Re: [PATCH] Add documentation on meaning of -EPROBE_DEFER

[Saravana Kannan]

On Fri, Mar 27, 2020 at 10:01 AM Grant Likely <grant.likely@xxxxxxx> wrote:
>
> Add a bit of documentation on what it means when a driver .probe() hook
> returns the -EPROBE_DEFER error code, including the limitation that
> -EPROBE_DEFER should be returned as early as possible, before the driver
> starts to register child devices.
>
> Also: minor markup fixes in the same file
>
> Signed-off-by: Grant Likely <grant.likely@xxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Saravana Kannan <saravanak@xxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
>  .../driver-api/driver-model/driver.rst        | 32 ++++++++++++++++---
>  1 file changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/driver-api/driver-model/driver.rst b/Documentation/driver-api/driver-model/driver.rst
> index baa6a85c8287..63057d9bc8a6 100644
> --- a/Documentation/driver-api/driver-model/driver.rst
> +++ b/Documentation/driver-api/driver-model/driver.rst
> @@ -4,7 +4,6 @@ Device Drivers
>
>  See the kerneldoc for the struct device_driver.
>
> -
>  Allocation
>  ~~~~~~~~~~
>
> @@ -167,9 +166,26 @@ the driver to that device.
>
>  A driver's probe() may return a negative errno value to indicate that
>  the driver did not bind to this device, in which case it should have
> -released all resources it allocated::
> +released all resources it allocated.
> +
> +Optionally, probe() may return -EPROBE_DEFER if the driver depends on
> +resources that are not yet available (e.g., supplied by a driver that
> +hasn't initialized yet).  The driver core will put the device onto the
> +deferred probe list and will try to call it again later. If a driver
> +must defer, it should return -EPROBE_DEFER as early as possible to
> +reduce the amount of time spent on setup work that will need to be
> +unwound and reexecuted at a later time.
> +
> +.. warning::
> +      -EPROBE_DEFER must not be returned if probe() has already created
> +      child devices, even if those child devices are removed again
> +      in a cleanup path. If -EPROBE_DEFER is returned after a child
> +      device has been registered, it may result in an infinite loop of
> +      .probe() calls to the same driver.

The infinite loop is a current implementation behavior. Not an
intentional choice. So, maybe we can say the behavior is undefined
instead?

-Saravana