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.
