Re: [RFC v2 0/2] gpio: Support for shared GPIO lines on boards

[Philipp Zabel <p.zabel@pengutronix.de>]

On Wed, 2019-11-06 at 11:23 +0200, Peter Ujfalusi wrote:
> 
> On 05/11/2019 20.07, Grygorii Strashko wrote:
> > > > (but hey - if this is boot only then gpio-hogs should work. Are they?)
> > > 
> > > That is another thing which almost works ;)
> > > w/o gpio binding deferred probing is not possible if the GPIO controller
> > > is probed later.
> > > In some cases it might be even impossible to make sure that the GPIO
> > > controller would probe first (GPIO extender on different i2c bus than
> > > the user(s) of the gpio line)
> > > In some cases moving around nodes in DT might artificially make things
> > > work, but then someone compiles the expander as module, or some 'small'
> > > change in kernel and the probe order on the bus changes.
> > > I don't think it is a valid thing to have commits on the DT files
> > > saying: move the expander front/after the hog affected user since since
> > > Monday the probe order has changed. Then move it back two weeks later ;)
> > > 
> > 
> > Ok. Above sounds like real problem. The implicit dependence is exist,
> > but can't
> > be resolved if any driver depends on gpio-hog of some gpio-controller.
> > Probe deferring of gpio-controller will not lead to probe differing of
> > dependent driver.
> > 
> > Question: will gpio-hog mechanism resolve your case if it works (and
> > probe differing issues)?
> 
> I see gpio-hog to fulfill different role, use cases. It is more like
> controlling muxes on boards to select between different exclusive
> features. Things like route the I2S lines to analog codec or HDMI, route
> RGB video to LCD panel or to HDMI, etc.
> 
> But, if it would work it could be used for components which can be
> enabled all the time. On the other hand, if a device has reset/enable
> line then the driver should have a way to control it.

I wonder if it would be useful to differentiate between required and
suggested state in the consumer facing GPIO API for nonexclusive GPIOs.

A driver that is ok with the enable line going into active state at any
time while the device is suspended could use

	gpiod_set_value(en_gpio, 1);

to resume, but

	gpiod_politely_suggest_value(en_gpio, 0);

or similar to suspend, and the core could allow other drivers to
override this state. Similarly to how the regulator framework allows
consumers to set a voltage range, and the core decides on the actual
voltage that fits the constraints.

regards
Philipp