MCP23008 bias settings with libgpiod

[mdeneen]

I'm using a microchip 8-port IO expander which supports bias pull-ups on the digital input lines. I can set these pull-ups in the device tree, but I'd really like to apply them on the fly in userspace software using libgpiod.

When I reconfigure the line with libgpiod it shows up in gpioinfo with the appropriate bias setting printed in the listing, but the pull-up is not actually configured on the MCP23008.

MCP23008, in the Kernel, is a pinctrl device and not a gpio controller. I had read a few things about this, and had come to the conclusion that it was previously not possible because the uAPI did not exist. The uAPI exists now, and I think that what I'm doing should be possible with the versions of libgpiod (v2.1.1) and Kernel (6.6.23).

What is the glue between gpiolib and pinctrl that I am missing. Is what I am trying to do impossible?

[brgl]

Which driver exactly are you using, I will take a look?

[mdeneen]

drivers/pinctrl/pinctrl-mcp23s08.c
drivers/pinctrl/pinctrl-mcp23s08_i2c.c

I should have access to the hardware again soon. My initial thought was to add

mcp->chip.set_config = gpiochip_generic_config;

in the mcp23s08_probe_one function.

[brgl]

Yes, that should work. I see it doesn't define the gc->set_config() callback so no configuration that goes trough the GPIO subsystem can be applied unless you assign this callback.

[mdeneen]

Would it be worth adding this callback to other pinctrl drivers which support line treatment?

[brgl]

I would start by sending out a patch that extends this driver and we may discuss it under it on the list. How does it sound?

[mdeneen]

Sounds good. I'll report back once I have my hands on the hardware again and have tested the change.

Thanks again, Bart.