Re: Raspberry Pi 3B: /dev/mem mmap GPIO works on earmv7hf but not onaarch64

[Yuri Honegger <yuri.honegger%mailbox.org@localhost>]

> > What's wrong with gpioctl(8)?
>
> In my use case I am not only toggling a single GPIO bit but need to
> update 8-bit bus data / 2-bit control signals atomically for the
> PSG accesses (unlike FreeBSD we don't provide GPIOACCESS32 ioctl).
>
> I also need to program BCM283x GPCLK0 (clock manager registers)
> as the PSG master 2MHz clock, and I have not found a stable API
> for that on NetBSD/evbarm (please correct me if I missed one).
>
> That tends to push userland implementations toward MMIO register access.
>
> So the main question is: is the intended answer
> (A) "/dev/mem is RAM-only for aarch64; peripheral MMIO via /dev/mem
> is unsupported", or
> (B) "peripheral MMIO via /dev/mem is acceptable if correct device
> attributes are implemented"?

Driving a 8-bit bus at 2MHz will be hard with any abstraction between
your software and the GPIOs, even with a GPIOACCESS32 (unless
you only need to write new values at a much much lower rate than 2MHz)

In my opinion, the cleanest way would be to write a kernel module
specifically for this application so you can mmio as you like.
You might want to deactivate the other gpio driver in your config/dtb though.

- yuri