Re: [PATCH 1/4] gpiolib: add support for bias pull disable

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Wed, Jul 13, 2022 at 7:37 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> On Wed, Jul 13, 2022 at 03:14:18PM +0200, Nuno Sá wrote:
> > This change prepares the gpio core to look at firmware flags and set
> > 'FLAG_BIAS_DISABLE' if necessary. It works in similar way to
> > 'GPIO_PULL_DOWN' and 'GPIO_PULL_UP'.
>
> ...
>
> >       GPIO_PULL_UP                    = (1 << 4),
> >       GPIO_PULL_DOWN                  = (1 << 5),
> > +     GPIO_PULL_DISABLE               = (1 << 6),
>
> To me it seems superfluous. You have already two flags:
> PUp
> PDown
> When none is set --> Pdisable

What happens in the pin control case for some drivers at least is that
the machine
comes up with some pull up/downs enabled (by power-on default or from
the boot loader), and some systems need to explicitly disable these
pulls.

In these (device tree) cases they set bias-disable; in the device tree, and
the driver will actively disable any pull up/down.

OK this is maybe not the most elegant system engineering. But some of
those users are hobbyists and cannot affect what the ASIC or firmware
is doing, because vendors are not really listening.

Another semantic reason is that pins can also be set to bias-high-impedance;
which is what "some people" would assume is the default if you disable
both pull up and pull down. (Yeah ... semantics...)

Device tree also has bias-pull-pin-default; to make things more complicated.
This should *really* leave it at power-on default. Explicitly.

I think for Nuno's usecase (using a random pin from userspace) the state
of biasing cannot be assumed, the driver will not change bias to
disabled just because neither pull up or down is specified, so the driver
needs an explicit kick saying "disable any bias".

Yours,
Linus Walleij