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

[Andy Duan]

From: Leonard Crestez <leonard.crestez@xxxxxxx> Sent: Tuesday, March 24, 2020 11:27 PM
> 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.
If DMA driver is not enabled, we should disable DMA controller node in
dts file to match current sw environment, then driver doesn't do defer probe.

So I still suggest to check -EPROBE_DEFER for dma_request_slave_channel() in
.probe() function.

Andy
> >
> > 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:
> >