counter: add GPIO-based counter driver

This series adds a counter subsystem driver that does quadrature
encoder position tracking with plain GPIO pins and edge interrupts.
Compared to interrupt-cnt.c (pulse-only) it provides full A/B/Index
decoding and exposes the counter sysfs ABI.  Target hardware is
low- to medium-speed rotary encoders on SoCs without a free eQEP /
FTM / etc.  Benchmark rig: github.com/wafgo/qenc-bench.

Following William's v5 review [1][2] the driver now exposes two
Counts on the same wires:

  Count 1: Synapses on A and B (+ optional Index 1). Supports
           QUADRATURE_X1_{A,B} / X2_{A,B} / X4, PULSE_DIRECTION,
           INCREASE and DECREASE.
  Count 2: Synapse on B (+ optional Index 2). INCREASE / DECREASE.

== Changes in v6 ==

Driver:
  - Two-Count layout with per-Count state struct
    (gpio_counter_count_priv) so future per-Count features can
    be added without parallel arrays.
  - ISR rewrite: STATE_CHANGED check used in all quadrature modes
    (not only X4); the *_delta helpers are gone.
  - INCREASE / DECREASE now count on both edges of their input
    (A for Count 1, B for Count 2).
  - PULSE_DIRECTION: direction is sampled in the B-ISR, the A-ISR
    only steps the value.
  - direction is initialised in function_write so the first read
    after a mode switch returns a defined value.
  - enable_write: added enable_lock mutex and reordered the
    enable/disable sequence so the spinlock is dropped before
    disable_irq() (which can sleep). irq_b is shared by Count 1
    and Count 2, so it is refcounted; the per-Count irqs (A,
    Index 1, Index 2) are toggled directly.

Binding:
  - index-gpios takes 1..2 entries: Index for Count 1, optional
    Index for Count 2.
  - Description and Counts wording updated for the two-Count
    layout.
  - signal-b stays required.  The single-line case is already
    covered by interrupt-cnt.  Oleksij suggested [3] extending
    interrupt-cnt with a multi-line variant; I kept this as its
    own driver because the Generic Counter ABI (Counts /
    Synapses / Signals, ceiling, preset, action) does not fit
    interrupt-cnt's pulse hot path well.
  - Conor's v5 Acked-by [4] is dropped because of the binding
    changes -- a fresh Ack would be appreciated.

[1] https://lore.kernel.org/all/20260617074929.333876-1-wbg@k...
[2] https://lore.kernel.org/all/20260623053800.120909-1-wbg@k...
[3] https://lore.kernel.org/all/ajoXoYc1CYKZyQ5l@pengutronix.de/
[4] https://lore.kernel.org/all/20260525-register-bogus-b1985...

Wadim Mueller (3):
  dt-bindings: counter: add gpio-counter binding
  counter: add GPIO-based counter driver
  MAINTAINERS: add entry for GPIO counter driver

 .../bindings/counter/gpio-counter.yaml        |  59 ++
 MAINTAINERS                                   |   7 +
 drivers/counter/Kconfig                       |  17 +
 drivers/counter/Makefile                      |   1 +
 drivers/counter/gpio-counter.c                | 983 ++++++++++++++++++
 5 files changed, 1067 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/counter/gpio-counter.yaml
 create mode 100644 drivers/counter/gpio-counter.c


base-commit: 3cd8b194bf3428dfa53120fee47e827a7c495815
-- 
2.52.0