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

[Grant Likely <grant.likely@arm.com>]

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

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.