dp8393x: fix receiving buffer exhaustion

The card is not able to exit from exhaustion state, because
while the drive consumes the buffers, the RRP is incremented
(when the driver clears the ISR RBE bit), so it stays equal
to RWP, and while RRP == RWP, the card thinks it is always
in exhaustion state. So the driver consumes all the buffers,
but the card cannot receive new ones.

This patch fixes the problem by not incrementing RRP when
the driver clears the ISR RBE bit.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/net/dp8393x.c

@@ -304,7 +304,7 @@ static void dp8393x_do_load_cam(dp8393xState *s)
     dp8393x_update_irq(s);
 }
 
-static void dp8393x_do_read_rra(dp8393xState *s)
+static void dp8393x_do_read_rra(dp8393xState *s, int next)
 {
     int width, size;
 
@@ -323,19 +323,20 @@ static void dp8393x_do_read_rra(dp8393xState *s)
         s->regs[SONIC_CRBA0], s->regs[SONIC_CRBA1],
         s->regs[SONIC_RBWC0], s->regs[SONIC_RBWC1]);
 
-    /* Go to next entry */
-    s->regs[SONIC_RRP] += size;
+    if (next) {
+        /* Go to next entry */
+        s->regs[SONIC_RRP] += size;
 
-    /* Handle wrap */
-    if (s->regs[SONIC_RRP] == s->regs[SONIC_REA]) {
-        s->regs[SONIC_RRP] = s->regs[SONIC_RSA];
-    }
+        /* Handle wrap */
+        if (s->regs[SONIC_RRP] == s->regs[SONIC_REA]) {
+            s->regs[SONIC_RRP] = s->regs[SONIC_RSA];
+        }
 
-    /* Check resource exhaustion */
-    if (s->regs[SONIC_RRP] == s->regs[SONIC_RWP])
-    {
-        s->regs[SONIC_ISR] |= SONIC_ISR_RBE;
-        dp8393x_update_irq(s);
+        /* Check resource exhaustion */
+        if (s->regs[SONIC_RRP] == s->regs[SONIC_RWP]) {
+            s->regs[SONIC_ISR] |= SONIC_ISR_RBE;
+            dp8393x_update_irq(s);
+        }
     }
 
     /* Done */
@@ -549,7 +550,7 @@ static void dp8393x_do_command(dp8393xState *s, uint16_t command)
     if (command & SONIC_CR_RST)
         dp8393x_do_software_reset(s);
     if (command & SONIC_CR_RRRA)
-        dp8393x_do_read_rra(s);
+        dp8393x_do_read_rra(s, 1);
     if (command & SONIC_CR_LCAM)
         dp8393x_do_load_cam(s);
 }
@@ -640,7 +641,7 @@ static void dp8393x_write(void *opaque, hwaddr addr, uint64_t data,
             data &= s->regs[reg];
             s->regs[reg] &= ~data;
             if (data & SONIC_ISR_RBE) {
-                dp8393x_do_read_rra(s);
+                dp8393x_do_read_rra(s, 0);
             }
             dp8393x_update_irq(s);
             if (dp8393x_can_receive(s->nic->ncs)) {
@@ -846,7 +847,7 @@ static ssize_t dp8393x_receive(NetClientState *nc, const uint8_t * buf,
 
         if (s->regs[SONIC_RCR] & SONIC_RCR_LPKT) {
             /* Read next RRA */
-            dp8393x_do_read_rra(s);
+            dp8393x_do_read_rra(s, 1);
         }
     }