On Mon, Jul 9, 2018 at 9:52 AM Russell King - ARM Linux
[off-list ref] wrote:

On Mon, Jul 09, 2018 at 09:41:48AM -0600, Rob Herring wrote:

quoted

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).

So what happens if we have a set of modules which use deferred probing
in order to work?

It is opt-in by subsystem or drivers and mainly intended for
subsystems which can be optional or only support built-in drivers.
However, I don't really envision many other users other than the ones
I converted (pinctrl, iommu, pm-domains). If you look at patch 3,
you'll see it is dependent on !CONFIG_MODULES.

For the timeout, well, that's for debugging only. If you get to the
point of loading sound modules, you probably don't need the timeout.
It's for debugging not booting.

For example, with sound stuff built as modules, and auto-loaded in
parallel by udev, the modules get added in a random order.  The
modules have non-udev obvious dependencies between them (resource
dependencies) which result in deferred probing being necessary to
bring the device up.