• Saravana Kannan's avatar
    of: property: Add functional dependency link from DT bindings · a3e1d1a7
    Saravana Kannan authored
    Add device links after the devices are created (but before they are
    probed) by looking at common DT bindings like clocks and
    interconnects.
    
    Automatically adding device links for functional dependencies at the
    framework level provides the following benefits:
    
    - Optimizes device probe order and avoids the useless work of
      attempting probes of devices that will not probe successfully
      (because their suppliers aren't present or haven't probed yet).
    
      For example, in a commonly available mobile SoC, registering just
      one consumer device's driver at an initcall level earlier than the
      supplier device's driver causes 11 failed probe attempts before the
      consumer device probes successfully. This was with a kernel with all
      the drivers statically compiled in. This problem gets a lot worse if
      all the drivers are loaded as modules without direct symbol
      dependencies.
    
    - Supplier devices like clock providers, interconnect providers, etc
      need to keep the resources they provide active and at a particular
      state(s) during boot up even if their current set of consumers don't
      request the resource to be active. This is because the rest of the
      consumers might not have probed yet and turning off the resource
      before all the consumers have probed could lead to a hang or
      undesired user experience.
    
      Some frameworks (Eg: regulator) handle this today by turning off
      "unused" resources at late_initcall_sync and hoping all the devices
      have probed by then. This is not a valid assumption for systems with
      loadable modules. Other frameworks (Eg: clock) just don't handle
      this due to the lack of a clear signal for when they can turn off
      resources. This leads to downstream hacks to handle cases like this
      that can easily be solved in the upstream kernel.
    
      By linking devices before they are probed, we give suppliers a clear
      count of the number of dependent consumers. Once all of the
      consumers are active, the suppliers can turn off the unused
      resources without making assumptions about the number of consumers.
    
    By default we just add device-links to track "driver presence" (probe
    succeeded) of the supplier device. If any other functionality provided
    by device-links are needed, it is left to the consumer/supplier
    devices to change the link when they probe.
    
    kbuild test robot reported clang error about missing const
    Reported-by: default avatarkbuild test robot <lkp@intel.com>
    Signed-off-by: default avatarSaravana Kannan <saravanak@google.com>
    Link: https://lore.kernel.org/r/20190904211126.47518-4-saravanak@google.comSigned-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    a3e1d1a7
kernel-parameters.rst 8.07 KB