Re: [RFC PATCH 3/3] tty: serial: fsl_lpuart: fix possible console deadlock

[Leonard Crestez]

On 2020-03-24 8:48 PM, Michael Walle wrote:
> If the kernel console output is on this console any
> dev_{err,warn,info}() may result in a deadlock if the sport->port.lock
> spinlock is already held. This is because the _console_write() try to
> aquire this lock, too. Remove any error messages where the spinlock is
> taken or print after the lock is released.
> 
> Reported-by: Leonard Crestez <leonard.crestez@xxxxxxx>
> Signed-off-by: Michael Walle <michael@xxxxxxxx>

It seems that this was an issue even before commit 159381df1442 ("tty: 
serial: fsl_lpuart: fix DMA operation when using IOMMU") but these error 
prints never triggered.

Would it be possible to move all the dma alloc/config/prep outside the 
serial port lock? As it stands this still calls into dmaengine coode and 
that might decide to print as well.

Really I don't think the lock needs to protect more than bits like 
TDMAE/RDMAE.

BTW: You should add more people in CC for reviews, for example 
linux-imx@xxxxxxx is checked by a lot of people.

> ---
>   drivers/tty/serial/fsl_lpuart.c | 35 +++++++--------------------------
>   1 file changed, 7 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index bbba298b68a4..0910308b38b1 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -420,7 +420,6 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
>   {
>   	struct circ_buf *xmit = &sport->port.state->xmit;
>   	struct scatterlist *sgl = sport->tx_sgl;
> -	struct device *dev = sport->port.dev;
>   	struct dma_chan *chan = sport->dma_tx_chan;
>   	int ret;
>   
> @@ -442,10 +441,8 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
>   
>   	ret = dma_map_sg(chan->device->dev, sgl, sport->dma_tx_nents,
>   			 DMA_TO_DEVICE);
> -	if (!ret) {
> -		dev_err(dev, "DMA mapping error for TX.\n");
> +	if (!ret)
>   		return;
> -	}
>   
>   	sport->dma_tx_desc = dmaengine_prep_slave_sg(chan, sgl,
>   					ret, DMA_MEM_TO_DEV,
> @@ -453,7 +450,6 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
>   	if (!sport->dma_tx_desc) {
>   		dma_unmap_sg(chan->device->dev, sgl, sport->dma_tx_nents,
>   			      DMA_TO_DEVICE);
> -		dev_err(dev, "Cannot prepare TX slave DMA!\n");
>   		return;
>   	}
>   
> @@ -520,21 +516,12 @@ static int lpuart_dma_tx_request(struct uart_port *port)
>   	struct lpuart_port *sport = container_of(port,
>   					struct lpuart_port, port);
>   	struct dma_slave_config dma_tx_sconfig = {};
> -	int ret;
>   
>   	dma_tx_sconfig.dst_addr = lpuart_dma_datareg_addr(sport);
>   	dma_tx_sconfig.dst_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
>   	dma_tx_sconfig.dst_maxburst = 1;
>   	dma_tx_sconfig.direction = DMA_MEM_TO_DEV;
> -	ret = dmaengine_slave_config(sport->dma_tx_chan, &dma_tx_sconfig);
> -
> -	if (ret) {
> -		dev_err(sport->port.dev,
> -				"DMA slave config failed, err = %d\n", ret);
> -		return ret;
> -	}
> -
> -	return 0;
> +	return dmaengine_slave_config(sport->dma_tx_chan, &dma_tx_sconfig);
>   }
>   
>   static bool lpuart_is_32(struct lpuart_port *sport)
> @@ -1074,8 +1061,8 @@ static void lpuart_copy_rx_to_tty(struct lpuart_port *sport)
>   
>   	dmastat = dmaengine_tx_status(chan, sport->dma_rx_cookie, &state);
>   	if (dmastat == DMA_ERROR) {
> -		dev_err(sport->port.dev, "Rx DMA transfer failed!\n");
>   		spin_unlock_irqrestore(&sport->port.lock, flags);
> +		dev_err(sport->port.dev, "Rx DMA transfer failed!\n");
>   		return;
>   	}
>   
> @@ -1179,23 +1166,17 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
>   	sg_init_one(&sport->rx_sgl, ring->buf, sport->rx_dma_rng_buf_len);
>   	nent = dma_map_sg(chan->device->dev, &sport->rx_sgl, 1,
>   			  DMA_FROM_DEVICE);
> -
> -	if (!nent) {
> -		dev_err(sport->port.dev, "DMA Rx mapping error\n");
> +	if (!nent)
>   		return -EINVAL;
> -	}
>   
>   	dma_rx_sconfig.src_addr = lpuart_dma_datareg_addr(sport);
>   	dma_rx_sconfig.src_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
>   	dma_rx_sconfig.src_maxburst = 1;
>   	dma_rx_sconfig.direction = DMA_DEV_TO_MEM;
> -	ret = dmaengine_slave_config(chan, &dma_rx_sconfig);
>   
> -	if (ret < 0) {
> -		dev_err(sport->port.dev,
> -				"DMA Rx slave config failed, err = %d\n", ret);
> +	ret = dmaengine_slave_config(chan, &dma_rx_sconfig);
> +	if (ret < 0)
>   		return ret;
> -	}
>   
>   	sport->dma_rx_desc = dmaengine_prep_dma_cyclic(chan,
>   				 sg_dma_address(&sport->rx_sgl),
> @@ -1203,10 +1184,8 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
>   				 sport->rx_sgl.length / 2,
>   				 DMA_DEV_TO_MEM,
>   				 DMA_PREP_INTERRUPT);
> -	if (!sport->dma_rx_desc) {
> -		dev_err(sport->port.dev, "Cannot prepare cyclic DMA\n");
> +	if (!sport->dma_rx_desc)
>   		return -EFAULT;
> -	}
>   
>   	sport->dma_rx_desc->callback = lpuart_dma_rx_complete;
>   	sport->dma_rx_desc->callback_param = sport;
>