stm32l4-multi/libuart: optimize infiniteRxHandler

[lukileczo]

Description

Optimize libuart_infiniteRxHandler:

• return early when rxbuf is NULL - previously the function updated positions in buffer even when no one was waiting to read from UART. This caused us to unnecessary execute most of the interrupt handling code, which probably resulted in delay of interrupt handling when rxbuf was ready. This early return fixes the problem with overrun.
• for safety, swapped memcpy to for loop on volatile DMA buffer.
• check if in IDLE interrupt we have any new data to receive - previously handler was entered even without any data ready which also introduced additional delay in processing interrupts from DMA

Examples from testing:

00000384_ABCDEFGHIJKLMNOPQRSTUVWXYZ123469
00000385_ABCNOPQRSTUVWXYZ123469
00000386_ABCDEFGHIJKLMNOPQRSTUVWXYZ123469

In 385 overrun was detected.

00000284_ABCDEFGHIJKLMNOPQRSTUVWXYZ123469NOPQRSTUVWXYZ123469

\n\r and start of next line dropped, overrun was detected.

Motivation and Context

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• [ ] Already covered by automatic testing.
• [ ] New test added: (add PR link here).
• [x] Tested by hand on: armv7m4-stm32l4x6-at

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing linter checks and tests passed.
• [x] My changes generate no new compilation warnings for any of the targets.

Special treatment

• [ ] This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• [ ] I will merge this PR by myself when appropriate.

[github-actions[bot]]

Unit Test Results

7 155 tests  ±0   6 448 :white_check_mark: ±0   33m 46s :stopwatch: -18s   397 suites ±0     707 :zzz: ±0      1 files   ±0       0 :x: ±0 

Results for commit 71bfc0be. ± Comparison against base commit dd197dbc.

:recycle: This comment has been updated with latest results.