regmap-gpio: Support set_config and other not quite so standard ICs?

[Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>]

[-- Attachment #1: Type: text/plain, Size: 2950 bytes --]

Hi Linus, All,

On Thu, 2021-03-25 at 12:32 +0200, Matti Vaittinen wrote:
> On Thu, 2021-03-25 at 10:35 +0100, Linus Walleij wrote:

snip

> > It could potentially (like the other Rohm GPIO MFD PMIC drivers)
> > make some use of the gpio regmap library, but we have some
> > pending changes for that so look into it after the next merge
> > window.
> > 
> > I.e. for your TODO: look at the GPIO_REGMAP helper.
> 
> I just took a quick peek at gpio_regmap and it looks pretty good to
> me!
> 
> Any particular reason why gpio_regmap is not just part of gpio_chip?
> I
> guess providing the 'gpio_regmap_direction_*()', 'gpio_regmap_get()',
> 'gpio_regmap_set()' as exported helpers and leaving calling the
> (devm_)gpiochip_add_data() to IC driver would have allowed more
> flexibility. Drivers could then use the gpio_regamap features which
> fit
> the IC (by providing pointers to helper functions in gpio_chip) - and
> handle potential oddball-features by using pointers to some
> customized
> functions in gpio_chip.

So, v5.13-rc1 is out. I started wondering the gpio_regamap - and same
question persists. Why hiding the gpio_chip from gpio_regmap users?

Current IF makes it very hard (impossible?) for driver to override any
of the regmap_gpio functions (or provide own alternatives) for cases
which do not fit the generic regmap_gpio model.

My first obstacle is providing gpio_chip.set_config for BD71815.

1) I guess the method fitting current design would be adding drive-mode 
register/mask(s) to the gpio_regmap_config. Certainly doable - but I
have a bad feeling of this approach. I am afraid this leads to bloating
the gpio_regmap_config with all kinds of IC specific workarounds (when
HW designers have invented new cool control registers setups) - or then
just not using the regmap_gpio for any ICs which have any quirks - even
if 90% of regmap_gpio logic would fit...

2) Other possibility is allowing IC driver to provide function pointers
for some operations (in my case for example for the set_config) - if
the default operation the regmap_gpio provides does not fit the IC.
This would require the regmap_gpio to be visible to IC drivers so that
IC drivers can access the regmap, device & register information - or
some way to convert the gpio_chip pointer to IC specific private data
pointer. Doable but still slightly bloat.

3) The last option would be adding pointer to regmap_gpio to gpio_chip
- and exporting the regmap_gpio functions as helpers - leaving the gpio
registration to be done by the IC driver. That would allow IC driver to
use the regmap_gpio helpers which suit the IC and write own functions
for rest of the stuff.

I'd like to hear opinions - should I draft some changes according to
these proposals (which one, 1,2,3 or something else?) - or as this all
been already discussed and am I just missing something?

Best Regards
	Matti Vaittinen

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 488 bytes --]