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

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Date:   Wed, 5 Jun 2019 19:36:13 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Joerg Roedel <joro@...tes.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-pm@...r.kernel.org, linux-gpio@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] driver: core: Allow subsystems to continue deferring
 probe

On Wed, Jun 05, 2019 at 04:23:12PM +0200, Thierry Reding 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>
> ---
>  drivers/base/dd.c            | 17 ++++++++++++-----
>  drivers/base/power/domain.c  |  2 +-
>  drivers/iommu/of_iommu.c     |  2 +-
>  drivers/pinctrl/devicetree.c | 10 ++++++----
>  include/linux/device.h       |  2 +-
>  5 files changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 0df9b4461766..25ffbadf4187 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -238,23 +238,30 @@ __setup("deferred_probe_timeout=", deferred_probe_timeout_setup);
>  /**
>   * driver_deferred_probe_check_state() - Check deferred probe state
>   * @dev: device to check
> + * @persist: Boolean flag indicating whether drivers should keep trying to
> + *   probe after built-in drivers have had a chance to probe. This is useful
> + *   for built-in drivers that rely on resources provided by modular drivers.

Functionality aside, having random boolean flags in a function parameter
list is a horrid way to have an api.  Now when you see a call to this
function with either "true" or "false" as an option, you have no idea
what that means.  So you need to go look up this definition and then
work backwards.

So even if the idea is sane (I'm not saying that), this implementation
is not acceptable at all.

thanks,

greg k-h