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

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On 27/03/2020 18:10, Saravana Kannan wrote:
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?

If you feel strongly about it, but I don't have any problem with documenting it as the current implementation behaviour, and then changing the text if that ever changes.

g.




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