Re: [PATCH v4 1/4] tty: serial: fsl_lpuart: fix DMA operation when using IOMMU

[Leonard Crestez]

On 06.03.2020 23:44, Michael Walle wrote:
> The DMA channel might not be available at probe time. This is esp. the
> case if the DMA controller has an IOMMU mapping.
> 
> There is also another caveat. If there is no DMA controller at all,
> dma_request_chan() will also return -EPROBE_DEFER. Thus we cannot test
> for -EPROBE_DEFER in probe(). Otherwise the lpuart driver will fail to
> probe if, for example, the DMA driver is not enabled in the kernel
> configuration.
> 
> To workaround this, we request the DMA channel in _startup(). Other
> serial drivers do it the same way.
> 
> Signed-off-by: Michael Walle <michael@xxxxxxxx>

This appears to cause boot hangs on imx8qxp-mek (boards boots fine on 
next-20200324 if this patch is reverted)

> ---
>   drivers/tty/serial/fsl_lpuart.c | 88 +++++++++++++++++++++------------
>   1 file changed, 57 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index c31b8f3db6bf..33798df4d727 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -1493,36 +1493,67 @@ static void rx_dma_timer_init(struct lpuart_port *sport)
>   static void lpuart_tx_dma_startup(struct lpuart_port *sport)
>   {
>   	u32 uartbaud;
> +	int ret;
>   
> -	if (sport->dma_tx_chan && !lpuart_dma_tx_request(&sport->port)) {
> -		init_waitqueue_head(&sport->dma_wait);
> -		sport->lpuart_dma_tx_use = true;
> -		if (lpuart_is_32(sport)) {
> -			uartbaud = lpuart32_read(&sport->port, UARTBAUD);
> -			lpuart32_write(&sport->port,
> -				       uartbaud | UARTBAUD_TDMAE, UARTBAUD);
> -		} else {
> -			writeb(readb(sport->port.membase + UARTCR5) |
> -				UARTCR5_TDMAS, sport->port.membase + UARTCR5);
> -		}
> +	sport->dma_tx_chan = dma_request_chan(sport->port.dev, "tx");
> +	if (IS_ERR(sport->dma_tx_chan)) {
> +		dev_info_once(sport->port.dev,
> +			      "DMA tx channel request failed, operating without tx DMA (%ld)\n",
> +			      PTR_ERR(sport->dma_tx_chan));

It seems that this since this is called from lpuart32_startup with 
&sport->port.lock held and lpuart32_console_write takes the same lock it 
can and hang.

As a workaround I can just remove this print but there are other 
possible error conditions in dmaengine code which can cause a printk.

Maybe the port lock should only be held around register manipulation?

> +		sport->dma_tx_chan = NULL;
> +		goto err;
> +	}
> +
> +	ret = lpuart_dma_tx_request(&sport->port);
> +	if (!ret)
> +		goto err;

This is backwards: lpuart_dma_tx_request returns negative errno on failure.

> +
> +	init_waitqueue_head(&sport->dma_wait);
> +	sport->lpuart_dma_tx_use = true;
> +	if (lpuart_is_32(sport)) {
> +		uartbaud = lpuart32_read(&sport->port, UARTBAUD);
> +		lpuart32_write(&sport->port,
> +			       uartbaud | UARTBAUD_TDMAE, UARTBAUD);
>   	} else {
> -		sport->lpuart_dma_tx_use = false;
> +		writeb(readb(sport->port.membase + UARTCR5) |
> +		       UARTCR5_TDMAS, sport->port.membase + UARTCR5);
>   	}
> +
> +	return;
> +
> +err:
> +	sport->lpuart_dma_tx_use = false;
>   }
>   
>   static void lpuart_rx_dma_startup(struct lpuart_port *sport)
>   {
> -	if (sport->dma_rx_chan && !lpuart_start_rx_dma(sport)) {
> -		/* set Rx DMA timeout */
> -		sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
> -		if (!sport->dma_rx_timeout)
> -			sport->dma_rx_timeout = 1;
> +	int ret;
>   
> -		sport->lpuart_dma_rx_use = true;
> -		rx_dma_timer_init(sport);
> -	} else {
> -		sport->lpuart_dma_rx_use = false;
> +	sport->dma_rx_chan = dma_request_chan(sport->port.dev, "rx");
> +	if (IS_ERR(sport->dma_rx_chan)) {
> +		dev_info_once(sport->port.dev,
> +			      "DMA rx channel request failed, operating without rx DMA (%ld)\n",
> +			      PTR_ERR(sport->dma_rx_chan));
> +		sport->dma_rx_chan = NULL;
> +		goto err;
>   	}
> +
> +	ret = lpuart_start_rx_dma(sport);
> +	if (ret)
> +		goto err;

This is not backwards.

> +
> +	/* set Rx DMA timeout */
> +	sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
> +	if (!sport->dma_rx_timeout)
> +		sport->dma_rx_timeout = 1;
> +
> +	sport->lpuart_dma_rx_use = true;
> +	rx_dma_timer_init(sport);
> +
> +	return;
> +
> +err:
> +	sport->lpuart_dma_rx_use = false;
>   }
>   
>   static int lpuart_startup(struct uart_port *port)
> @@ -1615,6 +1646,11 @@ static void lpuart_dma_shutdown(struct lpuart_port *sport)
>   			dmaengine_terminate_all(sport->dma_tx_chan);
>   		}
>   	}
> +
> +	if (sport->dma_tx_chan)
> +		dma_release_channel(sport->dma_tx_chan);
> +	if (sport->dma_rx_chan)
> +		dma_release_channel(sport->dma_rx_chan);
>   }
>   
>   static void lpuart_shutdown(struct uart_port *port)
> @@ -2520,16 +2556,6 @@ static int lpuart_probe(struct platform_device *pdev)
>   
>   	sport->port.rs485_config(&sport->port, &sport->port.rs485);
>   
> -	sport->dma_tx_chan = dma_request_slave_channel(sport->port.dev, "tx");
> -	if (!sport->dma_tx_chan)
> -		dev_info(sport->port.dev, "DMA tx channel request failed, "
> -				"operating without tx DMA\n");
> -
> -	sport->dma_rx_chan = dma_request_slave_channel(sport->port.dev, "rx");
> -	if (!sport->dma_rx_chan)
> -		dev_info(sport->port.dev, "DMA rx channel request failed, "
> -				"operating without rx DMA\n");
> -
>   	return 0;
>   
>   failed_attach_port:
>