opencores_eth: flush queue whenever can_receive can go from false to true

The following registers control whether MAC can receive frames:
- MODER.RXEN bit that enables/disables receiver;
- TX_BD_NUM register that specifies number of RX descriptors.
Notify QEMU networking core when the MAC is ready to receive frames.
Discard frame and raise BUSY interrupt when the frame arrives but the
current RX descriptor is not empty.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

hw/net/opencores_eth.c

@@ -169,6 +169,7 @@ enum {
 };
 
 enum {
+    INT_SOURCE_BUSY = 0x10,
     INT_SOURCE_RXB = 0x4,
     INT_SOURCE_TXB = 0x1,
 };
@@ -351,8 +352,7 @@ static int open_eth_can_receive(NetClientState *nc)
     OpenEthState *s = qemu_get_nic_opaque(nc);
 
     return GET_REGBIT(s, MODER, RXEN) &&
-        (s->regs[TX_BD_NUM] < 0x80) &&
-        (rx_desc(s)->len_flags & RXD_E);
+        (s->regs[TX_BD_NUM] < 0x80);
 }
 
 static ssize_t open_eth_receive(NetClientState *nc,
@@ -402,6 +402,12 @@ static ssize_t open_eth_receive(NetClientState *nc,
         desc *desc = rx_desc(s);
         size_t copy_size = GET_REGBIT(s, MODER, HUGEN) ? 65536 : maxfl;
 
+        if (!(desc->len_flags & RXD_E)) {
+            open_eth_int_source_write(s,
+                    s->regs[INT_SOURCE] | INT_SOURCE_BUSY);
+            return size;
+        }
+
         desc->len_flags &= ~(RXD_CF | RXD_M | RXD_OR |
                 RXD_IS | RXD_DN | RXD_TL | RXD_SF | RXD_CRC | RXD_LC);
 
@@ -551,6 +557,15 @@ static uint64_t open_eth_reg_read(void *opaque,
     return v;
 }
 
+static void open_eth_notify_can_receive(OpenEthState *s)
+{
+    NetClientState *nc = qemu_get_queue(s->nic);
+
+    if (open_eth_can_receive(nc)) {
+        qemu_flush_queued_packets(nc);
+    }
+}
+
 static void open_eth_ro(OpenEthState *s, uint32_t val)
 {
 }
@@ -567,6 +582,7 @@ static void open_eth_moder_host_write(OpenEthState *s, uint32_t val)
 
     if (set & MODER_RXEN) {
         s->rx_desc = s->regs[TX_BD_NUM];
+        open_eth_notify_can_receive(s);
     }
     if (set & MODER_TXEN) {
         s->tx_desc = 0;
@@ -592,6 +608,18 @@ static void open_eth_int_mask_host_write(OpenEthState *s, uint32_t val)
             s->regs[INT_SOURCE] & s->regs[INT_MASK]);
 }
 
+static void open_eth_tx_bd_num_host_write(OpenEthState *s, uint32_t val)
+{
+    if (val < 0x80) {
+        bool enable = s->regs[TX_BD_NUM] == 0x80;
+
+        s->regs[TX_BD_NUM] = val;
+        if (enable) {
+            open_eth_notify_can_receive(s);
+        }
+    }
+}
+
 static void open_eth_mii_command_host_write(OpenEthState *s, uint32_t val)
 {
     unsigned fiad = GET_REGFIELD(s, MIIADDRESS, FIAD);
@@ -630,6 +658,7 @@ static void open_eth_reg_write(void *opaque,
         [MODER] = open_eth_moder_host_write,
         [INT_SOURCE] = open_eth_int_source_host_write,
         [INT_MASK] = open_eth_int_mask_host_write,
+        [TX_BD_NUM] = open_eth_tx_bd_num_host_write,
         [MIICOMMAND] = open_eth_mii_command_host_write,
         [MIITX_DATA] = open_eth_mii_tx_host_write,
         [MIISTATUS] = open_eth_ro,