Re: [PATCH v9 8/8] serial: 8250: retain DMA resources when termination fails

[Andy Shevchenko]

On Mon, Nov 02, 2026 at 04:14:35PM -0700, James Hilliard wrote:
> serial8250_release_dma() ignores the return from dmaengine_terminate_sync()
> before freeing the coherent RX buffer, unmapping the TX FIFO and releasing
> the channels. If the provider fails to stop a channel, the DMAengine
> wrapper returns the error without synchronizing its callbacks. Hardware
> or a detached callback can then access the released storage and state.
> 
> Checking the error and returning early would not preserve their lifetime:
> serial core frees the TX FIFO after the driver's shutdown callback returns,
> and port removal can then release the callback context as well.
> 
> Require successful synchronous termination of both channels before
> releasing any DMA resources. Retry failed channels after a sleep outside
> the port lock, leaving channels that have already stopped and drained
> alone. Keep the shutdown gate and storage reservation in place throughout
> the retries.
> 
> Start retries with a 1 ms sleep and double it up to one second. This
> makes short-lived failures responsive while bounding sustained retry
> activity; neither delay is a hardware timeout. Report the initial failure,
> rate-limit ongoing failures with the elapsed time and current RX/TX error
> codes, and report recovery once both channels have stopped and drained.
> 
> A permanently failed DMA engine blocks close or removal. This deliberately
> retains the buffers, mappings, channels and callback context until the
> provider can establish that they are no longer in use.

...

> +static void serial8250_terminate_dma(struct uart_8250_port *p)
> +{
> +	struct uart_8250_dma *dma = p->dma;
> +	unsigned int delay_us = USEC_PER_MSEC;

Don't use multipliers without a number, it's harder to read (when syntax
highlighting is on) and follow in general.

1 * USEC_PER_MSEC

> +	ktime_t start;
> +	int rx_ret, tx_ret;
> +
> +	rx_ret = dmaengine_terminate_sync(dma->rxchan);
> +	tx_ret = dmaengine_terminate_sync(dma->txchan);
> +	if (!rx_ret && !tx_ret)
> +		return;
> +
> +	start = ktime_get();
> +	dev_err(p->port.dev,
> +		"DMA termination failed: RX %d, TX %d; teardown blocked\n",
> +		rx_ret, tx_ret);
> +
> +	/*
> +	 * Shutdown cannot return while DMA owns storage or callback state:
> +	 * serial core frees the TX FIFO afterwards, and removal frees the port.
> +	 * Retain both channels and retry outside the port lock. A permanently
> +	 * failed DMA engine therefore prevents teardown from completing.
> +	 */
> +	while (rx_ret || tx_ret) {
> +		fsleep(delay_us);
> +		/* Retry promptly at first, then limit sustained retry activity. */
> +		delay_us = min(delay_us * 2, USEC_PER_SEC);

Ditto.

> +		if (rx_ret)
> +			rx_ret = dmaengine_terminate_sync(dma->rxchan);
> +		if (tx_ret)
> +			tx_ret = dmaengine_terminate_sync(dma->txchan);
> +		if (rx_ret || tx_ret)
> +			dev_err_ratelimited(p->port.dev,
> +					    "DMA termination still failing after %lld ms: RX %d, TX %d\n",
> +					    ktime_ms_delta(ktime_get(), start), rx_ret, tx_ret);
> +	}
> +	dev_info(p->port.dev, "DMA termination recovered after %lld ms\n",
> +		 ktime_ms_delta(ktime_get(), start));
> +}

-- 
With Best Regards,
Andy Shevchenko