UART should be enabled in general and have RX enabled specifically to be
able to receive data from peripheral device. Same goes for transmitting
data to peripheral device and a TXE flag.

Check if UART CR register has EN and RXE or TXE bits enabled before
trying to receive or transmit data.

Signed-off-by: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
---
 hw/char/pl011.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/hw/char/pl011.c b/hw/char/pl011.c
index 3184949d69..522f36e4f3 100644
--- a/hw/char/pl011.c
+++ b/hw/char/pl011.c
@@ -54,6 +54,11 @@
 #define INT_E (INT_OE | INT_BE | INT_PE | INT_FE)
 #define INT_MS (INT_RI | INT_DSR | INT_DCD | INT_CTS)
 
+/* UARTCR bits */
+#define PL011_CR_UARTEN (1 << 0)
+#define PL011_CR_TXE    (1 << 8)
+#define PL011_CR_RXE    (1 << 9)
+
 static const unsigned char pl011_id_arm[8] =
   { 0x11, 0x10, 0x14, 0x00, 0x0d, 0xf0, 0x05, 0xb1 };
 static const unsigned char pl011_id_luminary[8] =
@@ -203,6 +208,11 @@ static void pl011_trace_baudrate_change(const PL011State *s)
                                 s->ibrd, s->fbrd);
 }
 
+static inline bool pl011_can_transmit(PL011State *s)
+{
+    return s->cr & PL011_CR_UARTEN && s->cr & PL011_CR_TXE;
+}
+
 static void pl011_write(void *opaque, hwaddr offset,
                         uint64_t value, unsigned size)
 {
@@ -213,7 +223,9 @@ static void pl011_write(void *opaque, hwaddr offset,
 
     switch (offset >> 2) {
     case 0: /* UARTDR */
-        /* ??? Check if transmitter is enabled.  */
+        if (!pl011_can_transmit(s)) {
+            break;
+        }
         ch = value;
         /* XXX this blocks entire thread. Rewrite to use
          * qemu_chr_fe_write and background I/O callbacks */
@@ -284,7 +296,11 @@ static int pl011_can_receive(void *opaque)
     PL011State *s = (PL011State *)opaque;
     int r;
 
-    r = s->read_count < pl011_get_fifo_depth(s);
+    if (!(s->cr & PL011_CR_UARTEN) || !(s->cr & PL011_CR_RXE)) {
+        r = 0;
+    } else {
+        r = s->read_count < pl011_get_fifo_depth(s);
+    }
     trace_pl011_can_receive(s->lcr, s->read_count, r);
     return r;
 }
@@ -429,7 +445,7 @@ static void pl011_reset(DeviceState *dev)
     s->fbrd = 0;
     s->read_trigger = 1;
     s->ifl = 0x12;
-    s->cr = 0x300;
+    s->cr = PL011_CR_RXE | PL011_CR_TXE;
     pl011_reset_pipe(s);
 
     for (i = 0; i < ARRAY_SIZE(s->irq); i++) {
-- 
2.34.1

On Tue, 17 Jan 2023 at 22:05, Evgeny Iakovlev
<eiakovlev@linux.microsoft.com> wrote:
>
> UART should be enabled in general and have RX enabled specifically to be
> able to receive data from peripheral device. Same goes for transmitting
> data to peripheral device and a TXE flag.
>
> Check if UART CR register has EN and RXE or TXE bits enabled before
> trying to receive or transmit data.
>
> Signed-off-by: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
> ---
>  hw/char/pl011.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM