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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Sat, 23 Jan 2021 23:52:12 +0100
Message-ID: <CACRpkdYtToLVxyMxC=24AoGVk89c9FyQt899OTz9Jx8yJeu+mw@mail.gmail.com>
Subject: Re: [PATCH v5] gpiolib: Bind gpio_device to a driver to enable
 fw_devlink=on by default
To:     Saravana Kannan <saravanak@google.com>
Cc:     Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Marc Zyngier <maz@kernel.org>,
        Jisheng Zhang <Jisheng.Zhang@synaptics.com>,
        Kever Yang <kever.yang@rock-chips.com>,
        Android Kernel Team <kernel-team@android.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, Jan 22, 2021 at 8:36 PM Saravana Kannan <saravanak@google.com> wrote:

> There are multiple instances of GPIO device tree nodes of the form:
>
> foo {
>         compatible = "acme,foo";
>         ...
>
>         gpio0: gpio0@xxxxxxxx {
>                 compatible = "acme,bar";
>                 ...
>                 gpio-controller;
>         };
>
>         gpio1: gpio1@xxxxxxxx {
>                 compatible = "acme,bar";
>                 ...
>                 gpio-controller;
>         };
>
>         ...
> }
>
> bazz {
>         my-gpios = <&gpio0 ...>;
> }
>
> Case 1: The driver for "foo" populates struct device for these gpio*
> nodes and then probes them using a driver that binds with "acme,bar".
> This driver for "acme,bar" then registers the gpio* nodes with gpiolib.
> This lines up with how DT nodes with the "compatible" property are
> typically converted to struct devices and then registered with driver
> core to probe them. This also allows the gpio* devices to hook into all
> the driver core capabilities like runtime PM, probe deferral,
> suspend/resume ordering, device links, etc.
>
> Case 2: The driver for "foo" doesn't populate struct devices for these
> gpio* nodes before registering them with gpiolib. Instead it just loops
> through its child nodes and directly registers the gpio* nodes with
> gpiolib.
>
> Drivers that follow case 2 cause problems with fw_devlink=on. This is
> because fw_devlink will prevent bazz from probing until there's a struct
> device that has gpio0 as its fwnode (because bazz lists gpio0 as a GPIO
> supplier). Once the struct device is available, fw_devlink will create a
> device link with gpio0 device as the supplier and bazz device as the
> consumer. After this point, since the gpio0 device will never bind to a
> driver, the device link will prevent bazz device from ever probing.
>
> Finding and refactoring all the instances of drivers that follow case 2
> will cause a lot of code churn and it is not something that can be done
> in one shot. In some instances it might not even be possible to refactor
> them cleanly. Examples of such instances are [1] [2].
>
> This patch works around this problem and avoids all the code churn by
> simply setting the fwnode of the gpio_device and creating a stub driver
> to bind to the gpio_device. This allows all the consumers to continue
> probing when the driver follows case 2.
>
> [1] - https://lore.kernel.org/lkml/20201014191235.7f71fcb4@xhacker.debian/
> [2] - https://lore.kernel.org/lkml/e28e1f38d87c12a3c714a6573beba6e1@kernel.org/
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
> Cc: Kever Yang <kever.yang@rock-chips.com>
> Fixes: e590474768f1 ("driver core: Set fw_devlink=on by default")
> Signed-off-by: Saravana Kannan <saravanak@google.com>

This v5 version is a beauty!

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij