[Qemu-devel] [PULL 42/52] char/cadence_uart: Remove TX timer & add TX FIFO state

[Peter Maydell]

From: Peter Crosthwaite <address@hidden>

This tx timer implementation is flawed. Despite the controller
attempting to time the guest visable assertion of the TX-empty status
bit (and corresponding interrupt) the controller is still transmitting
characters instantaneously. There is also no sense of multiple character
delay.

The only side effect of this timer is assertion of tx-empty status. So
just remove the timer completely and hold tx-empty as permanently
asserted (its reset status). This matches the actual behaviour of
instantaneous transmission.

While we are VMSD version bumping, add the tx_fifo as device state to
prepare for upcomming TxFIFO flow control. Implement the interrupt
generation logic for the TxFIFO occupancy.

Signed-off-by: Peter Crosthwaite <address@hidden>
Message-id: address@hidden
Signed-off-by: Peter Maydell <address@hidden>
---
 hw/char/cadence_uart.c | 44 +++++++++++++-------------------------------
 1 file changed, 13 insertions(+), 31 deletions(-)

diff --git a/hw/char/cadence_uart.c b/hw/char/cadence_uart.c
index 216eed7..3eeadb1 100644
--- a/hw/char/cadence_uart.c
+++ b/hw/char/cadence_uart.c
@@ -121,13 +121,14 @@ typedef struct {
     MemoryRegion iomem;
     uint32_t r[R_MAX];
     uint8_t rx_fifo[RX_FIFO_SIZE];
+    uint8_t tx_fifo[TX_FIFO_SIZE];
     uint32_t rx_wpos;
     uint32_t rx_count;
+    uint32_t tx_count;
     uint64_t char_tx_time;
     CharDriverState *chr;
     qemu_irq irq;
     QEMUTimer *fifo_trigger_handle;
-    QEMUTimer *tx_time_handle;
 } UartState;
 
 static void uart_update_status(UartState *s)
@@ -138,8 +139,12 @@ static void uart_update_status(UartState *s)
     s->r[R_SR] |= !s->rx_count ? UART_SR_INTR_REMPTY : 0;
     s->r[R_SR] |= s->rx_count >= s->r[R_RTRIG] ? UART_SR_INTR_RTRIG : 0;
 
-    s->r[R_SR] |= UART_SR_INTR_TEMPTY;
+    s->r[R_SR] |= s->tx_count == TX_FIFO_SIZE ? UART_SR_INTR_TFUL : 0;
+    s->r[R_SR] |= !s->tx_count ? UART_SR_INTR_TEMPTY : 0;
+    s->r[R_SR] |= s->tx_count >= s->r[R_TTRIG] ? UART_SR_TTRIG : 0;
+
     s->r[R_CISR] |= s->r[R_SR] & UART_SR_TO_CISR_MASK;
+    s->r[R_CISR] |= s->r[R_SR] & UART_SR_TTRIG ? UART_INTR_TTRIG : 0;
     qemu_set_irq(s->irq, !!(s->r[R_IMR] & s->r[R_CISR]));
 }
 
@@ -152,24 +157,6 @@ static void fifo_trigger_update(void *opaque)
     uart_update_status(s);
 }
 
-static void uart_tx_redo(UartState *s)
-{
-    uint64_t new_tx_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
-
-    timer_mod(s->tx_time_handle, new_tx_time + s->char_tx_time);
-
-    s->r[R_SR] |= UART_SR_INTR_TEMPTY;
-
-    uart_update_status(s);
-}
-
-static void uart_tx_write(void *opaque)
-{
-    UartState *s = (UartState *)opaque;
-
-    uart_tx_redo(s);
-}
-
 static void uart_rx_reset(UartState *s)
 {
     s->rx_wpos = 0;
@@ -181,6 +168,7 @@ static void uart_rx_reset(UartState *s)
 
 static void uart_tx_reset(UartState *s)
 {
+    s->tx_count = 0;
 }
 
 static void uart_send_breaks(UartState *s)
@@ -261,10 +249,6 @@ static void uart_ctrl_update(UartState *s)
 
     s->r[R_CR] &= ~(UART_CR_TXRST | UART_CR_RXRST);
 
-    if ((s->r[R_CR] & UART_CR_TX_EN) && !(s->r[R_CR] & UART_CR_TX_DIS)) {
-            uart_tx_redo(s);
-    }
-
     if (s->r[R_CR] & UART_CR_STARTBRK && !(s->r[R_CR] & UART_CR_STOPBRK)) {
         uart_send_breaks(s);
     }
@@ -447,9 +431,6 @@ static int cadence_uart_init(SysBusDevice *dev)
     s->fifo_trigger_handle = timer_new_ns(QEMU_CLOCK_VIRTUAL,
             (QEMUTimerCB *)fifo_trigger_update, s);
 
-    s->tx_time_handle = timer_new_ns(QEMU_CLOCK_VIRTUAL,
-            (QEMUTimerCB *)uart_tx_write, s);
-
     s->char_tx_time = (get_ticks_per_sec() / 9600) * 10;
 
     s->chr = qemu_char_get_next_serial();
@@ -473,17 +454,18 @@ static int cadence_uart_post_load(void *opaque, int 
version_id)
 
 static const VMStateDescription vmstate_cadence_uart = {
     .name = "cadence_uart",
-    .version_id = 1,
-    .minimum_version_id = 1,
-    .minimum_version_id_old = 1,
+    .version_id = 2,
+    .minimum_version_id = 2,
+    .minimum_version_id_old = 2,
     .post_load = cadence_uart_post_load,
     .fields = (VMStateField[]) {
         VMSTATE_UINT32_ARRAY(r, UartState, R_MAX),
         VMSTATE_UINT8_ARRAY(rx_fifo, UartState, RX_FIFO_SIZE),
+        VMSTATE_UINT8_ARRAY(tx_fifo, UartState, RX_FIFO_SIZE),
         VMSTATE_UINT32(rx_count, UartState),
+        VMSTATE_UINT32(tx_count, UartState),
         VMSTATE_UINT32(rx_wpos, UartState),
         VMSTATE_TIMER(fifo_trigger_handle, UartState),
-        VMSTATE_TIMER(tx_time_handle, UartState),
         VMSTATE_END_OF_LIST()
     }
 };
-- 
1.8.5