escc: always set STATUS_TXEMPTY in R_STATUS on device reset

The "Transmit Interrupts and Transmit Buffer Empty Bit" section of the ESCC
datasheet states the following about the STATUS_TXEMPTY bit: "After a hardware
reset (including a hardware reset by software), or a channel reset, this bit
is set to 1".

Update escc_reset() to set the STATUS_TXEMPTY bit in the R_STATUS register
on device reset as described which fixes a regression whereby the Sun PROM
checks this bit early on startup and gets stuck in an infinite loop if it is
not set.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20211118181835.18497-2-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/hw/char/escc.c b/hw/char/escc.c
index 0fce4f6..a7d9050 100644
--- a/hw/char/escc.c
+++ b/hw/char/escc.c
@@ -354,6 +354,17 @@ static void escc_reset(DeviceState *d)
             cs->rregs[j] = 0;
             cs->wregs[j] = 0;
         }
+
+        /*
+         * ...but there is an exception. The "Transmit Interrupts and Transmit
+         * Buffer Empty Bit" section on page 50 of the ESCC datasheet says of
+         * the STATUS_TXEMPTY bit in R_STATUS: "After a hardware reset
+         * (including a hardware reset by software), or a channel reset, this
+         * bit is set to 1". The Sun PROM checks this bit early on startup and
+         * gets stuck in an infinite loop if it is not set.
+         */
+        cs->rregs[R_STATUS] |= STATUS_TXEMPTY;
+
         escc_reset_chn(cs);
     }
 }