Commit 1645b8ee authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

serial: only resample THR interrupt on rising edge of IER.THRI 

There is disagreement on whether LSR.THRE should be resampled when
IER.THRI goes from 1 to 1.  Bochs only does it if IER.THRI goes from 0
to 1; PCE does it even if IER.THRI is unchanged.  But the Windows driver
seems to always go from 1 to 0 and back to 1, so do things in agreement
with Bochs, because the handling of thr_ipending was reported in 2010
(https://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01914.html

)
as breaking DR-DOS Plus.

Reported-by: default avatarRoy Tam <roytam@gmail.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 023c3a97

hw/char/serial.c

+16 −9
Original line number Diff line number Diff line
@@ -336,10 +336,12 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val, 
            s->divider = (s->divider & 0x00ff) | (val << 8);

            serial_update_parameters(s);

        } else {

            uint8_t changed = (s->ier ^ val) & 0x0f;

            s->ier = val & 0x0f;

            /* If the backend device is a real serial port, turn polling of the modem

               status lines on physical port on or off depending on UART_IER_MSI state */

            if (s->poll_msl >= 0) {

             * status lines on physical port on or off depending on UART_IER_MSI state.

             */

            if ((changed & UART_IER_MSI) && s->poll_msl >= 0) {

                if (s->ier & UART_IER_MSI) {

                     s->poll_msl = 1;

                     serial_update_msl(s);
@@ -354,19 +356,24 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val, 
             * This is not in the datasheet, but Windows relies on it.  It is

             * unclear if THRE has to be resampled every time THRI becomes

             * 1, or only on the rising edge.  Bochs does the latter, and Windows

             * always toggles IER to all zeroes and back to all ones.  But for

             * now leave it as it has always been in QEMU.

             * always toggles IER to all zeroes and back to all ones, so do the

             * same.

             *

             * If IER.THRI is zero, thr_ipending is not used.  Set it to zero

             * so that the thr_ipending subsection is not migrated.

             */

            if (changed & UART_IER_THRI) {

                if ((s->ier & UART_IER_THRI) && (s->lsr & UART_LSR_THRE)) {

                    s->thr_ipending = 1;

                } else {

                    s->thr_ipending = 0;

                }

            }



            if (changed) {

                serial_update_irq(s);

            }

        }

        break;

    case 2:

        /* Did the enable/disable flag change? If so, make sure FIFOs get flushed */