Fix index and ability to configure LPUART_RX due to typo

[haslinghuis]

• fixes #13567
• ref: https://github.com/betaflight/betaflight/pull/13306
• fix index when assigning LPUART resource due to typo
• fix misleading _MAX_INDEX with zero-based C indexing / when mixing resource id with C arrays

Some history:

• 10704

• 11821

• 11786

Tested on MAMBAG4AIO

resource SERIAL_TX 1 A09
resource SERIAL_TX 2 B03
resource SERIAL_TX 4 C10
resource SERIAL_RX 1 A10
resource SERIAL_RX 2 B04
resource SERIAL_RX 3 B09
resource SERIAL_RX 4 C11
resource LPUART_TX 1 B11
resource LPUART_RX 1 B10

# serial
serial 20 1 115200 57600 0 115200
serial 0 0 115200 57600 0 115200
serial 1 0 115200 57600 0 115200
serial 2 0 115200 57600 0 115200
serial 3 0 115200 57600 0 115200
serial 40 0 115200 57600 0 115200

[github-actions[bot]]

Do you want to test this code? You can flash it directly from Betaflight Configurator:

• Simply put #13565 (this pull request number) in the Select commit field of the Configurator firmware flasher tab (you need to Enable expert mode, Show release candidates and Development).

WARNING: It may be unstable. Use only for testing!

[hydra]

I'd just like to say that while being accurate, the PR title leaves a lot out. Perhaps, 'Fix ability to configure LPUART_RX due to typo'. The description is a bit lacking too, some information about the original report and the source would have been nice too. When searching through commits, history and using git blame it really helps to have good commit messages and PR titles.

Here's my orginal report that led to this PR, posted in discord. https://discord.com/channels/868013470023548938/924169278926770207/1231700278089089117

[ledvinap]

Actually, the code is still wrong. SERIAL_PORT_MAX_INDEX is count of uarts + lpuarts. So it should be

    DEFA( OWNER_LPUART_TX,     PG_SERIAL_PIN_CONFIG, serialPinConfig_t, ioTagTx[SERIAL_PORT_MAX_INDEX - SERIAL_LPUART_MAX_INDEX], SERIAL_LPUART_MAX_INDEX ),
    DEFA( OWNER_LPUART_RX,     PG_SERIAL_PIN_CONFIG, serialPinConfig_t, ioTagRx[SERIAL_PORT_MAX_INDEX - SERIAL_LPUART_MAX_INDEX], SERIAL_LPUART_MAX_INDEX ),
#endif

And _MAX_INDEX is totally misleading with zero-based C indexing / when mixing resource id with C arrays

[haslinghuis]

@ledvinap @hydra

[hydra]

@ledvinap @hydra

I gave this a test on the SPRacingH7NEO target I'm working on and was able to remap UART 1 RX/TX pins and assign/de-assign/swap LPUART 1 RX and TX pins ok.

suggest merging this as a quick-fix until someone has more time to cleanup the indexing further with regards to the other changes in #13569

[hydra]

I would just like to mention that it sure would be nice to have some unit tests to check for expected behavior.

[ledvinap]

@hydra : That would mean unittesting different target #defines .. not impossible, bue there is very little scaffolding in place now