[PATCH v7 5/7] hw/char/pl011: Consider TX FIFO overrun error

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

When transmission is disabled, characters are still queued
to the FIFO which eventually overruns. Report that error
condition in the status register.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/char/pl011.c      | 22 ++++++++++++++++++++++
 hw/char/trace-events |  2 ++
 2 files changed, 24 insertions(+)

diff --git a/hw/char/pl011.c b/hw/char/pl011.c
index 34a5cb3af5d..f67ce951ac9 100644
--- a/hw/char/pl011.c
+++ b/hw/char/pl011.c
@@ -61,6 +61,9 @@ DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr)
 /* Data Register, UARTDR */
 #define DR_BE   (1 << 10)
 
+/* Receive Status Register/Error Clear Register, UARTRSR/UARTECR */
+#define RSR_OE  (1 << 3)
+
 /* Interrupt status bits in UARTRIS, UARTMIS, UARTIMSC */
 #define INT_OE (1 << 10)
 #define INT_BE (1 << 9)
@@ -157,6 +160,18 @@ static inline unsigned pl011_get_fifo_depth(PL011State *s)
     return pl011_is_fifo_enabled(s) ? PL011_FIFO_DEPTH : 1;
 }
 
+static bool pl011_is_tx_fifo_full(PL011State *s)
+{
+    bool fifo_enabled = pl011_is_fifo_enabled(s);
+    bool tx_fifo_full = fifo_enabled
+                        ? fifo8_is_full(&s->xmit_fifo)
+                        : !fifo8_is_empty(&s->xmit_fifo);
+
+    trace_pl011_fifo_tx_is_full(fifo_enabled ? "FIFO" : "CHAR", tx_fifo_full);
+
+    return tx_fifo_full;
+}
+
 static inline void pl011_reset_rx_fifo(PL011State *s)
 {
     s->read_count = 0;
@@ -275,6 +290,13 @@ static void pl011_write_txdata(PL011State *s, uint8_t data)
                       "PL011 data written to disabled TX UART\n");
     }
 
+    if (pl011_is_tx_fifo_full(s)) {
+        /* The FIFO is already full. Content remains valid. */
+        trace_pl011_fifo_tx_overrun();
+        s->rsr |= RSR_OE;
+        return;
+    }
+
     trace_pl011_fifo_tx_put(data);
     pl011_loopback_tx(s, data);
     fifo8_push(&s->xmit_fifo, data);
diff --git a/hw/char/trace-events b/hw/char/trace-events
index c857f4c4b38..d52f511a1e2 100644
--- a/hw/char/trace-events
+++ b/hw/char/trace-events
@@ -66,9 +66,11 @@ pl011_can_receive(uint32_t lcr, unsigned rx_fifo_used, size_t rx_fifo_depth, uns
 pl011_fifo_enable(bool enable) "enable:%u"
 pl011_fifo_rx_put(uint32_t c, unsigned read_count, size_t rx_fifo_depth) "RX FIFO push char [0x%02x] %d/%zu depth used"
 pl011_fifo_rx_full(void) "RX FIFO now full, RXFF set"
+pl011_fifo_tx_is_full(const char *desc, bool full) "mode:%s full:%u"
 pl011_fifo_tx_put(uint8_t byte) "TX FIFO push char [0x%02x]"
 pl011_fifo_tx_xmit_used(unsigned sent) "TX FIFO used %u chars"
 pl011_fifo_tx_xmit_consumed(unsigned sent) "TX FIFO consumed %u chars"
+pl011_fifo_tx_overrun(void) "TX FIFO overrun"
 pl011_baudrate_change(unsigned int baudrate, uint64_t clock, uint32_t ibrd, uint32_t fbrd) "new baudrate %u (clk: %" PRIu64 "hz, ibrd: %" PRIu32 ", fbrd: %" PRIu32 ")"
 pl011_receive(int size) "recv %d chars"
 
-- 
2.47.1