Re: [PATCH v6 1/1] USB: serial: cp210x: Adding GPIO support for CP2105

[Martyn Welch <martyn.welch@collabora.co.uk>]

On Fri, Oct 07, 2016 at 04:31:26PM +0100, Martyn Welch wrote:
> Also, if an output can only be open-drain, attempting to set the pin as
> push-pull succeeds because gpiolib (currently) assumes that a pin can
> always be p-p and doesn't even check the return value of it's call to
> .set_single_ended:
> 
> 
>                /* Make sure to disable open drain/source hardware, if any */
>                 if (gc->set_single_ended)
>                         gc->set_single_ended(gc,
>                                              gpio_chip_hwgpio(desc),
>                                              LINE_MODE_PUSH_PULL);
> 
> 
> This is clearly a separate issue.
> 

A bit of testing also show that a "open-drain only" output will succeed when
an open-source output driven high is requested. Rather than having a pin
pulled hard to VCC, we have a pin pulled to VCC via a resistor and thus
(if there are other devices connected to the pin pulling to GND via a
resistor, which would kinda stand to reason given the pin is being
configured for open-source mode) we have the signal floating at a voltage
determined by the effect of the two sets of resistors acting as a voltage
divider.

Martyn