[RFC PATCH 0/5] gpio: add PMIO support to gpio-mmio

[Jose Javier Rodriguez Barbarin <dev-josejavier.rodriguez@duagon.com>]

This series is an RFC for adding port-mapped I/O (PMIO) support to
gpio-mmio.

The initial motivation was to add PMIO support to gpio-menz127, as we
plan to support this across MCB client drivers. Since gpio-menz127
currently relies on the gpio_generic_chip API, adding PMIO support only
in that driver would require a significant refactoring, including
separate callbacks for memory-mapped and port-mapped accesses.

While looking into this, I noticed a TODO item added by Linus Walleij
about extending gpio-mmio to support port-mapped devices. Based on that,
this series explores adding PMIO support to gpio-mmio instead of
handling it in individual drivers.

The main goal of this approach is to preserve compatibility with
existing MMIO drivers using gpio_generic_chip while extending the API to
also describe PMIO-backed registers. To achieve that, the series extends
struct gpio_generic_chip_config with dedicated fields for port-mapped
registers.

To handle the different register address types used by MMIO
(void __iomem *) and PMIO (unsigned long), this series introduces a
small wrapper structure that can represent both. The read_reg() and
write_reg() callbacks are then updated to operate on this common
representation.

This series has been tested with gpio-menz127, and the driver worked
correctly with both MMIO and PMIO devices.

This is being sent as RFC because I would like feedback on the overall
approach before proceeding further.

In particular, feedback would be appreciated on:
- whether extending gpio_generic_chip_config is the right direction;
- whether introducing a common MMIO/PMIO register descriptor is
  acceptable;
- whether PMIO support should instead be implemented differently in
  gpio-mmio.

Jose Javier Rodriguez Barbarin (5):
  gpio: generic: add a generic register wrapper for MMIO and PMIO
  gpio: generic: extend gpio_generic_chip_config with PMIO register
    fields
  gpio: generic: add io_port to struct gpio_generic_chip
  gpio: mmio: convert accessors to generic register descriptors
  gpio: mmio: add port-mapped read/write callbacks

 drivers/gpio/gpio-mmio.c     | 277 +++++++++++++++++++++++++----------
 include/linux/gpio/generic.h |  49 +++++--
 2 files changed, 243 insertions(+), 83 deletions(-)

-- 
2.53.0