Introduce the Counter character device interface

Changes in v17:
- A couple minor improvements to documentation from review suggestions
- chrdev_lock redeclared as atomic_t; BITMAP was not necessary because chrdev_lock is a single flag
- test_and_set_bit_lock(), clear_bit_unlock(), and clear_bit(), replaced respectively with atomic_add_unless(), atomic_dec(), and atomic_set()
- counter_comp_read_is_equal() and counter_comp_read_is_set() macros implemented in order to properly test counter_comp structures' read callback states
- counter_sysfs_add() call performed before counter_chrdev_add() call in counter_register() in order to match unwinding sequence
- for-loop utilized in counter-example.c in order to simplify code
- counter-example.c returns 1 on error instead of -errno; errno may be modified after a subsequent library call so we can't depend on it

For convenience, this patchset is also available on my personal git
repo: https://gitlab.com/vilhelmgray/iio/-/tree/counter_chrdev_v17

A Counter character device interface is introduced that allows Counter events and associated data to be read() by userspace; the events_configure() and watch_validate() driver callbacks are introduced to support Counter events; and IRQ support is added to the 104-QUAD-8 driver, serving as an example of how to support the new Counter events functionality.

William Breathitt Gray (9):
counter: Move counter enums to uapi header
counter: Add character device interface
docs: counter: Document character device interface tools/counter: Create Counter tools
counter: Implement signalZ_action_component_id sysfs attribute
counter: Implement *_component_id sysfs attributes
counter: Implement events_queue_size sysfs attribute
counter: 104-quad-8: Replace mutex with spinlock
counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8

 Documentation/ABI/testing/sysfs-bus-counter   |  29 +
 Documentation/driver-api/generic-counter.rst  | 177 ++++--
 .../userspace-api/ioctl/ioctl-number.rst      |   1 +
 MAINTAINERS                                   |   2 +
 drivers/counter/104-quad-8.c                  | 256 ++++++--
 drivers/counter/Kconfig                       |   6 +-
 drivers/counter/Makefile                      |   2 +-
 drivers/counter/counter-chrdev.c              | 578 ++++++++++++++++++
 drivers/counter/counter-chrdev.h              |  14 +
 drivers/counter/counter-core.c                |  56 +-
 drivers/counter/counter-sysfs.c               | 123 +++-
 include/linux/counter.h                       |  98 +--
 include/uapi/linux/counter.h                  | 154 +++++
 tools/Makefile                                |  13 +-
 tools/counter/Build                           |   1 +
 tools/counter/Makefile                        |  53 ++
 tools/counter/counter_example.c               |  92 +++
 17 files changed, 1509 insertions(+), 146 deletions(-)
 create mode 100644 drivers/counter/counter-chrdev.c
 create mode 100644 drivers/counter/counter-chrdev.h
 create mode 100644 include/uapi/linux/counter.h
 create mode 100644 tools/counter/Build
 create mode 100644 tools/counter/Makefile
 create mode 100644 tools/counter/counter_example.c

base-commit: a5ae0cfd53aaf031c2e9ba048281776fa67047c2

2.33.0