Bias compile settings
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I had to use CFLAGS=-DPERIPHERY_GPIO_CDEV_SUPPORT -DGPIOHANDLE_REQUEST_BIAS_PULL_UP -DGPIOHANDLE_REQUEST_BIAS_PULL_DOWN -DGPIOHANDLE_REQUEST_BIAS_DISABLE or I'd get "Kernel version does not support configuring GPIO line bias". Is that the correct way to use bias?
If your kernel version / headers are older than 5.5, then they don't support GPIO line bias. You'll either need to build with a new kernel version, or c-periphery will need to add conditional compilation for that specific gpio-cdev feature. gpio-cdev was originally introduced (without the line bias feature) in kernel version 4.8.
gpio-cdev has been under a lot of flux across kernel versions, which is good for new features, but adds some complexity to libraries like c-periphery downstream. There appears to be a new v2 API coming to kernel version 5.10. It's going to take a bit of effort to seamlessly support multiple kernel versions.
Ah, totally makes sense since I'm testing on 5.4 kernel under Armbian. I believe they stick with LTS versions only, so it will be next LTS.
Oh, actually, there's 5.8 mainline, I will try this now...
OK this works Linux nanopiduo 5.8.16-sunxi #20.08.14 SMP Tue Oct 20 22:15:32 CEST 2020 armv7l armv7l armv7l GNU/Linux even though the Duo doesn't have PUDs :) At least the kernel error went away.