linux-kernel - Re: [PATCH v2] serial: imx: Fix handling of TC irq in combination with DMA

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Message-ID: <a0b56107-bea0-7105-3021-030b181b2260@kontron.de>
Date:   Tue, 9 Jun 2020 08:23:30 +0000
From:   Schrempf Frieder <frieder.schrempf@...tron.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Fabio Estevam <festevam@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v2] serial: imx: Fix handling of TC irq in combination
 with DMA

On 09.06.20 09:23, Schrempf Frieder wrote:
> From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> 
> commit 1866541492641c02874bf51f9d8712b5510f2c64 upstream
> 
> When using RS485 half duplex the Transmitter Complete irq is needed to
> determine the moment when the transmitter can be disabled. When using
> DMA this irq must only be enabled when DMA has completed to transfer all
> data. Otherwise the CPU might busily trigger this irq which is not
> properly handled and so the also pending irq for the DMA transfer cannot
> trigger.
> 
> Cc: <stable@...r.kernel.org> # v4.14.x
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> [Backport to v4.14]
> Signed-off-by: Frieder Schrempf <frieder.schrempf@...tron.de>
> ---

Sorry, forgot the changelog:

Changes for backport v2:
* Remove an unrelated additional blank line.

> When using RS485 with DMA enabled simply transmitting some data on our
> i.MX6ULL based boards often freezes the system completely. The higher
> the baudrate, the easier it is to reproduce the issue. To test this I
> simply used:
> 
> stty -F /dev/ttymxc1 speed 115200
> while true; do echo TEST > /dev/ttymxc1; done
> 
> Without the patch this leads to an almost immediate system freeze,
> with the patch applied, everything keeps working as expected.
> ---
>   drivers/tty/serial/imx.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 3f2605edd855..70c737236870 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -538,6 +538,11 @@ static void dma_tx_callback(void *data)
>   
>   	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&sport->port))
>   		imx_dma_tx(sport);
> +	else if (sport->port.rs485.flags & SER_RS485_ENABLED) {
> +		temp = readl(sport->port.membase + UCR4);
> +		temp |= UCR4_TCEN;
> +		writel(temp, sport->port.membase + UCR4);
> +	}
>   
>   	spin_unlock_irqrestore(&sport->port.lock, flags);
>   }
> @@ -555,6 +560,10 @@ static void imx_dma_tx(struct imx_port *sport)
>   	if (sport->dma_is_txing)
>   		return;
>   
> +	temp = readl(sport->port.membase + UCR4);
> +	temp &= ~UCR4_TCEN;
> +	writel(temp, sport->port.membase + UCR4);
> +
>   	sport->tx_bytes = uart_circ_chars_pending(xmit);
>   
>   	if (xmit->tail < xmit->head || xmit->head == 0) {
> @@ -617,10 +626,15 @@ static void imx_start_tx(struct uart_port *port)
>   		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
>   			imx_stop_rx(port);
>   
> -		/* enable transmitter and shifter empty irq */
> -		temp = readl(port->membase + UCR4);
> -		temp |= UCR4_TCEN;
> -		writel(temp, port->membase + UCR4);
> +		/*
> +		 * Enable transmitter and shifter empty irq only if DMA is off.
> +		 * In the DMA case this is done in the tx-callback.
> +		 */
> +		if (!sport->dma_is_enabled) {
> +			temp = readl(port->membase + UCR4);
> +			temp |= UCR4_TCEN;
> +			writel(temp, port->membase + UCR4);
> +		}
>   	}
>   
>   	if (!sport->dma_is_enabled) {
>