Add default buses to board

[caternuson]

From the discussion raised in this issue, w.r.t I2C bus:
#340

[dhalbert]

I gently want to suggest that we revisit the idea of board.I2C(), board.UART(), board.SPI() as implemented recently in PR's #842, #844, and #845. I think it introduces a new, somewhat weak, shortcut API that doesn't work out well if you need to change your program to use different pins. I suggest instead that we add defaults to the pin args for busio.I2C(), busio.UART(), and busio.SPI().

I'll take board.I2C() as an example. It returns a singleton instance that is busio.l2C(board.SCL, board.SDA). So now how might it be used? Well, instead of

i2c = board.I2C(board.SCL, board.SDA)
...
i2c.write_to(...)

you can do:

board.I2C().write_to(...)

But suppose you later decide you want to use different pins? The singleton call doesn't take pin args, so you don't do this (which is too verbose anyway):

board.I2C(board.D0, board.D1).write_to(...)    # DOESN'T WORK

Instead you need to go back to the original way and rewrite it as i2c = busio.I2C(board.D0, board.D1) You have to change all the uses of board.I2C() throughout your program to the first way.

You might argue that instead one should just assign the value:

i2c = board.I2C()
...
i2c.write_to(...)

but in that case, I think the better API is to have busio.I2C() have default args (as suggested by @deshipu in #340 (comment), and @ladyada in an offline discussion), i.e. the equivalent of:

    def I2C(scl=board.SCL, sda=board.SDA, *, frequency=400000):

so that you can just do:

i2c = busio.I2C()     # Note busio, not board.
                      # Will use the defaults of board.SCL and board.SDA.

It's true you lose the convenience of the singleton, but you don't need to specify the pin names or remember their order, and you don't need to import board (vs. not having to import busio).

The motivation for the original is issue #340, which was much more about adafruit_bus_device than about providing a convenience singleton. Note also that (usually?) busio.SOMEBUS is implemented to invisibly fall over to bitbangio.SOMEBUS if busio.SOMEBUS doesn't exist on the particular board.

Tagging @matt-land, @tannewt, @deshipu, @caternuson, and @tannewt for comments.

(This bothered me enough that it woke me up. On the other hand, my middle-of-the-night thoughts might not be so coherent.)

[deshipu]

Of course I would much prefer the solution proposed here by @dhalbert. Not only will it use less memory when you are not using I2C, not only adding it will result in smaller code size, but also it has the advantage of using an existing Python mechanism, which everybody will have to learn about anyways, instead of being something entirely new created specially for the purpose, which has to be learned separately from the documentation.

[tannewt]

One case this is designed to ease is initializing multiple I2C devices on the same bus, which is common when using multiple feather wings. With a singleton in board the device drivers can default to the same bus:

d1 = device1.Device1()
d2 = device2.Device2()

With the proposed default args we'd need:

bus = busio.I2C()
d1 = device1.Device1(bus)
d2 = device2.Device2(bus)

The second option does make it easier to change the pins but how common will this be? With featherwings in particular, this is highly unlikely. Are we ok with the extra step of the second option? Is the first too magical?

I think it'd be tricky to have a shared default bus like the first option without having it built into a core module such as board or busio. If we put it in adafruit_bus_device we prevent mixing bus device drivers and non-bus device drivers.

[deshipu]

Can't we cache the bus, so that creating a bus with the same parameters just gets you the same bus back?
Then the device can also default to it.

I think that's what MicroPython does, anyways.

[dhalbert]

I was composing a message with the same idea the @deshipu just posted.

Suppose busio took care of the singleton? Right now:

i2c1 = busio.I2C(board.SCL, board.SDA)
i2c2 = busio.I2C(board.SCL, board.SDA)

will throw a "Pin xxx in use" right now.

We could have busio.I2C keep track of the pins and return the same object. If this is an issue of upward compatibility, we could have a new SharedI2C class. But I think it's probably fine to create and return a singleton as necessary.

[tannewt]

Ok, that's fine with me. I think it'll be a bit tricky because the memory is allocated in shared-bindings and not common-hal.

[dhalbert]

Yes, this would be easier to implement in Python than C 😃

[deshipu]

As an easier solution, we could cache only the argument-less call.

[dhalbert]

That might be kind of confusing, since then sometimes a double call would work and sometimes not. I think it might not be too hard to keep track. The I2C objects already keep the pins, so we just have to look over the existing objects. Just have to keep a list of them. We could even use list primitives to manage the storage if we don't want to manage it in a C way on the heap.

[deshipu]

And expose the list as busio.buses? That could actually help in debugging when you get errors about used pins and you are not sure where from.