• Rob Herring's avatar
    driver core: allow stopping deferred probe after init · 25b4e70d
    Rob Herring authored
    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).
    
    Cc: Alexander Graf <agraf@suse.de>
    Signed-off-by: default avatarRob Herring <robh@kernel.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    25b4e70d
kernel-parameters.txt 170 KB