linux-kernel - Re: [PATCH v3] driver: core: Allow subsystems to continue deferring probe

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Date:   Tue, 25 Jun 2019 00:26:56 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Rob Herring <robh@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] driver: core: Allow subsystems to continue deferring probe

On Fri, Jun 21, 2019 at 5:17 PM Thierry Reding <thierry.reding@...il.com> wrote:

> From: Thierry Reding <treding@...dia.com>
>
> Some subsystems, such as pinctrl, allow continuing to defer probe
> indefinitely. This is useful for devices that depend on resources
> provided by devices that are only probed after the init stage.
>
> One example of this can be seen on Tegra, where the DPAUX hardware
> contains pinmuxing controls for pins that it shares with an I2C
> controller. The I2C controller is typically used for communication
> with a monitor over HDMI (DDC). However, other instances of the I2C
> controller are used to access system critical components, such as a
> PMIC. The I2C controller driver will therefore usually be a builtin
> driver, whereas the DPAUX driver is part of the display driver that
> is loaded from a module to avoid bloating the kernel image with all
> of the DRM/KMS subsystem.
>
> In this particular case the pins used by this I2C/DDC controller
> become accessible very late in the boot process. However, since the
> controller is only used in conjunction with display, that's not an
> issue.
>
> Unfortunately the driver core currently outputs a warning message
> when a device fails to get the pinctrl before the end of the init
> stage. That can be confusing for the user because it may sound like
> an unwanted error occurred, whereas it's really an expected and
> harmless situation.
>
> In order to eliminate this warning, this patch allows callers of the
> driver_deferred_probe_check_state() helper to specify that they want
> to continue deferring probe, regardless of whether we're past the
> init stage or not. All of the callers of that function are updated
> for the new signature, but only the pinctrl subsystem passes a true
> value in the new persist parameter if appropriate.
>
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
> Changes in v3:
> - add new function rather than extend the existing function with flags

I see you need something like this and I can't think of anything
better so:
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

Yours,
Linus Walleij