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SubjectRe: [PATCH] Add documentation on meaning of -EPROBE_DEFER
On Fri, Mar 27, 2020 at 10:01 AM Grant Likely <grant.likely@arm.com> 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@arm.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Saravana Kannan <saravanak@google.com>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
> .../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

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