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

[Kent Gibson <warthog618@gmail.com>]

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.

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.

If there are cases of drivers not fully or correctly supporting those
PIN_CONFIG_BIAS flags, then that is an issue with those drivers.

Cheers,
Kent.