[PATCH v4 0/3] counter: add GPIO-based quadrature encoder driver

[Wadim Mueller <wafgo01@gmail.com>]

This series adds a new counter subsystem driver which implements
quadrature encoder position tracking using plain GPIO pins with
edge-triggered interrupts.

The driver targets low- to medium-speed rotary encoders on SoCs where
hardware quadrature counter peripherals (eQEP, FTM, ...) are either
not available or already in use. It complements interrupt-cnt.c by
providing full A/B/Index quadrature decoding instead of just pulse
counting.

The strategic question from v3 -- "why kernel and not libgpiod in
userspace?" -- was already discussed in the v3 cover thread. A
reproducible benchmark on AM64x (kernel counter vs gpiomon/libgpiod
2.1.2, 3-run sweeps from 1 kHz up to 200 kHz) was posted there and
is also mirrored at https://github.com/wafgo/qenc-bench. William
accepted the case for a kernel implementation based on this, so i
will not repeat the numbers here, only the link.

Changes in v4 address the detailed review feedback from William on
PATCH 3/3 of v3:

Counter ABI / generic semantics:
  - Drop the private gpio_qenc_function enum, store and exchange the
    function as enum counter_function directly. function_read/write
    become trivial accessors.
  - Support the full set of generic Count functions:
    INCREASE, DECREASE, PULSE_DIRECTION,
    QUADRATURE_X1_{A,B}, QUADRATURE_X2_{A,B}, QUADRATURE_X4.
  - Add COUNTER_SYNAPSE_ACTION_FALLING_EDGE to the synapse action
    list and report it from action_read for the function modes
    where it does apply.
  - In QUADRATURE_X1_{A,B} the reported synapse action is now
    direction-dependent (RISING_EDGE when going forward, FALLING_EDGE
    when going backward) to match the Counter ABI semantics.
  - Restructure action_read: default to NONE, handle the Index
    synapse as a single early-return case, drop the per-function
    if/else cascade.
  - Use a dedicated action list for the Index synapse with only NONE
    and RISING_EDGE, since the other synapse actions do not apply
    there.
  - Migrate the Index feature from a custom "index_enabled"
    COUNTER_COMP_COUNT_BOOL to the generic COUNTER_COMP_PRESET +
    COUNTER_COMP_PRESET_ENABLE pair. The Index ISR now loads the
    preset value into count when preset_enable is set, like
    intel-qep and other drivers do.

Driver internals:
  - Change priv->count from s64 to u64. ceiling now defaults to
    U64_MAX instead of using ceiling == 0 as a sentinel for "no
    ceiling", which i think is also more clean.
  - Rewrite gpio_qenc_update_count direction-first, +/-1, with
    proper saturation at 0 and at ceiling.
  - Introduce CREATE_QE_STATE(prev_a, prev_b, curr_a, curr_b) and a
    single 16-entry X4 transition table indexed by the macro. The
    delta == 2 "invalid transition" path is gone, the table just
    has 0 in that slot.
  - Use default: in the ISR switches instead of listing every
    ignored case.
  - Drop the now-redundant enabled flag in the ISRs. Gating happens
    via enable_irq/disable_irq anyway and enable_read derives the
    state from irq_get_irq_data() so there is only one source of
    truth.
  - Simplify enable_write: no !!, assign directly, split the two
    branches by return.
  - Drop the no-op events_configure() hook from counter_ops, this
    one was just left over.
  - Do not register a synapse for the Index signal when no Index
    GPIO is wired (no zombie entries) and drop the corresponding
    !gpio guard in signal_read which was only there to catch the
    zombie case.
  - Declare priv->cnts as a single-element array and use
    ARRAY_SIZE(priv->cnts) for counter->num_counts, for consistency
    with the rest of the subsystem.
  - Rename the Count from "Position" to the more generic "Count",
    since positioning is only one of the use cases for a quadrature
    encoder.

Documentation / style:
  - Add a short comment to the priv spinlock.

Krzysztofs "drop const on scalar parameters" note from PATCH 2/3
of v2 is also taken care of in this rewrite -- v3 was Acked-by
Conor only and did not carry code changes yet, so the const scalar
parameters survived there. In v4 there are no const-qualified scalar
parameters left in the driver.

The follow-up scope which was discussed with William -- COMPARE/FLOOR
components plus OVERFLOW/UNDERFLOW/THRESHOLD/DIRECTION_CHANGE events
-- is on purpose not included in v4. I will send those as a separate
series on top of this one once it lands, so the diff size stays
reviewable.

Changes in v4:
  - Major review-driven rewrite of the driver (see above).

Changes in v3:
  - Pick up Acked-by: Conor Dooley on the DT binding patch.
  - No code changes.

Changes in v2:
  - DT binding: rephrase description to describe the hardware, not
    the driver/sysfs behaviour (Conor Dooley)
  - DT binding: drop redundant example without index GPIO (Conor Dooley)

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

 .../counter/gpio-quadrature-encoder.yaml      |  60 ++
 MAINTAINERS                                   |   7 +
 drivers/counter/Kconfig                       |  15 +
 drivers/counter/Makefile                      |   1 +
 drivers/counter/gpio-quadrature-encoder.c     | 739 ++++++++++++++++++
 5 files changed, 822 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/counter/gpio-quadrature-encoder.yaml
 create mode 100644 drivers/counter/gpio-quadrature-encoder.c


base-commit: 3cd8b194bf3428dfa53120fee47e827a7c495815
-- 
2.52.0