alexcekay opened a new pull request, #17203:
URL: https://github.com/apache/nuttx/pull/17203
## Summary
When closing a serial port an ongoing TX DMA transfer will be stopped. This
can cause one (or multiple) the following:
- dev->dmatx.length != 0
- dev->dmatx.nlength != 0
- stm32_dmaresidual returning a non-zero residual
This is caused by length/nlength not being set to 0 at startup or during
closing. In addition the DMA_SxNDTR register is not set to 0 at startup or
during closing.
This commit solves the issue by setting the variables and register to 0
during closing.
## Testing
Tested on an Auterion FMU v6x. The issue could be triggered by triggering a
large DMA transmit and closing the port afterwards. After opening the port
again no TX DMA will be started, due to `uart_xmitchars_dma` not being executed
as the condition would always remain false.
```
if (priv->dev.dmatx.length == 0 &&
priv->dev.dmatx.nlength == 0 &&
stm32_dmaresidual(priv->txdma) == 0)
{
uart_xmitchars_dma(dev);
}
```
When applying the fix `uart_xmitchars_dma` will be executed as expected.
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