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

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