[linux-sunxi] Re: [PATCH 3/3] sunxi: A20-OLinuXino-LIME2: Add i2c2 bus in DTS

[haas@computerlinguist.org (Michael Haas)]

Hello Maxime,

thank you for taking the time to review this patch set.

On 04/02/2016 12:34 PM, Maxime Ripard wrote:
> Hi,
>
> On Fri, Mar 25, 2016 at 08:04:07PM +0100, Michael Haas wrote:
>> The A20 processor provides a third I2C bus on pins PB20 and PB21.
>> The A20-OLinuXino-LIME2 exposes this bus via its GPIO1 port.
>>
>> Olimex also provide a breakout board called the
>> A20-OLinuXino-LIME2-UEXT. This change is required to properly
>> support I2C on the UEXT connector found there.
>>
>> Signed-off-by: Michael Haas <haas@computerlinguist.org>
>
> What are the other options on that pin? Is it something exclusively
> dedicated to i2c on the pin headers and the documentation, or is
> anyone free to use that pin for whatever he wishes?
>
> Thanks,
> Maxime

I assume you are talking about the UEXT connector, not about PB20 and
PB21. These are regular GPIO pins multiplexed with I2C, but no other
function.

Regarding UEXT: the A20-OLinuXino-LIME2-UEXT breakout board supports two
use cases here.

The first one: provide a simple adapter from the 0.05" pin headers on
the olinuxino to the more common world of 0.1" headers. In this use
case, you can plug the A20-OLinuXino-LIME2-UEXT intoany of LCD_CON or
GIO-{1,2,3} and you're good to go.

The second one: here, you want to use UEXT modules provided by Olimex.
It's a simple connector exposing I2C, SPI and UART as mentioned by
Priit. To use UEXT, you have to use GPIO-1 to get the correct pin mapping.

Regarding the official documentation provided by Olimex. The
A20-OLinuXino-Lime2_Rev_c.pdf shows the pins on GPIO-1 as PB20 and PB21.
In the schematics for the breakout board,
A20-OLinuXino-Lime2-UEXT_sch.pdf, the pins for the UEXT connector are
labeled as PB20/SCK and PB21/SDA. This acknowledges the double use.
However, PB20 and PB21 have pull-ups by default according to
A20-OLinuXino-Lime2_Rev_c.pdf,
which makes them a lot more useful as I2C than as GPIO.

What originally prompted me to submit this patch: I bought a breakout
board and a small LCD with UEXT connector together with my board. Then I
found out that it doesn't work out of the box because i2c2 is not
enabled. Since using UEXT is probably common for that particular
vendor, I believe it should be enabled by default.

Michael