On Thu, Aug 29, 2019 at 9:43 AM Rob Herring [off-list ref] wrote:

On Thu, Aug 29, 2019 at 2:46 AM Saravana Kannan [off-list ref] wrote:

quoted

Add device-links to track functional dependencies between devices
after they are created (but before they are probed) by looking at
their common DT bindings like clocks, interconnects, etc.

Having functional dependencies automatically added before the devices
are probed, 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.

v1 -> v2:
- Drop patch to speed up of_find_device_by_node()
- Drop depends-on property and use existing bindings

v2 -> v3:
- Refactor the code to have driver core initiate the linking of devs
- Have driver core link consumers to supplier before it's probed
- Add support for drivers to edit the device links before probing

v3 -> v4:
- Tested edit_links() on system with cyclic dependency. Works.
- Added some checks to make sure device link isn't attempted from
  parent device node to child device node.
- Added way to pause/resume sync_state callbacks across
  of_platform_populate().
- Recursively parse DT node to create device links from parent to
  suppliers of parent and all child nodes.

v4 -> v5:
- Fixed copy-pasta bugs with linked list handling
- Walk up the phandle reference till I find an actual device (needed
  for regulators to work)
- Added support for linking devices from regulator DT bindings
- Tested the whole series again to make sure cyclic dependencies are
  broken with edit_links() and regulator links are created properly.

v5 -> v6:
- Split, squashed and reordered some of the patches.
- Refactored the device linking code to follow the same code pattern for
  any property.

v6 -> v7:
- No functional changes.
- Renamed i to index
- Added comment to clarify not having to check property name for every
  index
- Added "matched" variable to clarify code. No functional change.
- Added comments to include/linux/device.h for add_links()

v7 -> v8:
- Rebased on top of linux-next to handle device link changes in [1]

v8 -> v9:
- Fixed kbuild test bot reported errors (docs and const)

v9->v10:
- Changes made based on reviews on LKML [2] and discussions at ELC [3]
- Dropped the edit_links() patch
- Dropped the patch that skips linking for default bus nodes
- 1/7: Changed from bus.add_links() to fwnode.ops.add_links()
- 1/7: Update device link doc
- 1/7: Lots of comments/fn doc updates
- 1/7: Renamed device_link_check_waiting_consumers() to
  device_link_add_missing_supplier_links()
- 2/7: Moved DT parsing/linking code from of/platform.c to of/property.c

Why? You'll notice that of/property.c doesn't know anything about
platform_device (and struct device):

$ git grep platform_device -- drivers/of/property.c
$

Everything related to platform_device goes in of/platform.c.
Everything related to struct device only goes in of/device.c. I'd be
okay with a new file for this too.

The only platform_device related code in what got moved to
of/property.c is the call to of_find_device_by_node(). And that's
because I forgot that function returns a platform_device --- it should
really have been called of_find_plat_device_by_node() or something
similar. Outside of that, of/property.c makes sense because that's
where the fwnode ops are implemented.

As you mentioned in the other email, just searching platform_bus is
not sufficient. So I'll have to figure something out for that. Once I
do, I think the code will be fine in of/property.c as it shouldn't
have any reference to platform_device.

Thanks for catching what I missed.

-Saravana