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

[Grant Likely]

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.