On Mon, Nov 02, 2026 at 04:14:33PM -0700, James Hilliard wrote:
> uart_flush_buffer() resets the transmit kfifo before calling the driver.
> serial8250_tx_dma_flush() terminates DMA asynchronously and clears
> tx_running, allowing a new write to replace tx_size while a detached
> callback has not yet run. That callback can then consume bytes
> from the new transmission. Reusing the reset kfifo can also overwrite the
> source of a transfer that has not stopped yet.
>
> Reserve the flushed FIFO storage through serial core before returning
> from the flush callback. Keep tx_running set until a worker synchronizes
> the terminated channel, suppressing old callbacks and new submissions
> during that interval. Return the mapping to CPU ownership before releasing
> the reservation and waking writers. Normal transfers continue to use the
> FIFO directly.
>
> Reuse the locked completion processing after retirement, including OMAP
> delayed register restoration and transmitter-empty handling. Take a runtime
> PM reference around worker UART accesses and drain the worker before
> releasing the channels. Shutdown releases the FIFO reservation after
> stopping DMA and unmapping its source. Serial core keeps writes and wakeups
> gated until startup or resume makes the port ready again.
>
> Track a flushed FIFO separately from the transfer length. If termination
> fails, retain the reservation until normal completion and account the
> completed bytes without advancing the reset FIFO. After successful
> cancellation, process a zero-length completion once callbacks have drained.
>
> The race was found by inspecting cancellation and restart while preparing
> H616 UART DMA support.
>
> Retain the tx_running check in both completion callbacks. Releasing the
> reservation relies on the DMA provider draining detached callbacks when
> dmaengine_synchronize() returns; provider callback-lifetime fixes supply
> that guarantee for controllers which previously lacked it.
...
> - if (!dma->tx_running)
> + if (!dma->tx_running || serial8250_tx_dma_blocked(dma))
> return;
> - if (!dma->tx_running)
> + if (!dma->tx_running || serial8250_tx_dma_blocked(dma))
> return;
...
> guard(uart_port_lock_irqsave)(&p->port);
>
> - if (dma->shutting_down)
> + if (serial8250_tx_dma_blocked(dma) || !dma->tx_running)
> return;
Try to make these checks consistent. It's always better to choose the order and
be aware of the potential side-effects of the bit-wise operations.
...
> - if (dma->shutting_down || dma->tx_running)
> + if (serial8250_tx_dma_blocked(dma) || dma->tx_running)
> return 0;
> if (uart_tx_stopped(&p->port) || kfifo_is_empty(&tport->xmit_fifo)) {
--
With Best Regards,
Andy Shevchenko
