On Thu, Feb 06, 2025 at 08:55:09PM +0200, Andy Shevchenko wrote:
> 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?
I'll add one in v2.
> > +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;
Will change in v2.
> > + /*
> > + * 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);
> > + }
> > +}
Thanks for the review!
Regards,
John
