linux-kernel - Re: [PATCH v2 2/7] gpio: regmap: Add configurable dir/value order

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Date:   Tue, 18 May 2021 13:39:12 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Michael Walle <michael@...le.cc>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-gpio@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-kernel@...r.kernel.org,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>
Subject: Re: [PATCH v2 2/7] gpio: regmap: Add configurable dir/value order

Hi Andrew,

On Tue, 2021-05-18 at 03:40 +0200, Andrew Lunn wrote:
> On Mon, May 17, 2021 at 09:28:04PM +0200, Sander Vanheule wrote:
> > GPIO chips may not support setting the output value when a pin is
> > configured as an input
> 
> Could you describe what happens with the hardware you are playing
> with. Not being able to do this means you will get glitches when
> enabling the output so you should not use these GPIOs with bit banging
> busses like i2c.

As I was testing this driver, I noticed that output settings for GPIO LEDs,
connected to the RTL8231, weren't being properly set. The actual LED brightness
didn't correspond to the one reported by sysfs. Changing the operation order
fixed this.

However, the vendor code uses I2C bitbanging quite extensively on these chips,
so I decided to have another look.

>From u-boot on my device, I can manipulate the RTL8231 registeres relatively
easily. I performed the following short tests:
 * Set pin to input, pull pin high, write output low, change direction: pin
   output changes to low value
 * Set pin to input pull pin low, write output high, change direction: pin
   output changes to high value

Which seems to indicate that I _can_ set output values on input pins... I'll
need to look into this in more detail when I have a bit more time, later this
week.


Best,
Sander