The loader bringup now has some IRQ testing built into it.
Couldn't actually get an IRQ to trigger, but it has to do something
more with me not getting the RV IRQ machine bits configured right
than anything else. However, the stub is useful for confirming
which IRQ bits actually fire on the UDMA UART implementation,
which is the big point. There's an ambiguity in the wiring
of the UDMA block which isn't explained at all, which is the
fact that there's an RX interrupt and an RX_CHAR interrupt.
I still don't know what RX_CHAR does, but the RX interrupt
does fire when a character is received, so that's the one
we're going to use.
- make the "base" field no longer public. It is accessible, but only
through an `unsafe` method, because, it is unsafe.
- add accessors to offest/mask fields. Because, it's not at all
unsafe to access these, and it can be useful for bit-ops that are
not part of the generic set of UTRA.
- add Arc to AtomicCSR, and require `std` for AtomicCSR types.
This should not break anything, I checked inside @foundation-devices
keyOS and they don't seem to be using AtomicCSR anywhere.
The Arc makes it a bit safer to clone the AtomicCSR base pointer
and share it between threads, but it will have an impact on performance.
The USB stack (which requires AtomicCSR) was regression testing and the
impact seems minimal.
Recall that the main purpose of AtomicCSR is to provide a CSR accessor
that does not require a mutable reference to the base object. This
can make certain Rust idioms a bit cleaner. We wrap the base accessor
in Arc+Atomic to try and improve the safety of throwing the borrow
checker under the bus, but it's still not perfect because hardware
mutable state can have side-effects within hardware itself that cannot
be modeled or checked by Rust's borrow checker.
