[PATCH 00/12] introduce support for early platform drivers

[Bartosz Golaszewski]

This series is a follow-up to the RFC[1] posted a couple days ago.

NOTE: this series applies on top of my recent patches[2] that move the previous
implementation of early platform devices to arch/sh.

Problem:

Certain class of devices, such as timers, certain clock drivers and irq chip
drivers need to be probed early in the boot sequence. The currently preferred
approach is using one of the OF_DECLARE() macros. This however does not create
a platform device which has many drawbacks - such as not being able to use
devres routines, dev_ log functions or no way of deferring the init OF function
if some other resources are missing.

For drivers that use both platform drivers and OF_DECLARE the situation is even
more complicated as the code needs to take into account that there can possibly
be no struct device present. For a specific use case that we're having problems
with, please refer to the recent DaVinci common-clock conversion patches and
the nasty workaround that this problem implies[3].

We also used to have an early platform drivers implementation but they were not
integrated with the linux device model at all - they merely used the same data
structures. The users could not use devres, defer probe and the early devices
never became actual platform devices later on.

Proposed solution:

This series aims at solving this problem by (re-)introducing the concept of
early platform drivers and devices - this time however in a way that seamlessly
integrates with the existing platform drivers and also offers device-tree
support.

The idea is to provide a way for users to probe devices early, while already
being able to use devres, devices resources and properties and also deferred
probing.

New structures are introduced: the early platform driver contains the
early_probe callback which has the same signature as regular platform_device
probe. This callback is called early on. The user can have both the early and
regular probe speficied or only one of them and they both receive the same
platform device object as argument. Any device data allocated early will be
carried over to the normal probe.

The architecture code is responsible for calling early_platform_start() in
which the early drivers will be registered and devices populated from DT.

Once the device and kobject mechanisms are ready, all early drivers and devices
will be converted into real platform drivers and devices. Also: if any of the
early platform registration functions will be called once early initialization
is done, these functions will work like regular platform_device/driver ones.

Patches 1-9/12 introduce changes to existing code that are necessary before
adding support for early drivers. Patch 10/12 contains the new framwork in an
isolated file which can be compiled only if needed by the architecture.

Patch 11/12 contains a dummy early platform driver that serves as a code
example and can be used for simple testing.

The last patch finally makes the of/platform code aware of early platform
drivers.

If accepted, this new mechanism could potentially lead to consolidation of the
code currently used by users of OF_DECLARE, since they could be converted to
actual platform drivers.

[1] https://lkml.org/lkml/2018/4/26/657
[2] https://lkml.org/lkml/2018/4/30/547
[3] https://lkml.org/lkml/2018/4/26/1094

Bartosz Golaszewski (12):
  platform/early: add a new field to struct device
  platform/early: don't WARN() on non-empty devres list for early
    devices
  platform/early: export platform_match() locally
  platform: provide a separate function for initializing platform
    devices
  platform: export platform_device_release() locally
  of: add a new flag for OF device nodes
  of/platform: provide a separate routine for setting up device
    resources
  of/platform: provide a separate routine for device initialization
  platform/early: add an init section for early driver data
  platform/early: implement support for early platform drivers
  misc: implement a dummy early platform driver
  of/platform: make the OF code aware of early platform drivers

 drivers/base/Kconfig              |   3 +
 drivers/base/Makefile             |   1 +
 drivers/base/base.h               |   4 +
 drivers/base/dd.c                 |   2 +-
 drivers/base/early.c              | 332 ++++++++++++++++++++++++++++++
 drivers/base/platform.c           |  28 ++-
 drivers/misc/Kconfig              |   8 +
 drivers/misc/Makefile             |   2 +
 drivers/misc/dummy-early.c        |  82 ++++++++
 drivers/of/platform.c             |  85 +++++---
 include/asm-generic/vmlinux.lds.h |  11 +
 include/linux/device.h            |   1 +
 include/linux/early_platform.h    |  75 +++++++
 include/linux/of.h                |   1 +
 include/linux/of_platform.h       |   2 +
 include/linux/platform_device.h   |   2 +
 16 files changed, 604 insertions(+), 35 deletions(-)
 create mode 100644 drivers/base/early.c
 create mode 100644 drivers/misc/dummy-early.c
 create mode 100644 include/linux/early_platform.h

-- 
2.17.0