Volatile memory access for MMIO

• Use pointer::read_volatile and pointer::write_volatile.
• Never hold a reference.

• Use &raw to get fields of structs without creating an intermediate reference.

• Volatile access: read or write operations may have side-effects, so prevent the compiler or hardware from reordering, duplicating or eliding them.

  • Usually if you write and then read, e.g. via a mutable reference, the compiler may assume that the value read is the same as the value just written, and not bother actually reading memory.
• Some existing crates for volatile access to hardware do hold references, but this is unsound. Whenever a reference exist, the compiler may choose to dereference it.
• Use &raw to get struct field pointers from a pointer to the struct.
• For compatibility with old versions of Rust you can use the addr_of! macro instead.