About devices and MMIO regions

[carlocaione]

Until a few months ago Zephyr was mainly targeted to platforms with no MMU. Things are quickly changing with now at least ARM64 and X86 supporting MMU.

This is creating a new class of problems: how drivers should ask for the kernel page-table mapping for their IO device region.

Currently there are two ways of doing this in Zephyr:

• Automatically using the MMIO helper macros in https://github.com/zephyrproject-rtos/zephyr/blob/main/include/sys/device_mmio.h. This is handy and it works nicely but the driver must be written to support that. Currently only 3 drivers in Zephyr are written to use these helpers
• If the driver is not using the MMIO helper macros then each platform must carry a file to map the device regions at boot time, for example https://github.com/zephyrproject-rtos/zephyr/blob/main/soc/arm64/qemu_cortex_a53/mmu_regions.c

What I'd like to discuss here is:

• Can we enforce the use of the MMIO helpers in all the new drivers that are going to be submitted to Zephyr? It's an additional tiny amount of work that can make life easier down the road
• We should also try to push this on the old drivers already in the tree, but I don't have high expectations about this TBH

tagging some good people @npitre @dcpleung @MaureenHelm @ioannisg @galak @nashif @povergoing @carlescufi