[PATCH] 8250: add workaround for MPC8[356]xx UART break IRQ storm

[Kumar Gala]

On Feb 26, 2010, at 1:25 PM, Paul Gortmaker wrote:

> Sending a break on the SOC UARTs found in some MPC83xx/85xx/86xx
> chips seems to cause a short lived IRQ storm (/proc/interrupts
> typically shows somewhere between 300 and 1500 events).  Unfortunately
> this renders SysRQ over the serial console completely inoperable.
> Testing with obvious things like ACKing the event doesn't seem to
> change anything vs. a completely dumb approach of just ignoring
> it and waiting for it to stop, so that is what is implemented here.
> 
> Signed-off-by: Paul Gortmaker <paul.gortmaker at windriver.com>
> ---
> 
> This is a refresh of a patch I'd done earlier -- I've tried to make
> the bug support as generic as possible to minimize having board
> specific ifdef crap in 8250.c -- any suggestions on how to further
> improve it are welcome.
> 
> drivers/serial/8250.c      |    6 ++++++
> drivers/serial/8250.h      |   20 ++++++++++++++++++++
> drivers/serial/Kconfig     |   14 ++++++++++++++
> include/linux/serial_reg.h |    2 ++
> 4 files changed, 42 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
> index e9b15c3..850b0e9 100644
> --- a/drivers/serial/8250.c
> +++ b/drivers/serial/8250.c
> @@ -1531,6 +1531,11 @@ static void serial8250_handle_port(struct uart_8250_port *up)
> 
> 	status = serial_inp(up, UART_LSR);
> 
> +	if ((up->bugs & UART_BUG_PPC) && (status == UART_LSR_RFE_ERROR_BITS)) {
> +		spin_unlock_irqrestore(&up->port.lock, flags);
> +		return;
> +	}
> +
> 	DEBUG_INTR("status = %x...", status);
> 
> 	if (status & (UART_LSR_DR | UART_LSR_BI))
> @@ -1948,6 +1953,7 @@ static int serial8250_startup(struct uart_port *port)
> 
> 	up->capabilities = uart_config[up->port.type].flags;
> 	up->mcr = 0;
> +	up->bugs |= UART_KNOWN_BUGS;
> 
> 	if (up->port.iotype != up->cur_iotype)
> 		set_io_from_upio(port);
> diff --git a/drivers/serial/8250.h b/drivers/serial/8250.h
> index 6e19ea3..2074ce1 100644
> --- a/drivers/serial/8250.h
> +++ b/drivers/serial/8250.h
> @@ -49,6 +49,7 @@ struct serial8250_config {
> #define UART_BUG_TXEN	(1 << 1)	/* UART has buggy TX IIR status */
> #define UART_BUG_NOMSR	(1 << 2)	/* UART has buggy MSR status bits (Au1x00) */
> #define UART_BUG_THRE	(1 << 3)	/* UART has buggy THRE reassertion */
> +#define UART_BUG_PPC	(1 << 4)	/* UART has buggy PPC break IRQ storm */
> 
> #define PROBE_RSA	(1 << 0)
> #define PROBE_ANY	(~0)
> @@ -78,3 +79,22 @@ struct serial8250_config {
> #else
> #define ALPHA_KLUDGE_MCR 0
> #endif
> +
> +/*
> + * The following UART bugs are currently dynamically detected and not
> + * required to be contingent on any particular compile time options.
> + */
> +#define HAS_BUG_QUOT	0	/* assign UART_BUG_QUOT to enable */
> +#define HAS_BUG_TXEN	0	/* assign UART_BUG_TXEN to enable */
> +#define HAS_BUG_NOMSR	0	/* assign UART_BUG_NOMSR to enable */
> +#define HAS_BUG_THRE	0	/* assign UART_BUG_THRE to enable */
> +
> +#ifdef CONFIG_SERIAL_8250_PPC_BUG
> +#define HAS_BUG_PPC	UART_BUG_PPC
> +#else
> +#define HAS_BUG_PPC	0
> +#endif
> +
> +#define UART_KNOWN_BUGS (HAS_BUG_QUOT | HAS_BUG_TXEN | HAS_BUG_NOMSR | \
> +			HAS_BUG_THRE | HAS_BUG_PPC)
> +
> diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
> index 9ff47db..e01a411 100644
> --- a/drivers/serial/Kconfig
> +++ b/drivers/serial/Kconfig
> @@ -70,6 +70,20 @@ config SERIAL_8250_CONSOLE
> 
> 	  If unsure, say N.
> 
> +config SERIAL_8250_PPC_BUG
> +	bool "Fix 8250/16550 to handle IRQ storm after receipt of a break"
> +	depends on SERIAL_8250 && PPC32
> +	---help---
> +	  If you say Y here, addional checks will be added in the handling of
> +	  interrupts on the serial ports which will prevent ill effects of
> +	  an interrupt storm triggered by a break on the serial line. Without
> +	  this enabled, a Sysrq via the serial console can be unusable on
> +	  some systems.
> +
> +	  This is commonly observed on PPC32 MPC83xx/85xx/86xx based boards.
> +
> +	  If unsure, say N.
> +

is there harm caused if we have SERIAL_8250_PPC_BUG set and dont need it?

- k