE: [PATCH] i2c-gpio.c: correct logic of pdata->scl_is_open

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Message-ID: <ADE657CA350FB648AAC2C43247A983F001F39DF77673@AUSP01VMBX24.collaborationhost.net>
Date:	Tue, 1 Nov 2011 11:29:32 -0500
From:	H Hartley Sweeten <hartleys@...ionengravers.com>
To:	"Voss, Nikolaus" <N.Voss@...nmann.de>,
	'Håvard Skinnemoen' <hskinnemoen@...il.com>
CC:	'Linux Kernel' <linux-kernel@...r.kernel.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH] i2c-gpio.c: correct logic of pdata->scl_is_open_drain

On Tuesday, November 01, 2011 1:02 AM, Voss, Nikolaus wrote:
>> It is never correct to use push-pull I/O for i2c, so the flag does not
>> specify the desired behavior of the driver, it specifies what the
>> hardware has been configured to do so that the driver can choose the
>> cheapest way to do open-drain I/O.
>
> Ok, seems rather logical after all.
>
>> 
>> And even if you could argue that the flag should be inverted, it has
>> had the same meaning since the driver was introduced several years
>> ago, so changing it now will break every single platform which
>> currently uses i2c-gpio.
>
> I completely agree.
>

Just for the record...

The GPIO pins used for I2C on the ep93xx platform can be configured as
open-drain I/O pins.  When they are, and external pull-ups are applied
to the signals, the I2C bus works correctly when the "is_open_drain"
flags are set.  If the pull-ups are missing the bus does not work.
If the "is_open_drain" flags are not set, the I2C bus works correctly
regardless of the existence of the pull-ups.

Based on that I think the i2c-gpio driver's use of the "is_open_drain"
flags is correct.

Regards,
Hartley
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