Re: MCP23S08 Pincontroller Bias Configuration and GpioLib

[Kent Gibson <warthog618@gmail.com>]

On Wed, Sep 18, 2024 at 09:21:11PM -0400, Matt Walker wrote:
> Hi all -- I'm trying to do GPIO from userspace and have run into some
> confusion I could use some clarification on.
>
> Assumption: It is not possible to set the pull up bias for a specific
> line from the device tree and expose the line to user space. And it is
> not possible to set the bias from the old sysfs gpio configuration.
>
> I'm therefore trying to use the GPIO_V2_LINE_SET_CONFIG_IOCTL to set
> the bias of the line. However, the MCP23S08 (pinctrl-mcp23s08.c)
> driver does not have its implementation of struct
> pinconf_ops.pin_config_set called. Instead I had to add an
> implementation of struct gpiochip.set_config.
>
> Confusion: I thought the pin controller was solely responsible for
> configuration of bias and that the gpiolib would magically know that
> it needs to call pinconf_ops.pin_config_set. Since it seems like an
> unnecessary duplication of effort to set bias config in both pinctrl
> and gpiochip, I figure I'm missing something. I thought it might be
> that the MCP driver does not have an explicit call to
> gpiochip_add_pin_range, but that wasn't it. I also tried adding the
> pinctrl definitions suggested in the device tree mapping, and that
> also didn't work.
>
> Thanks for any insights anyone might have! And if there's not a better
> way, I'll clean up the driver and submit it as a patch.
>

Not my bailiwick, but looking through the code (both gpiolib and drivers)
it seems to me you should set

	mcp->chip.set_config = gpiochip_generic_config;

to hook into the magic that you are expecting.

Cheers,
Kent.