Re: [PATCH 1/2] serial: 8250: add UPF_NO_EMPTY_FIFO_READ flag

[Tony Lindgren <tony@atomide.com>]

* Vikram Pandita <vikram.pandita@ti.com> [091116 15:00]:
> OMAP3630 and OMAP4430 UART IP blocks have a restriction wrt RX FIFO.
> Empty RX fifo read causes an abort. OMAP1/2/3 do not have this restriction.
> 
> So interoduce a flag in 8250 driver: UPF_NO_EMPTY_FIFO_READ
> 
> If this flag is specified for an 8250 port, then first check if rx fifo has
> contents, and only then proceed to read the fifo.
> 
> The change affects only boards passing this flag in port data and hence is
> non-disruptive for other boards.
> 
> First reported/fixed on omap4430 by Shilimkar, Santosh

This should go to linux-serial@vger.kernel.org and Alan Cox.

Regards,

Tony
 
> Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
> Cc: Shilimkar, Santosh <santosh.shilimkar@ti.com>
> ---
>  drivers/serial/8250.c       |   30 +++++++++++++++++++++++++-----
>  include/linux/serial_core.h |    1 +
>  2 files changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
> index 737b4c9..03c890e 100644
> --- a/drivers/serial/8250.c
> +++ b/drivers/serial/8250.c
> @@ -1245,7 +1245,11 @@ static void autoconfig(struct uart_8250_port *up, unsigned int probeflags)
>  #endif
>  	serial_outp(up, UART_MCR, save_mcr);
>  	serial8250_clear_fifos(up);
> -	serial_in(up, UART_RX);
> +	if (up->port.flags & UPF_NO_EMPTY_FIFO_READ) {
> +		if (serial_inp(up, UART_LSR) & UART_LSR_DR)
> +			serial_in(up, UART_RX);
> +	} else
> +		serial_in(up, UART_RX);
>  	if (up->capabilities & UART_CAP_UUE)
>  		serial_outp(up, UART_IER, UART_IER_UUE);
>  	else
> @@ -1289,7 +1293,11 @@ static void autoconfig_irq(struct uart_8250_port *up)
>  	}
>  	serial_outp(up, UART_IER, 0x0f);	/* enable all intrs */
>  	(void)serial_inp(up, UART_LSR);
> -	(void)serial_inp(up, UART_RX);
> +	if (up->port.flags & UPF_NO_EMPTY_FIFO_READ) {
> +		if (serial_inp(up, UART_LSR) & UART_LSR_DR)
> +			(void)serial_inp(up, UART_RX);
> +	} else
> +		(void)serial_inp(up, UART_RX);
>  	(void)serial_inp(up, UART_IIR);
>  	(void)serial_inp(up, UART_MSR);
>  	serial_outp(up, UART_TX, 0xFF);
> @@ -1984,7 +1992,11 @@ static int serial8250_startup(struct uart_port *port)
>  	 * Clear the interrupt registers.
>  	 */
>  	(void) serial_inp(up, UART_LSR);
> -	(void) serial_inp(up, UART_RX);
> +	if (up->port.flags & UPF_NO_EMPTY_FIFO_READ) {
> +		if (serial_inp(up, UART_LSR) & UART_LSR_DR)
> +			(void) serial_inp(up, UART_RX);
> +	} else
> +		(void) serial_inp(up, UART_RX);
>  	(void) serial_inp(up, UART_IIR);
>  	(void) serial_inp(up, UART_MSR);
>  
> @@ -2141,7 +2153,11 @@ dont_test_tx_en:
>  	 * routines or the previous session.
>  	 */
>  	serial_inp(up, UART_LSR);
> -	serial_inp(up, UART_RX);
> +	if (up->port.flags & UPF_NO_EMPTY_FIFO_READ) {
> +		if (serial_inp(up, UART_LSR) & UART_LSR_DR)
> +			serial_inp(up, UART_RX);
> +	} else
> +		serial_inp(up, UART_RX);
>  	serial_inp(up, UART_IIR);
>  	serial_inp(up, UART_MSR);
>  	up->lsr_saved_flags = 0;
> @@ -2207,7 +2223,11 @@ static void serial8250_shutdown(struct uart_port *port)
>  	 * Read data port to reset things, and then unlink from
>  	 * the IRQ chain.
>  	 */
> -	(void) serial_in(up, UART_RX);
> +	if (up->port.flags & UPF_NO_EMPTY_FIFO_READ) {
> +		if (serial_inp(up, UART_LSR) & UART_LSR_DR)
> +			(void) serial_inp(up, UART_RX);
> +	} else
> +		(void) serial_in(up, UART_RX);
>  
>  	del_timer_sync(&up->timer);
>  	up->timer.function = serial8250_timeout;
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index db532ce..0d65bb3 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -308,6 +308,7 @@ struct uart_port {
>  #define UPF_SPD_WARP		((__force upf_t) (0x1010))
>  #define UPF_SKIP_TEST		((__force upf_t) (1 << 6))
>  #define UPF_AUTO_IRQ		((__force upf_t) (1 << 7))
> +#define UPF_NO_EMPTY_FIFO_READ	((__force upf_t) (1 << 8))
>  #define UPF_HARDPPS_CD		((__force upf_t) (1 << 11))
>  #define UPF_LOW_LATENCY		((__force upf_t) (1 << 13))
>  #define UPF_BUGGY_UART		((__force upf_t) (1 << 14))
> -- 
> 1.6.5.1.69.g36942
> 
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