Re: [PATCH] gpio: mcp23s08: support setting pullups from device tree data

[Alexander Stein <alexander.stein@systec-electronic.com>]

On Monday 07 December 2015 09:22:59, Timo Teras wrote:
> On Mon, 07 Dec 2015 07:53:08 +0100
> Alexander Stein <alexander.stein@systec-electronic.com> wrote:
> 
> > On Friday 04 December 2015 13:46:13, Timo Teräs wrote:
> > > Make pullup configurable from device tree data.
> > > 
> > > Signed-off-by: Timo Teräs <timo.teras@iki.fi>
> > > ---
> > > I'm not fully certain how the pullup data should be made
> > > configurable via OF. gpio-twl4030.c uses similar approach as this,
> > > but gpio-samsung.c makes the pullup configuration seems to be part
> > > of the custom data in gpio specifiers.  
> > 
> > I think this should be specified for each individual GPIO. e.g.
> > > gpios = <&gpio-ctrl 0 GPIO_ACTIVE_HIGH GPIO_PULLUP>;  
> > 
> > You don't know (yet) which pins might need a internal pull-up when
> > specifying the mcp23s08 node itself. You might override the
> > controller node in your dts, but so you need to adjust 2 entries, one
> > for the user and one for the pullup, rather than have all options at
> > one place.
> 
> I think this is debatable. So consider this as opening a bit wider
> discussion about the preferred way of exposing pullup/down configuration
> via OF. And hopefully documenting it. (As mentioned there's already
> code in for either way.)
> 
> Agreeably one might want to override it on PIN basis. Perhaps the GPIO
> function can be changed or similar.

I think for this you need only to add an additional cell entry or just add an additional mask bit like active low/high.
This would add the pull up/down feature to each GPIO user.

> The other side is: Why would my gpio-keys input mapping specification
> need know to what kind of GPIO input it is connected to? What if I
> switch the GPIO expander - or the hardware schema?  I need to edit 10
> different places rather than one - places that are about the high-level
> functionality, not the hardware. Normally the pullup settings are
> hardware layout dependant, so GPIO configuration would be the logical
> place.

gpio-keys need to know this when selecting active low or active high. You usually need the corresponding pull up/down for this to work.

> One generally wants to configure pullups correctly for all GPIOs
> regardless of if they are connected or not; or if the relevant
> high-level gpio driver is loaded or not. And this should be done as
> early as possible - even before the driver for that specific GPIO pin
> functionality is loaded.

This is also a valid point.

> Perphaps others have more arguments for the per-pin configuration?

I get the impression both ways would be needed... which is kinda error-prone.

Best regards,
Alexander
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