Re: [PATCH v2] serial: 8250: 8250_omap: Fix possible interrupt storm on K3 SoCs

[Jan Kiszka <jan.kiszka@siemens.com>]

On 22.06.21 16:57, Vignesh Raghavendra wrote:
> On K3 family of SoCs (which includes AM654 SoC), it is observed that RX
> TIMEOUT is signalled after RX FIFO has been drained, in which case a
> dummy read of RX FIFO is required to clear RX TIMEOUT condition.
> Otherwise, this would lead to an interrupt storm.
> 
> Fix this by introducing UART_RX_TIMEOUT_QUIRK flag and doing a dummy
> read in IRQ handler when RX TIMEOUT is reported with no data in RX FIFO.
> 
> Fixes: be70874498f3 ("serial: 8250_omap: Add support for AM654 UART controller")
> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
> Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
> ---
> v2:
> Restrict workaround to K3 family of devices only (ti,am654-uart) where
> issue was reported.
> 
> v1: https://lore.kernel.org/r/20210511151955.28071-1-vigneshr@ti.com
> 
>  drivers/tty/serial/8250/8250_omap.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> index c06631ced414..79418d4beb48 100644
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -43,6 +43,7 @@
>  #define UART_ERRATA_CLOCK_DISABLE	(1 << 3)
>  #define	UART_HAS_EFR2			BIT(4)
>  #define UART_HAS_RHR_IT_DIS		BIT(5)
> +#define UART_RX_TIMEOUT_QUIRK		BIT(6)
>  
>  #define OMAP_UART_FCR_RX_TRIG		6
>  #define OMAP_UART_FCR_TX_TRIG		4
> @@ -104,6 +105,9 @@
>  #define UART_OMAP_EFR2			0x23
>  #define UART_OMAP_EFR2_TIMEOUT_BEHAVE	BIT(6)
>  
> +/* RX FIFO occupancy indicator */
> +#define UART_OMAP_RX_LVL		0x64
> +
>  struct omap8250_priv {
>  	int line;
>  	u8 habit;
> @@ -611,6 +615,7 @@ static int omap_8250_dma_handle_irq(struct uart_port *port);
>  static irqreturn_t omap8250_irq(int irq, void *dev_id)
>  {
>  	struct uart_port *port = dev_id;
> +	struct omap8250_priv *priv = port->private_data;
>  	struct uart_8250_port *up = up_to_u8250p(port);
>  	unsigned int iir;
>  	int ret;
> @@ -625,6 +630,18 @@ static irqreturn_t omap8250_irq(int irq, void *dev_id)
>  	serial8250_rpm_get(up);
>  	iir = serial_port_in(port, UART_IIR);
>  	ret = serial8250_handle_irq(port, iir);
> +
> +	/*
> +	 * On K3 SoCs, it is observed that RX TIMEOUT is signalled after
> +	 * FIFO has been drained, in which case a dummy read of RX FIFO
> +	 * is required to clear RX TIMEOUT condition.
> +	 */
> +	if (priv->habit & UART_RX_TIMEOUT_QUIRK &&
> +	    (iir & UART_IIR_RX_TIMEOUT) == UART_IIR_RX_TIMEOUT &&
> +	    serial_port_in(port, UART_OMAP_RX_LVL) == 0) {
> +		serial_port_in(port, UART_RX);
> +	}
> +
>  	serial8250_rpm_put(up);
>  
>  	return IRQ_RETVAL(ret);
> @@ -1218,7 +1235,8 @@ static struct omap8250_dma_params am33xx_dma = {
>  
>  static struct omap8250_platdata am654_platdata = {
>  	.dma_params	= &am654_dma,
> -	.habit		= UART_HAS_EFR2 | UART_HAS_RHR_IT_DIS,
> +	.habit		= UART_HAS_EFR2 | UART_HAS_RHR_IT_DIS |
> +			  UART_RX_TIMEOUT_QUIRK,
>  };
>  
>  static struct omap8250_platdata am33xx_platdata = {
> 

Tested-by: Jan Kiszka <jan.kiszka@siemens.com>

Thanks,
Jan

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