Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
This can be very useful for some purposes:
* It makes it possible to disable the UART in cases where it is not
needed or needs to be disabled to conserve power.
* It makes it possible to disable the serial output to reduce code
size, which may be important for some chips. Sometimes, a few kB can
be saved this way.
* It makes it possible to override the default, for example you might
want to use an actual UART to debug the USB-CDC implementation.
It also lowers the dependency on having machine.Serial defined, which is
often not defined when targeting a chip. Eventually, we might want to
make it possible to write `-target=nrf52` or `-target=atmega328p` for
example to target the chip itself with no board specific assumptions.
The defaults don't change. I checked this by running `make smoketest`
before and after and comparing the results.
This means that machine.UART0, machine.UART1, etc are of type
*machine.UART, not machine.UART. This makes them easier to pass around
and avoids surprises when they are passed around by value while they
should be passed around by reference.
There is a small code size impact in some cases, but it is relatively
minor.
It is always implemented exactly the same way (as an uint8) so there is
no reason to implement it in each target separately.
This also makes it easier to add some documentation to it.
Let's use the same default frequency everywhere, for consistency.
It could be any frequency, but 4MHz is already used for other chips and
it seems like a reasonable frequency to me (not too fast for most chips
but still reasonably fast). Oh, and 4MHz is slow enough that it can be
inspected by a Saleae Logic 4 (that sadly has been discontinued).
Only some pins (notably including GPIO2 aka machine.LED) have GPIO for
the default function 1. Other pins (such as GPIO 15) had a different
function by default. Function 3 means GPIO for all the pins, so always
use that when configuring a pin to use as a GPIO pin.
In the future, the mux configuration will need to be updated for other
functions such as SPI, I2C, etc.
This is only very minimal support. More support (such as tinygo flash,
or peripheral access) should be added in later commits, to keep this one
focused.
Importantly, this commit changes the LLVM repo from llvm/llvm-project to
tinygo-org/llvm-project. This provides a little bit of versioning in
case something changes in the Espressif fork. If we want to upgrade to
LLVM 11 it's easy to switch back to llvm/llvm-project until Espressif
has updated their fork.
