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

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Date:   Fri, 24 Jan 2020 16:02:29 -0600
From:   Frank Rowand <frowand.list@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Chris Brandt <chris.brandt@...esas.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC 2/2] gpio: of: Add DT overlay support for GPIO hogs

On 1/24/20 3:57 PM, Frank Rowand wrote:
> On 1/7/20 2:11 AM, Geert Uytterhoeven wrote:
>> Hi Frank,
>>
>> On Tue, Jan 7, 2020 at 8:10 AM Frank Rowand <frowand.list@...il.com> 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.
> 
> Verified by testing.
> 
> 
>>>> - 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.
> 
> Should be documented.
> 
> 
>>>>   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 was wrong in this case.
> 
> of_reconfig_get_state_change() does not simply report whether a node
> is added or removed, which confused me because it returns
> OF_RECONFIG_CHANGE_ADD and OF_RECONFIG_CHANGE_REMOVE (as well as
> no change), which I was incorrectly translating to node added or
> node removed.   OF_RECONFIG_CHANGE_ADD and OF_RECONFIG_CHANGE_REMOVE
> properly report a node becoming available or available due to changes
                                            ^^^^^^^^^^^^
                                            or unavailable

> in the "status" property, as well as accounting for a node being
> added or removed.
> 
> So the case that I was worried about is handled correctly.
> 
> 
>>> 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().
>>
>> Right.  of_device_is_available() should be called to check this.
>> Note that of_i2c_notify() and of_spi_notify() also lack such a check.
>> of_platform_notify() calls of_platform_device_create_pdata(), which does
>> have the check.
> 
> And thus I was wrong about this also, so of_gpio_notify() does not need to
> check the status property, since of_reconfig_get_state_change() already
> implicitly incorporates this check.
> 
>>
>> Gr{oetje,eeting}s,
>>
>>                         Geert
>>
> 
>