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SubjectRe: [PATCH] Add documentation on meaning of -EPROBE_DEFER
On Sat, Mar 28, 2020 at 1:57 AM Saravana Kannan <saravanak@google.com> wrote:
> On Fri, Mar 27, 2020 at 4:25 PM Grant Likely <grant.likely@arm.com> wrote:
> > 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.

...

> > >> +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?

Why? *Good* documentation must describe the actual behaviour, not hide it.

> > 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.
>
> Assuming Greg is okay with this doc update, I'm kinda leaning towards
> "undefined"

I think it should not distort the reality.

> because if documented as "infinite loop" people might be
> hesitant towards removing that behavior.

This is funny argument. Won't we do kernel better?

> But I'll let Greg make the
> final call. Not going to NACK for this point.

--
With Best Regards,
Andy Shevchenko

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