On Thu, Feb 06, 2025 at 03:55:51PM +0000, John Keeping wrote:
> When flushing the serial port's buffer, uart_flush_buffer() calls
> kfifo_reset() but if there is an outstanding DMA transfer then the
> completion function will consume data from the kfifo via
> uart_xmit_advance(), underflowing and leading to ongoing DMA as the
> driver tries to transmit another 2^32 bytes.
>
> This is readily reproduced with serial-generic and amidi sending even
> short messages as closing the device on exit will wait for the fifo to
> drain and in the underflow case amidi hangs for 30 seconds on exit in
> tty_wait_until_sent().
Sounds not good user experience...
> A trace of that gives:
>
> kworker/1:1-84 [001] 51.769423: bprint: serial8250_tx_dma: tx_size=3 fifo_len=3
> amidi-763 [001] 51.769460: bprint: uart_flush_buffer: resetting fifo
> irq/21-fe530000-76 [000] 51.769474: bprint: __dma_tx_complete: tx_size=3
> irq/21-fe530000-76 [000] 51.769479: bprint: serial8250_tx_dma: tx_size=4096 fifo_len=4294967293
> irq/21-fe530000-76 [000] 51.781295: bprint: __dma_tx_complete: tx_size=4096
> irq/21-fe530000-76 [000] 51.781301: bprint: serial8250_tx_dma: tx_size=4096 fifo_len=4294963197
> irq/21-fe530000-76 [000] 51.793131: bprint: __dma_tx_complete: tx_size=4096
> irq/21-fe530000-76 [000] 51.793135: bprint: serial8250_tx_dma: tx_size=4096 fifo_len=4294959101
> irq/21-fe530000-76 [000] 51.804949: bprint: __dma_tx_complete: tx_size=4096
>
> Since the port lock is held in when the kfifo is reset in
> uart_flush_buffer() and in __dma_tx_complete(), adding a flush_buffer
> hook to adjust the outstanding DMA byte count is sufficient to avoid the
> kfifo underflow.
Shouldn't this have a Fixes tag?
...
> +void serial8250_tx_dma_flush(struct uart_8250_port *p)
> +{
> + struct uart_8250_dma *dma = p->dma;
> + if (dma->tx_running) {
if (!dma->tx_running)
return;
> + /*
> + * kfifo_reset() has been called by the serial core, avoid
> + * advancing and underflowing in __dma_tx_complete().
> + */
> + dma->tx_size = 0;
> +
> + dmaengine_terminate_async(dma->rxchan);
> + }
> +}
--
With Best Regards,
Andy Shevchenko
