On Thu, Jun 28, 2018 at 11:43 PM, Rob Herring [off-list ref] wrote:

Deferred probe will currently wait forever on dependent devices to probe,
but sometimes a driver will never exist. It's also not always critical for
a driver to exist. Platforms can rely on default configuration from the
bootloader or reset defaults for things such as pinctrl and power domains.
This is often the case with initial platform support until various drivers
get enabled. There's at least 2 scenarios where deferred probe can render
a platform broken. Both involve using a DT which has more devices and
dependencies than the kernel supports. The 1st case is a driver may be
disabled in the kernel config. The 2nd case is the kernel version may
simply not have the dependent driver. This can happen if using a newer DT
(provided by firmware perhaps) with a stable kernel version. Deferred
probe issues can be difficult to debug especially if the console has
dependencies or userspace fails to boot to a shell.

There are also cases like IOMMUs where only built-in drivers are
supported, so deferring probe after initcalls is not needed. The IOMMU
subsystem implemented its own mechanism to handle this using OF_DECLARE
linker sections.

This commit adds makes ending deferred probe conditional on initcalls
being completed or a debug timeout. Subsystems or drivers may opt-in by
calling driver_deferred_probe_check_init_done() instead of
unconditionally returning -EPROBE_DEFER. They may use additional
information from DT or kernel's config to decide whether to continue to
defer probe or not.

The timeout mechanism is intended for debug purposes and WARNs loudly.

The remaining deferred probe pending list will also be dumped after the
timeout. Not that this timeout won't work for the console which needs
to be enabled before userspace starts. However, if the console's
dependencies are resolved, then the kernel log will be printed (as
opposed to no output).

There is another patch flying around with debugfs node to dump a list
of deferred probe queue.
I dunno if it makes sense to dump it here and there and if yes, some
unification in output, perhaps?

+       deferred_probe_timeout = simple_strtol(str, NULL, 0);

Hmm... I don't think 16-base or 8-base values are useful to support.
One subtle difference that people usually consider timeout values as
10-base and if at some point someone makes it as 0100 (no matter why),
it would be much less than expected.
08 wouldn't parsed at all.

+       if (deferred_probe_timeout > 0) {

Would it be harmful / useful if we skip this check and run the work immediately?

+               schedule_delayed_work(&deferred_probe_timeout_work,
+                       deferred_probe_timeout * HZ);
+       }

-- 
With Best Regards,
Andy Shevchenko