Fix one byte UART transmissions for lowrisc (maybe)

[bradjc]

Pull Request Overview

This pull request attempts to fix #1736. The issue is that the lowrisc UART TX driver can call a callback directly after a request is made, but for Tock's concurrency model that callback must be issued in response to an interrupt being handled so that the original caller can complete before the callback executes.

This change forces a TX interrupt to always happen, and only issues the callback in the handle_interrupt call if all bytes have been transmitted.

Testing Strategy

This is where I need help. I don't have an open titan setup, so this PR needs to be tested.

TODO or Help Wanted

Help with someone checking if this is a valid fix.

Documentation Updated

• Updated the relevant files in /docs, or no updates are required.

Formatting

• Ran make formatall.

[alistair23]

I tested this patch and don't see any regressions. I never saw the original issue so I'm unable to confirm it's fixed.

[phil-levis]

Looks good to me. I can't test the code, but conceptually making every write behave identically is a good idea, and issuing reentrant callbacks is a no-no.

[alistair23]

Ping! I hit this issue and this PR does fix it.

[ppannuto]

bors r+

[bors]

Build succeeded:

• ci-build
• ci-format
• ci-tests
• continuous-integration/travis-ci/push
• emulation-check