Re: [Qemu-devel] [Qemu-arm] [PATCH] pl011: do not put into fifo before enabled the interruption

[Peter Maydell <peter.maydell@linaro.org>]

On 26 January 2018 at 16:00, Wei Xu <xuwei5@hisilicon.com> wrote:
> If the user pressed some keys in the console during the guest booting,
> the console will be hanged after entering the shell.
> Because in the above case the pl011_can_receive will return 0 that
> the pl011_receive will not be called. That means no interruption will
> be injected in to the kernel and the pl011 state could not be driven
> further.
>
> This patch fixed that issue by checking the interruption is enabled or
> not before putting into the fifo.
>
> Signed-off-by: Wei Xu <xuwei5@hisilicon.com>
> ---
>  hw/char/pl011.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/hw/char/pl011.c b/hw/char/pl011.c
> index 2aa277fc4f..6296de9527 100644
> --- a/hw/char/pl011.c
> +++ b/hw/char/pl011.c
> @@ -229,6 +229,8 @@ static int pl011_can_receive(void *opaque)
>      PL011State *s = (PL011State *)opaque;
>      int r;
>
> +    if (!s->int_enabled)
> +       return 0;
>      if (s->lcr & 0x10) {
>          r = s->read_count < 16;
>      } else {
> --

This doesn't look right. You should be able to use the PL011
in a strictly polling mode, without ever enabling interrupts.
Returning false in can_receive if interrupts are disabled
would break that.

If the user presses keys before interrupts are enabled,
what ought to happen is:
 * we put the key in the FIFO, and update the int_level flags
 * when the FIFO is full, can_receive starts returning 0 and
   QEMU stops passing us new characters
 * when the guest driver for the pl011 initializes the
   device and enables interrupts then either:
    (a) it does something that clears the FIFO, which will
    mean can_receive starts allowing new chars again, or
    (b) it leaves the FIFO as it is, and we should thus
    immediately raise an interrupt for the characters still
    in the FIFO; when the guest handles this interrupt and
    gets the characters, can_receive will permit new ones

What is happening in your situation that means this is not
working as expected ?

thanks
-- PMM