alexcekay opened a new pull request, #18975:
URL: https://github.com/apache/nuttx/pull/18975
## Summary
The STM32 serial drivers already treat 0 as not present for optional GPIO
fields (CTS, RTS), guarding each call site with a `!= 0` check.
TX and RX GPIO fields lacked this guard, so boards that define
`GPIO_UARTx_TX` or `GPIO_UARTx_RX` as 0 (pin not routed) caused
`stm32_configgpio(0)` to be called, which silently configures PA0 as a floating
input, corrupting any other peripheral using that pin.
Having an optional TX/RX UART is useful for several cases:
- RX-only connections: e.g. radio control input on flight-controllers where
no TX is wired.
- Single-wire (half-duplex) connections: e.g. ESC telemetry connections.
- Output-only NSH which prints output but does not allow entering inputs.
Without a sentinel value for these cases one must assign the unused GPIO to
a NC pin, which is misleading when reading board configuration files.
This PR adds `!= 0` guards for `tx_gpio` and `rx_gpio` across all STM32
serial driver families.
Using 0 as the sentinel is safe for TX/RX because any valid UART config
requires `GPIO_ALT` bits set in the config word, so 0 can never represent a
real TX or RX pin configuration.
## Impact
- For users that already define a `GPIO_UARTx_TX` / `GPIO_UARTx_RX` as 0
this fixes the problem of unintendedly altering PA0
- As 0 is not a valid TX/RX pin config for any UART GPIO this change does
not break any existing behavior
## Testing
Tested on a v6s flight-controller (STM32H743VIH6) where we have the
following in the board configuration
```
#ifdef PX4_RESTRICTED_BUILD
# define GPIO_UART4_RX 0 /* PD0 */
#else
# define GPIO_UART4_RX GPIO_UART4_RX_5 /* PD0 */
#endif /* PX4_RESTRICTED_BUILD */
#define GPIO_UART4_TX (GPIO_UART4_TX_5 | GPIO_SPEED_2MHz) /* PD1 */
#define GPIO_UART5_RX (GPIO_UART5_RX_3 | GPIO_SPEED_2MHz) /* PD2 */
#define GPIO_UART5_TX 0 /* We did not route out the TX pin of UART5 */
```
With the change the following was observed:
- The NSH on UART4 correctly prints data in case `PX4_RESTRICTED_BUILD` is
set but does not allow the user to enter data
- So having a RX as 0 works as expected
- The RC input parser running on UART5 was able to read data correctly
- So having a TX as 0 works as expected
- The problem of PA0 being set to 0 is not observed anymore
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