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

[Alistair Francis <alistair23@gmail.com>]

On Mon, Aug 23, 2021 at 2:53 PM Bin Meng <bmeng.cn@gmail.com> wrote:
>
> 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.

Ok:

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

>
> Regards,
> Bin