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

[Kent Gibson <warthog618@gmail.com>]

On Thu, Jul 14, 2022 at 09:14:21AM +0200, Nuno Sá wrote:
> On Thu, 2022-07-14 at 12:20 +0800, Kent Gibson wrote:
> > On Wed, Jul 13, 2022 at 08:36:38PM +0300, Andy Shevchenko 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
> > > 
> > 
> > Agree with Andy on this.  The FLAG_BIAS_DISABLE was added, by me, to
> > allow the cdev interface to support bias.  cdev requires a "don't
> > care"
> > state, distinct from an explicit BIAS_DISABLE.
> > The FLAG_BIAS_DISABLE allows gpiolib-cdev to communicate that to
> > gpiolib, without altering the interpretation of the existing PULL_UP
> > and
> > PULL_DOWN flags.
> > That is not an issue on the machine interface, where the two
> > GPIO_PULL
> > flags suffice.
> > 
> 
> I see, but this means we can only disable the pin BIAS through
> userspace. I might be wrong but I don't see a reason why it wouldn't be
> valid to do it from an in kernel path as we do for PULL-UPS and PULL-
> DOWNS 
> 

> > If you are looking for the place where FLAG_BIAS_DISABLE is set it is
> > in
> > gpio_v2_line_config_flags_to_desc_flags() in gpiolib-cdev.c.
> > 
> > Referring to gpio_set_bias(), the only place in gpiolib the
> > FLAG_BIAS_DISABLE is used, if neither FLAG_PULL_UP, FLAG_PULL_DOWN,
> > nor FLAG_BIAS_DISABLE are set then the bias configuration remains
> > unchanged (the don't care case) - no change is passed to the driver.
> > Otherwise the corresponding PIN_CONFIG_BIAS flag is passed to the
> > driver.
> > 
> 
> Exactly, but note FLAG_BIAS_DISABLE can only be set from userspace at
> this point (IIUTC). If everyone agrees that should be case, so be it.
> But as I said, I just don't see why it's wrong to do it within the
> kernel.
> 

Believe it or not gpiolib-cdev is part of the kernel, not userspace - it
just provides an interface to userspace.

Bias can be disabled by calling gpiod_direction_input() or
gpiod_direction_output() after setting the FLAG_BIAS_DISABLE, as
gpiolib-cdev does.

Does that work for you?

Cheers,
Kent.


> > If there are cases of drivers not fully or correctly supporting those
> > PIN_CONFIG_BIAS flags, then that is an issue with those drivers.
> > 
> 
> Look at my reply to Andy in the cover for more details
> 
> - Nuno Sá