On Fri, 29 Aug 2025, Stefano Stabellini wrote:
> On Thu, 28 Aug 2025, dmuk...@xen.org wrote:
> > From: Denis Mukhin <dmuk...@ford.com>
> >
> > Add RBR/THR registers emulation to the I/O port handlder.
> >
> > Also, add RX/TX FIFO management code since RBR depends on RX FIFO and
> > THR depends on TX FIFO.
> >
> > FIFOs are not emulated as per UART specs for simplicity (not need to emulate
> > baud rate). Emulator does not emulate NS8250 (no FIFO), NS16550a (16 bytes)
> > or
> > NS16750 (64 bytes).
> >
> > FIFOs are emulated by means of using xencons_interface which conveniently
> > provides primitives for buffer management and later can be used for
> > inter-domain communication similarly to vpl011.
> >
> > Signed-off-by: Denis Mukhin <dmuk...@ford.com>
> > ---
> > Changes since v4:
> > - new patch
> > ---
> > xen/common/emul/vuart/ns16x50.c | 135 ++++++++++++++++++++++++++++++++
> > 1 file changed, 135 insertions(+)
> >
> > diff --git a/xen/common/emul/vuart/ns16x50.c
> > b/xen/common/emul/vuart/ns16x50.c
> > index 20597cc36b35..efb2f4c6441c 100644
> > --- a/xen/common/emul/vuart/ns16x50.c
> > +++ b/xen/common/emul/vuart/ns16x50.c
> > @@ -92,6 +92,119 @@ static bool ns16x50_fifo_rx_empty(const struct
> > vuart_ns16x50 *vdev)
> > return cons->in_prod == cons->in_cons;
> > }
> >
> > +static bool ns16x50_fifo_rx_full(const struct vuart_ns16x50 *vdev)
> > +{
> > + const struct xencons_interface *cons = &vdev->cons;
> > +
> > + return cons->in_prod - cons->in_cons == ARRAY_SIZE(cons->in);
> > +}
> > +
> > +static void ns16x50_fifo_rx_reset(struct vuart_ns16x50 *vdev)
> > +{
> > + struct xencons_interface *cons = &vdev->cons;
> > +
> > + cons->in_cons = cons->in_prod;
> > +}
> > +
> > +static int ns16x50_fifo_rx_getchar(struct vuart_ns16x50 *vdev)
> > +{
> > + struct xencons_interface *cons = &vdev->cons;
> > + int rc;
> > +
> > + if ( ns16x50_fifo_rx_empty(vdev) )
> > + {
> > + ns16x50_debug(vdev, "RX FIFO empty\n");
> > + rc = -ENODATA;
> > + }
> > + else
> > + {
> > + rc = cons->in[MASK_XENCONS_IDX(cons->in_cons, cons->in)];
> > + cons->in_cons++;
> > + }
> > +
> > + return rc;
>
> The signed integer to char conversion here is not great from a MISRA
> perspective. I think it would be better to keep rc as success/failure
> return value and take the read char as a pointer parameter.
>
>
> > +}
> > +
> > +static int ns16x50_fifo_rx_putchar(struct vuart_ns16x50 *vdev, char c)
> > +{
> > + struct xencons_interface *cons = &vdev->cons;
> > + int rc;
> > +
> > + /*
> > + * FIFO-less 8250/16450 UARTs: newly arrived word overwrites the
> > contents
> > + * of the THR.
> > + */
> > + if ( ns16x50_fifo_rx_full(vdev) )
> > + {
> > + ns16x50_debug(vdev, "RX FIFO full; resetting\n");
> > + ns16x50_fifo_rx_reset(vdev);
> > + rc = -ENOSPC;
> > + }
> > + else
> > + rc = 0;
> > +
> > + cons->in[MASK_XENCONS_IDX(cons->in_prod, cons->in)] = c;
> > + cons->in_prod++;
> > +
> > + return rc;
> > +}
> > +
> > +static bool ns16x50_fifo_tx_empty(const struct vuart_ns16x50 *vdev)
> > +{
> > + const struct xencons_interface *cons = &vdev->cons;
> > +
> > + return cons->out_prod == cons->out_cons;
> > +}
> > +
> > +static void ns16x50_fifo_tx_reset(struct vuart_ns16x50 *vdev)
> > +{
> > + struct xencons_interface *cons = &vdev->cons;
> > +
> > + cons->out_prod = 0;
> > + ASSERT(cons->out_cons == cons->out_prod);
>
> I am not sure about this.. why resetting the out_prod index? In theory
> it could keep increasing until wrap around and still go forward thanks
> to the mask
I get it now .. it is because it is called from ns16x50_fifo_tx_flush
right after printing to the real console.
I think it is better to simply do:
cons->out_cons = cons->out_prod;
which effectively clears out the whole buffer. It is dangerous to do:
cons->out_prod = 0;
without also doing:
cons->out_cons = 0;
Also, if ns16x50_fifo_tx_flush is the only caller of
ns16x50_fifo_tx_reset, I would open code the implementation directly
inside ns16x50_fifo_tx_flush to make it more obvious.
> > +}
> > +
> > +/*
> > + * Flush cached output to Xen console.
> > + */
> > +static void ns16x50_fifo_tx_flush(struct vuart_ns16x50 *vdev)
> > +{
> > + struct xencons_interface *cons = &vdev->cons;
> > + struct domain *d = vdev->owner;
> > +
> > + if ( ns16x50_fifo_tx_empty(vdev) )
> > + return;
> > +
> > + UART_IIR_THR ASSERT(cons->out_prod < ARRAY_SIZE(cons->out));
> > + cons->out[cons->out_prod] = '\0';
>
> should use MASK_XENCONS_IDX to access the array
>
>
> > + cons->out_prod++;
> > +
> > + guest_printk(d, guest_prefix "%s", cons->out);
> > +
> > + ns16x50_fifo_tx_reset(vdev);
>
> set UART_IIR_THR and call ns16x50_irq_check ?
>
>
> > +}
