Re: [PATCH 2/3] hw/char: cadence_uart: Disable transmit when input clock is disabled

[Bin Meng]

On Mon, Aug 23, 2021 at 12:43 PM Alistair Francis <alistair23@gmail.com> wrote:
>
> On Mon, Aug 23, 2021 at 12:11 PM Bin Meng <bmeng.cn@gmail.com> wrote:
> >
> > At present when input clock is disabled, any character transmitted
> > to tx fifo can still show on the serial line, which is wrong.
> >
> > Fixes: b636db306e06 ("hw/char/cadence_uart: add clock support")
> > Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
> > ---
> >
> >  hw/char/cadence_uart.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/hw/char/cadence_uart.c b/hw/char/cadence_uart.c
> > index b4b5e8a3ee..154be34992 100644
> > --- a/hw/char/cadence_uart.c
> > +++ b/hw/char/cadence_uart.c
> > @@ -327,6 +327,11 @@ static gboolean cadence_uart_xmit(void *do_not_use, 
> > GIOCondition cond,
> >  static void uart_write_tx_fifo(CadenceUARTState *s, const uint8_t *buf,
> >                                 int size)
> >  {
> > +    /* ignore characters when unclocked or in reset */
> > +    if (!clock_is_enabled(s->refclk) || device_is_in_reset(DEVICE(s))) {
> > +        return;
> > +    }
>
> Should we log a guest error here?
>

Not sure. Based on my past experience of many hardware, if the input
clock is disabled, accessing the whole register block might cause a
bus fault. But I believe such bus fault is not modeled in QEMU.

This change just mirrors the same check on the Rx side.

Regards,
Bin