Re: [PATCH 3/3] tty/serial: 8250_early: Add support for PXA UARTs

[Peter Hurley <peter@hurleysoftware.com>]

On 01/26/2015 11:50 PM, Rob Herring wrote:
> The PXA variant of the 8250 UART adds a UART enable bit which must not
> be cleared. Make the earlycon support maintain this bit if it is set.
> This implies some initialization of the UART, but we cannot
> unconditionally set the bit as some other variants require this bit to
> be clear for other functions.

So if the bootloader doesn't enable the uart, then earlycon doesn't work?
Or is there some other piece that is enabling the uart after earlycon setup
(besides the driver, of course)?

> Signed-off-by: Rob Herring <robh@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.cz>
> Cc: linux-serial@vger.kernel.org
> ---
>  drivers/tty/serial/8250/8250_early.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
> index ce2a8ab..c31a22b 100644
> --- a/drivers/tty/serial/8250/8250_early.c
> +++ b/drivers/tty/serial/8250/8250_early.c
> @@ -93,10 +93,10 @@ static void __init early_serial8250_write(struct console *console,
>  	struct uart_port *port = &early_device->port;
>  	unsigned int ier;
>  
> -	/* Save the IER and disable interrupts */
> +	/* Save the IER and disable interrupts preserving the UUE bit */
>  	ier = serial8250_early_in(port, UART_IER);
>  	if (ier)
> -		serial8250_early_out(port, UART_IER, 0);
> +		serial8250_early_out(port, UART_IER, ier & UART_IER_UUE);
>  
>  	uart_console_write(port, s, count, serial_putc);
>  
> @@ -127,9 +127,11 @@ static void __init init_port(struct earlycon_device *device)
>  	struct uart_port *port = &device->port;
>  	unsigned int divisor;
>  	unsigned char c;
> +	unsigned int ier;
>  
>  	serial8250_early_out(port, UART_LCR, 0x3);	/* 8n1 */
> -	serial8250_early_out(port, UART_IER, 0);	/* no interrupt */
> +	ier = serial8250_early_in(port, UART_IER);
> +	serial8250_early_out(port, UART_IER, ier & UART_IER_UUE); /* no interrupt */

The trailing comment seems no longer relevant; how about below?

	/* Mask interrupts (preserve XScale's uart enable bit) */
	serial8250_early_out(port, UART_IER, ier & UART_IER_UUE);

OTOH, if the uart is disabled, shouldn't earlycon be disabled?

Regards,
Peter Hurley

>  	serial8250_early_out(port, UART_FCR, 0);	/* no fifo */
>  	serial8250_early_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>  
>