Re: [Xen-devel] [PATCH] pl011: preserve RTS and DTR signal on UART init

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-09-03 at 16:00 +0200, Andre Przywara wrote:
> Although we do not support hardware flow control in the Xen driver
> for the PL011 UART, the other end may be configured to use it.
> In this case it waits in vain for the RTS signal to be asserted by
> the host and will never transmit any characters.
> So we leave RTS and DTR as they had been setup before.
> This fixes the UART input on Calxeda Midway, which uses hardware
> flow control for the serial-over-LAN functionality.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>

Acked + applied, thanks.

I wonder if we should turn of RXE/TXE/UARTEN early on before we do our
own setup? Just to get things in a known quiet state. Although if this
was going to be a problem it would have bit us all the way through
boot...

> ---
>  xen/drivers/char/pl011.c         | 7 +++++--
>  xen/include/asm-arm/pl011-uart.h | 4 ++++
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/drivers/char/pl011.c b/xen/drivers/char/pl011.c
> index 3ec6e10..3386e2b 100644
> --- a/xen/drivers/char/pl011.c
> +++ b/xen/drivers/char/pl011.c
> @@ -85,6 +85,7 @@ static void __init pl011_init_preirq(struct serial_port 
> *port)
>  {
>      struct pl011 *uart = port->uart;
>      unsigned int divisor;
> +    unsigned int cr;
>  
>      /* No interrupts, please. */
>      pl011_write(uart, IMSC, 0);
> @@ -120,8 +121,10 @@ static void __init pl011_init_preirq(struct serial_port 
> *port)
>      pl011_write(uart, IMSC, 0);
>      pl011_write(uart, ICR, ALLI);
>  
> -    /* Enable the UART for RX and TX; no flow ctrl */
> -    pl011_write(uart, CR, RXE | TXE | UARTEN);
> +    /* Enable the UART for RX and TX; keep RTS and DTR */
> +    cr = pl011_read(uart, CR);
> +    cr &= RTS | DTR;
> +    pl011_write(uart, CR, cr | RXE | TXE | UARTEN);
>  }
>  
>  static void __init pl011_init_postirq(struct serial_port *port)
> diff --git a/xen/include/asm-arm/pl011-uart.h 
> b/xen/include/asm-arm/pl011-uart.h
> index 3332c51..123f477 100644
> --- a/xen/include/asm-arm/pl011-uart.h
> +++ b/xen/include/asm-arm/pl011-uart.h
> @@ -38,6 +38,10 @@
>  #define DMACR  (0x48)
>  
>  /* CR bits */
> +#define CTSEN  (1<<15) /* automatic CTS hardware flow control */
> +#define RTSEN  (1<<14) /* automatic RTS hardware flow control */
> +#define RTS    (1<<11) /* RTS signal */
> +#define DTR    (1<<10) /* DTR signal */
>  #define RXE    (1<<9) /* Receive enable */
>  #define TXE    (1<<8) /* Transmit enable */
>  #define UARTEN (1<<0) /* UART enable */



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