Add support for memory mapped UARTs

[remimimimimi]

Unresolved questions:

• [x] Feature naming
• [x] Do we need atomics?
  • [x] Atomic ordering

[phil-opp]

Thanks for the PR!

I don't think that reusing the same SerialPort type for two very different things (MMIO vs port IO) is a good idea. There are even differences in the function signatures, e.g. the new method takes a pointer instead of a port number. I would prefer to introduce the MMIO-based port functionality through a separate MmioSerialPort type.

Regarding the features, I would prefer to keep the current stable and nightly features if possible. If necessary, we can add an additional mmio feature. We might need some trickery for the x86_64 dependency and its features, but it should be possible to implment this via target-specific dependencies somehow.

[remimimimimi]

Thanks for the PR!

Regarding the features, I would prefer to keep the current stable and nightly features if possible. If necessary, we can add an additional mmio feature. We might need some trickery for the x86_64 dependency and its features, but it should be possible to implment this via target-specific dependencies somehow.

I don't see other way to solve this problem besides these variants.

1. Add port, mmio_stable, mmio_nightly features and stay with stable nightly, but this kinda ugly.

   [features]
   default = [ "port", "nightly" ]
   port = []
   stable = [ "x86_64/external_asm" ]
   nightly = [ "x86_64/inline_asm" ]
   mmio_stable = []
   mmio_nightly = []

2. Small api break but this will make it more logic.

   [features]
   default = [ "port_nightly" ]
   port_stable = [ "x86_64/external_asm" ]
   port_nightly = [ "x86_64/inline_asm" ]
   mmio_stable = []
   mmio_nightly = []

3. Stay only with stable or nightly version of mmio. So there's no api break but less choice.

   [features]
   default = [ "port_nightly" ]
   port_stable = [ "x86_64/external_asm" ]
   port_nightly = [ "x86_64/inline_asm" ]
   mmio = []

4. Due #16 we can also do this

   
   [dependencies]
   x86_64 = { version = "0.14.0", default-features = false, features = ["instructions", "x86_64/external_asm"], optional = true }

[features] default = [ "port"] port = ["x86_64"] mmio = []

Can combined with mmio

stable = [] # Noop for port feature nightly = [] # Noop for port feature

There's issue about target-specific features but it opened since 2015, so this can be perfect version but nowadays it impossible.
```toml
[target.'cfg(target = "x86_64"'.features]
stable = ["x86_64/external_asm"]
nightly = ["x86_64/inline_asm"]
[target.'cfg(target = "riscv"'.features]
stable = []
nightly = []

[phil-opp]

I think we can simplify this by enabling the x86_64 dependency only on target_arch = x86_64 targets. Then we don't need any cargo features for disabling it. I prepared a patch to implement this in https://github.com/remimimimi/uart_16550/pull/1 (a PR against your PR).

[phil-opp]

Follow-up changes done in #18. Published as v0.2.15.