`Rng` API

[therealprof]

I'd like to suggest to suggest a new Rng API for a common way of getting random numbers from devices with some source of randomness.

I've already implemented this as a proof-of-concept in my nrf51-hal crate (https://github.com/therealprof/nrf51-hal) and an example use in my microbit crate (https://github.com/therealprof/microbit).

The suggested (blocking) API is similar to the I2c API in that a mutable u8 slice is passed as parameter which will be filled with the random data and a Result is returned:

//! Blocking hardware random number generator

/// Blocking read
pub trait Read {
    /// Error type
    type Error;

    /// Reads enough bytes from hardware random number generator to fill `buffer`
    fn read(&mut self, buffer: &mut [u8]) -> Result<(), Self::Error>;
}

Potentially this could also use an implementation for other (or even variable) array types, e.g. the rand crate requires a u32 slice as a seed.

I'd be happy to supply a PR for the the implementation.

[nickray]

I'm implementing this for STM32L4(32): https://github.com/nickray/stm32l4-hal/blob/rng/src/rng.rs
It "works", but I'd like to make it rustic. Some questions:

• should RngExt.enable be called .constrain in line with other traits? For me personally, that naming is a little weird (as the API is constrained, not the RNG), but maybe it just needs getting used to and having conventions is a good thing.
• turning HSI48 on depends on other RCC settings, so it should probably not be done implicitly in .enable. On the other hand, I'm not happy with either a) asserting, or b) not checking settings at all
• is there room for additional helper methods? I'm talking about things like wrapping self.rng.sr.read().cecs().bit() as .is_clock_error for people like me who don't know data sheets and register names by heart. Or does this go against HAL minimalism?
• the Error enum:
  • should this have only standardized errors, or reveal MCU-specific errors also?
  • is it efficient to check clock settings here on every call (admittedly, calling the RNG is not the fastest operation)?

[eldruin]

This trait was added. There is discussion about replacing/complementing this with rand_core at #128 which provide further methods.

[therealprof]

Hah! I knew there were previous discussions about this. Let's close this then.