Swatantra-66 opened a new pull request, #20894:
URL: https://github.com/apache/nuttx/pull/20894
## Summary
Fixes a critical bug where serial TX DMA sets up a zero-length second
transfer descriptor on buffer wrap, triggering illegal hardware configuration
errors on eDMA engines.
### Root Cause
In `drivers/serial/serial_dma.c` (`uart_xmitchars_dma()`), when the transmit
circular buffer wraps with `head == 0` and `tail > 0`:
```c
else
{
xfer->buffer = &dev->xmit.buffer[dev->xmit.tail];
xfer->length = dev->xmit.size - dev->xmit.tail;
xfer->nbuffer = dev->xmit.buffer;
xfer->nlength = dev->xmit.head;
}
```
`xfer->nbuffer` was assigned `dev->xmit.buffer` even though `xfer->nlength
== 0`.
Across chip drivers (`imxrt_serial.c`, `nxxx_lpuart.c`, `imx9_lpuart.c`,
`s32k1xx_serial.c`, `s32k3xx_serial.c`), the code checks:
```c
if (dev->dmatx.nbuffer)
```
Because `nbuffer != NULL`, the driver configures a second DMA descriptor
with `iter = nlength = 0` (or `CITER = BITER = 0`). On NXP eDMA controllers
(i.MX RT, S32K, MCX-N, i.MX9), `CITER = 0` is an illegal descriptor
configuration. The eDMA hardware detects this, asserts `ES[NCE]` (Non-zero
Configuration Error), and sets `CR[HALT] = 1`, which halts the entire eDMA
engine chip-wide and causes SPI, I2C, and UART peripherals to lock up.
### Fix
1. **`drivers/serial/serial_dma.c`**:
- In `uart_xmitchars_dma()`: Only set `xfer->nbuffer` and `xfer->nlength`
if `dev->xmit.head > 0`. If `head == 0`, explicitly set `xfer->nbuffer = NULL;
xfer->nlength = 0;`.
- In `uart_xmitchars_done()`: Clean up `xfer->buffer = xfer->nbuffer =
NULL;` on completion.
2. **`arch/arm` and `arch/arm64`**:
- Defend against zero-length descriptor setup in `imxrt`, `mcx-nxxx`,
`imx9`, `s32k1xx`, and `s32k3xx` serial drivers by checking `if
(dev->dmatx.nbuffer != NULL && dev->dmatx.nlength > 0)`.
Fixes: #20881
## Impact
- Prevents eDMA `ES[NCE]` configuration errors and chip-wide DMA halt on
i.MX RT, S32K, MCX, and i.MX9 during serial TX DMA operations.
- Eliminates zero-length DMA descriptor submissions across all architectures
using `CONFIG_SERIAL_TXDMA`.
## Testing
- Code style verified with `tools/checkpatch.sh -u`: all checks pass.
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