Break mbed-drivers dependency

[0xc0170]

If we want to remove dependency on mbed-drivers. I had a quick look at files within hal, which are referenced within hal tree:

Files from mbed-drivers used in target's HAL:

• pinmap common (uses error() if pinmap not found)
• mbed error (mbed_die() -> which requires wait_us)
• mbed assert - exit() and mbed_die()
• us ticker api

All these are currently in mbed-drivers module. There are some dependencies on other files as highlighted above. Thus to have HAL not depend on mbed-drivers, we would need to move files above, plus this additional files:

• error
• exit
• mbed_assert (use from core-util which depends on mbed-drivers)
• mbed_die
• mbed_interface - weak mac address (needs to be fixed)
• wait

@bogdanm @bremoran @salkinium

[bogdanm]

I think this problem appears because mbed-drivers is still too large:

1. error, mbed_assert can be part of a separate module (at least part of this functionality is already in core-util). However:
2. error calls mbed_die that uses the hardware unfortunately, so I don't see how we can easily break this dependency (unless you break mbed-drivers itself into mbed-drivers-gpio, mbed-drivers-spi, mbed-drivers-uart ...)
3. wait could be expressed in terms of a loosely calibrated busy wait loop for this specific scenario, so it won't depend on a hardware timer anymore
4. we already eliminated mbed_interface I think (or at least we were getting ready to eliminate it)

All in all, this is not an easy problem. Even if you refactor mbed-drivers a bit, you still have the dependency on hardware access functions for error via mbed_die.

[0xc0170]

What about - to have "clean" C layer, which we can use and build any upper layers on top of it (for example my-fancy-mbed-drivers).

Error, wait , assert would depend on HAL, can be in their own modules or we gather them somewhere as common c code.

[bogdanm]

I'm not sure if it's worth enhancing our HAL to something usable at C level because of this kind of dependency. It might be a solution though.

[rainierwolfcastle]

ARM Internal Ref: IOTSFW-1009