GPIO-lib provides a ready-to-use interface to initialize an IRQ-chip on
top of a GPIO chip. It's better from maintainability and readability
point of view to use one instead of supporting a hand-written Generic
IRQ-chip-based implementation. Moreover the new implementation won't
cause much functional overhead but will provide a cleaner driver code.
All of that makes the DW APB GPIO driver conversion pretty much justified
especially seeing a tendency of the other GPIO drivers getting converted
too.

Here is what we do in the framework of this commit to convert the driver
to using the GPIO-lib-based IRQ-chip interface:

1) IRQ ack, mask and unmask callbacks are locally defined instead of
using the Generic IRQ-chip ones.

2) An irq_chip structure instance is embedded into the dwapb_gpio
private data. Note we can't have a static instance of that structure since
GPIO-lib will add some hooks into it by calling gpiochip_set_irq_hooks().
A warning about that would have been printed by the GPIO-lib code if we
used a single irq_chip structure instance for multiple DW APB GPIO
controllers.

3) Initialize the gpio_irq_chip structure embedded into the gpio_chip
descriptor. By default there is no IRQ enabled so any event raised will be
handled by the handle_bad_irq() IRQ flow handler. If DW APB GPIO IP-core
is synthesized to have non-shared reference IRQ-lines, then as before the
hierarchical and cascaded cases are distinguished by checking how many
parental IRQs are defined. (Note irq_set_chained_handler_and_data() won't
initialize IRQs, which descriptors couldn't be found.) If DW APB GPIO IP
is used on a platform with shared IRQ line, then we simply won't let the
GPIO-lib to initialize the parental IRQs, but will handle them locally in
the driver.

4) Discard linear IRQ-domain and Generic IRQ-chip initialization, since
GPIO-lib IRQ-chip interface will create a new domain and accept a standard
IRQ-chip structure pointer based on the setting we provided in the
gpio_irq_chip structure.

5) Manually select a proper IRQ flow handler directly in the
irq_set_type() callback by calling irq_set_handler_locked() method, since
an ordinary (not Generic) irq_chip descriptor is now utilized. Note this
shalln't give any regression

6) Alter CONFIG_GPIO_DWAPB kernel config to select
CONFIG_GPIOLIB_IRQCHIP instead of CONFIG_GENERIC_IRQ_CHIP.

Note neither 4) nor 5) shall cause a regression of commit 6a2f4b7d
("gpio: dwapb: use a second irq chip"), since the later isn't properly
used here anyway.
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200730152808.2955-6-Sergey.Semin@baikalelectronics.ruSigned-off-by: Linus Walleij <linus.walleij@linaro.org>