[RFC] Short Circuit simplest Rust driver that touches real hardware?

[Sven Van Asbroeck <thesven73@gmail.com>]

(Is this the correct forum for this discussion? Should it go on
Rust-for-Linux's GitHub instead?)

Senior Linux people have expressed interest in seeing a Rust driver
that touches actual hardware. Nvme has been suggested as an excellent
candidate. But, that's going to be quite complex, and possibly a long
way off still.

Is there merit in "short circuiting" a Rust driver that touches actual
hardware? I.e. only do the minimum that's reasonably required, to
drive a trivial piece of hardware.

Suggestion:
1. implement `platform_device` in Rust, just enough to be `probe()`d
by the kernel, and receive a live `struct device *` - we should wrap
this pointer in a suitable Rust abstraction of course.
2. implement the simplest `IoMemBlock` in Rust. Since we have a live
`struct device *`, the `IoMemBlock` constructor can simply call
`devm_platform_get_and_ioremap_resource(our struct device *)` in C,
which will automatically use the right method to retrieve the `void
__iomem *`: devicetree, platform data, etc, we don't have to care.
3. use the `IoMemBlock` above to implement a trivial iomem-only driver
as a module. A few possible candidates:
* h/w random number generator in Raspberry Pi Zero: `brcm,bcm2835-rng`
* SoC thermal sensor in Raspberry Pi Zero: `brcm,bcm2835-thermal`
4. we don't have `hwrng` or `thermal_hwmon` devices in Rust yet, but
we could perhaps expose this as a `misc_device`, which is something we
already have. A single temp or randnum readout might do the trick.

The Raspberry Pi Zero can be purchased for $10. There is [QEMU
support](https://qemu.readthedocs.io/en/latest/system/arm/raspi.html)
for the peripherals above.

Feedback and ideas welcome !