Re: [PATCH 02/13] driver-core: defer all probes until late_initcall

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, June 23, 2015 04:17:29 PM Tomeu Vizoso wrote:
> On 23 June 2015 at 16:37, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > On Monday, June 22, 2015 07:07:08 PM Rob Herring wrote:
> >> On Fri, Jun 19, 2015 at 6:20 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> >> > On Friday, June 19, 2015 03:36:46 PM Tomeu Vizoso wrote:
> >> >> On 18 June 2015 at 23:50, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> >> >> > On Wednesday, June 17, 2015 03:42:12 PM Tomeu Vizoso wrote:
> >> >> >> To decrease the chances of devices deferring their probes because of
> >> >> >> dependencies not having probed yet because of their drivers not having
> >> >> >> registered yet, delay all probing until the late initcall level.
> >> >> >>
> >> >> >> This will allow us to avoid deferred probes completely later by probing
> >> >> >> dependencies on demand, or by probing them in dependency order.
> >> >> >>
> >> >> >> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> >> >> >> ---
> >> >> >>  drivers/base/dd.c | 8 +++++++-
> >> >> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >> >> >>
> >> >> >> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> >> >> >> index a638bbb..18438aa 100644
> >> >> >> --- a/drivers/base/dd.c
> >> >> >> +++ b/drivers/base/dd.c
> >> >> >> @@ -407,6 +407,12 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
> >> >> >>       if (!device_is_registered(dev))
> >> >> >>               return -ENODEV;
> >> >> >>
> >> >> >> +     /* Defer all probes until we start processing the queue */
> >> >> >> +     if (!driver_deferred_probe_enable) {
> >> >> >> +             driver_deferred_probe_add(dev);
> >> >> >
> >> >> > Do I think correctly that this will effectively force everybody to use deferred
> >> >> > probing?
> >> >>
> >> >> Guess it depends on the meaning of "using deferred probing". It will
> >> >> defer the probe of the first device to late_initcall (which will
> >> >> happen much earlier in time than before), but afterwards all built-in
> >> >> drivers will be available and depending on the order in which we try
> >> >> to probe devices, none may actually ask to defer its probe.
> >> >
> >> > So this will break things like the PNP system driver which relies on probing
> >> > stuff at the fs_initcall stage for correctness.  It may also break other
> >> > things with similar assumptions.
> >>
> >> Yes, but I think that this can be done for only OF based devices
> >> rather than globally for all platform devices and solve that problem.
> >> Matching is already dependent of the type of device.
> >
> > Well, the current patch is not OF-only, though.
> 
> Yeah, I'm currently looking at only delaying probing of devices
> created from OF data.

I'm not sure if tying it hard to OF is not too restrictive.

Maybe we can use some general opt-in mechanism that OF will just always use?

In fact, we have a similar problem in ACPI where we have the _DEP object which
is used by firmware to describe dependencies between devices.

> Note that calculating dependencies and trying to probe them before
> they are needed can be done independently of this patch, but it isn't
> that useful because devices will still defer their probes because the
> drivers of some dependencies won't have been registered until
> late_initcall.

I see.

Rafael