Re: [PATCH/RFC 2/2] gpio: of: Add DT overlay support for GPIO hogs

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Frank,

On Tue, Jan 7, 2020 at 8:26 AM Frank Rowand <frowand.list@gmail.com> wrote:
> On 1/7/20 1:10 AM, Frank Rowand wrote:
> > On 1/6/20 5:34 PM, Frank Rowand wrote:
> >> On 12/30/19 7:38 AM, Geert Uytterhoeven wrote:
> >>> As GPIO hogs are configured at GPIO controller initialization time,
> >>> adding/removing GPIO hogs in DT overlays does not work.
> >>>
> >>> Add support for GPIO hogs described in DT overlays by registering an OF
> >>> reconfiguration notifier, to handle the addition and removal of GPIO hog
> >>> subnodes to/from a GPIO controller device node.
> >>>
> >>> Note that when a GPIO hog device node is being removed, its "gpios"
> >>> properties is no longer available, so we have to keep track of which
> >>> node a hog belongs to, which is done by adding a pointer to the hog's
> >>> device node to struct gpio_desc.
> >>
> >> If I have read the patches and the existing overlay source correctly,
> >> then some observations:
> >>
> >> - A gpio hog node added in an overlay will be properly processed.
> >>
> >> - A gpio hog node already existing in the live devicetree, but with a
> >>   non-active status will be properly processed if the status of the
> >>   gpio hog node is changed to "ok" in the overlay.
> >>
> >> - If a gpio hog node already exists in the live devicetree with an
> >>   active status, then any updated or added properties in that gpio
> >>   hog node in the overlay will have no effect.
> >>
> >>   There is a scenario where the updated property would have an effect:
> >>   apply a second overlay that sets the status to inactive, then apply
> >>   a third overlay that sets the status back to active.  This is a
> >>   rather contrived example and I think it should be documented as
> >>   not supported and the result undefined.
> >
> > I went back and double checked the related code.  For gpio hog nodes
> > that are in a non-overlay, the status property is checked because
> > of_gpiochip_scan_gpios() uses for_each_available_child_of_node()
> > to search for gpio hog nodes, and for_each_available_child_of_node()
> > checks the status property.  But in the case of a gpio hog node
> > added by an overlay, of_gpio_notify() does not check the status
> > property in the gpio hog node.  The check for the status property
> > should be added to of_gpio_notify().
>
> One more thing I have not thought through is the case where the
> overlay contains both the gpio node and the gpio hog node.  I'll
> walk through that tomorrow.

I haven't veried that, but I assume that works already without my patch,
as GPIO hogs are configured at GPIO controller initialization time.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds