On Mon, Jul 9, 2018 at 9:52 AM Russell King - ARM Linux linux@armlinux.org.uk wrote:
On Mon, Jul 09, 2018 at 09:41:48AM -0600, Rob Herring 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).
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.