PCI: hotplug: Document TODOs

Contributions are solicited in particular to remedy the following issues:

cpcihp:

* There are no implementations of the ->hardware_test, ->get_power and

  ->set_power callbacks in struct cpci_hp_controller_ops.  Why were they

  introduced?  Can they be removed from the struct?

cpqphp:

* The driver spawns a kthread cpqhp_event_thread() which is woken by the

  hardirq handler cpqhp_ctrl_intr().  Convert this to threaded IRQ handling.

  The kthread is also woken from the timer pushbutton_helper_thread(),

  convert it to call irq_wake_thread().  Use pciehp as a template.

* A large portion of cpqphp_ctrl.c and cpqphp_pci.c concerns resource

  management.  Doesn't this duplicate functionality in the core?

ibmphp:

* Implementations of hotplug_slot_ops callbacks such as get_adapter_present()

  in ibmphp_core.c create a copy of the struct slot on the stack, then perform

  the actual operation on that copy.  Determine if this overhead is necessary,

  delete it if not.  The functions also perform a NULL pointer check on the

  struct hotplug_slot, this seems superfluous.

* Several functions access the pci_slot member in struct hotplug_slot even

  though pci_hotplug.h declares it private.  See get_max_bus_speed() for an

  example.  Either the pci_slot member should no longer be declared private

  or ibmphp should store a pointer to its bus in struct slot.  Probably the

  former.

* The functions get_max_adapter_speed() and get_bus_name() are commented out.

  Can they be deleted?  There are also forward declarations at the top of

  ibmphp_core.c as well as pointers in ibmphp_hotplug_slot_ops, likewise

  commented out.

* ibmphp_init_devno() takes a struct slot **, it could instead take a

  struct slot *.

* The return value of pci_hp_register() is not checked.

* iounmap(io_mem) is called in the error path of ebda_rsrc_controller()

  and once more in the error path of its caller ibmphp_access_ebda().

* The various slot data structures are difficult to follow and need to be

  simplified.  A lot of functions are too large and too complex, they need

  to be broken up into smaller, manageable pieces.  Negative examples are

  ebda_rsrc_controller() and configure_bridge().

* A large portion of ibmphp_res.c and ibmphp_pci.c concerns resource

  management.  Doesn't this duplicate functionality in the core?

sgi_hotplug:

* Several functions access the pci_slot member in struct hotplug_slot even

  though pci_hotplug.h declares it private.  See sn_hp_destroy() for an

  example.  Either the pci_slot member should no longer be declared private

  or sgi_hotplug should store a pointer to it in struct slot.  Probably the

  former.

shpchp:

* There is only a single implementation of struct hpc_ops.  Can the struct be

  removed and its functions invoked directly?  This has already been done in

  pciehp with commit 82a9e79ef132 ("PCI: pciehp: remove hpc_ops").  Clarify

  if there was a specific reason not to apply the same change to shpchp.

* The ->get_mode1_ECC_cap callback in shpchp_hpc_ops is never invoked.

  Why was it introduced?  Can it be removed?

* The hardirq handler shpc_isr() queues events on a workqueue.  It can be

  simplified by converting it to threaded IRQ handling.  Use pciehp as a

  template.