[PATCH v5 0/9] Introduce configfs-based interface for gpio-aggregator

[Koichiro Den <koichiro.den@xxxxxxxxxxxxx>]

This patch series introduces a configfs-based interface to gpio-aggregator
to address limitations in the existing 'new_device' interface.

The existing 'new_device' interface has several limitations:

  Issue#1. No way to determine when GPIO aggregator creation is complete.
  Issue#2. No way to retrieve errors when creating a GPIO aggregator.
  Issue#3. No way to trace a GPIO line of an aggregator back to its
           corresponding physical device.
  Issue#4. The 'new_device' echo does not indicate which virtual
           gpiochip<N> was created.
  Issue#5. No way to assign names to GPIO lines exported through an
           aggregator.

Although Issue#1 to #3 could technically be resolved easily without
configfs, using configfs offers a streamlined, modern, and extensible
approach, especially since gpio-sim and gpio-virtuser already utilize
configfs.

This v5 patch series includes 9 patches:

  Patch#1: Fix an issue that was spotted during v3 preparation.
  Patch#2: Reorder functions to prepare for configfs introduction.
  Patch#3: Add aggr_alloc() to reduce code duplication.
  Patch#4: Introduce basic configfs interface. Address Issue#1 to #4.
  Patch#5: Address Issue#5.
  Patch#6: Prepare for Patch#7.
  Patch#7: Expose devices created with sysfs to configfs.
  Patch#8: Suppress deferred probe for purely configfs-based aggregators.
  Patch#9: Documentation for the new configfs interface.

N.B. This v5 is based on the latest gpio/for-next commit as of writing this:
     * 45af02f06f69 ("gpio: virtuser: convert to use dev-sync-probe utilities")


v4->v5 changes:
  - Rebased off of the latest gpio/for-next, that includes the patch series:
    "Add synchronous fake device creation utility for GPIO drivers"
    (https://lore.kernel.org/all/20250221133501.2203897-1-koichiro.den@xxxxxxxxxxxxx/)

v3->v4 changes:
  - Splitted off the introduction of gpio-pseudo.[ch] and conversions.
  - Reordered commits to place a fix commit first.
  - Squashed the trivial update for gpio-aggregator's conversion to gpio-pseudo
    into the primary commit "gpio: aggregator: introduce basic configfs interface"
    as it is only meaningful when combined.

v2->v3 changes:
  - Addressed feedback from Bartosz:
    * Factored out the common mechanism for synchronizing platform device
      probe by adding gpio-pseudo.[ch].
    * Renamed "_auto." prefix to "_sysfs." for auto-generated
      configfs entries corresponding to sysfs-created devices.
    * Squashed v2 Patch#3 into its predecessor.
  - Addressed feedback from Geert:
    * Factored out duplicate code in struct gpio_aggregator initialization
      by adding gpio_alloc()/gpio_free() functions. Note that v2 Patch#7
      was dropped for other reasons as mentioned below, so aggr_free() in
      v3 is unrelated to the same-named function in v2.
    * Removed redundant parsing of gpio-line-names and unnecessary
      chip->names assignments; squashed v2 Patch#4 + v2 Patch#5 into v3
      Patch#9.
    * Updated to use sysfs_emit().
    * Updated Kconfig (select CONFIGFS_FS).
    * Fixed typos, coding style issues, missing const qualifiers, and other
      minor issues.
  - Resolved an issue that was spotted during v3 preparation. See Patch#2.
  - Reordered resource initialization order in gpio_aggregator_init() to
    both eliminate a potential race condition (as noted in the source code
    comment) and simplify the code. See Patch#8. This enabled:
    * Removal of v2 Patch#7.
    * Merging of aggr_unregister_lines() and aggr_free_lines() into a
      unified function.
  - Disabled 'delete_device' functionality for devices created via configfs
    for simplicity. It was mistakenly allowed in v2 and proved buggy. See
    Patch #8.

RFC->v2 changes:
  - Addressed feedback from Bartosz:
    * Expose devices created with sysfs to configfs.
    * Drop 'num_lines' attribute.
    * Fix bugs and crashes.
    * Organize internal symbol prefixes more cleanly.
  - Split diffs for improved reviewability.
  - Update kernel doc to reflect the changes.

v4: https://lore.kernel.org/all/20250217143531.541185-1-koichiro.den@xxxxxxxxxxxxx/
v3: https://lore.kernel.org/all/20250216125816.14430-1-koichiro.den@xxxxxxxxxxxxx/
v2: https://lore.kernel.org/all/20250203031213.399914-1-koichiro.den@xxxxxxxxxxxxx/
RFC (v1): https://lore.kernel.org/linux-gpio/20250129155525.663780-1-koichiro.den@xxxxxxxxxxxxx/T/#u


*** BLURB HERE ***

Koichiro Den (9):
  gpio: aggregator: protect driver attr handlers against module unload
  gpio: aggregator: reorder functions to prepare for configfs
    introduction
  gpio: aggregator: add aggr_alloc()/aggr_free()
  gpio: aggregator: introduce basic configfs interface
  gpio: aggregator: add 'name' attribute for custom GPIO line names
  gpio: aggregator: rename 'name' to 'key' in aggr_parse()
  gpio: aggregator: expose aggregator created via legacy sysfs to
    configfs
  gpio: aggregator: cancel deferred probe for devices created via
    configfs
  Documentation: gpio: document configfs interface for gpio-aggregator

 .../admin-guide/gpio/gpio-aggregator.rst      |  107 ++
 drivers/gpio/Kconfig                          |    2 +
 drivers/gpio/gpio-aggregator.c                | 1129 ++++++++++++++---
 3 files changed, 1050 insertions(+), 188 deletions(-)

-- 
2.45.2