dp8393x: Update LLFA and CRDA registers from rx descriptor

Follow the algorithm given in the National Semiconductor DP83932C
datasheet in section 3.4.7:

    At the next reception, the SONIC re-reads the last RXpkt.link field,
    and updates its CRDA register to point to the next descriptor.

The chip is designed to allow the host to provide a new list of
descriptors in this way.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit 5b0c98f)
*drop context dep on 19f7034
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/net/dp8393x.c

@@ -784,12 +784,13 @@ static ssize_t dp8393x_receive(NetClientState *nc, const uint8_t * buf,
         address = dp8393x_crda(s) + sizeof(uint16_t) * 5 * width;
         address_space_rw(&s->as, address, MEMTXATTRS_UNSPECIFIED,
                          (uint8_t *)s->data, size, 0);
-        if (dp8393x_get(s, width, 0) & SONIC_DESC_EOL) {
+        s->regs[SONIC_LLFA] = dp8393x_get(s, width, 0);
+        if (s->regs[SONIC_LLFA] & SONIC_DESC_EOL) {
             /* Still EOL ; stop reception */
             return -1;
-        } else {
-            s->regs[SONIC_CRDA] = s->regs[SONIC_LLFA];
         }
+        /* Link has been updated by host */
+        s->regs[SONIC_CRDA] = s->regs[SONIC_LLFA];
     }
 
     /* Save current position */
@@ -837,7 +838,7 @@ static ssize_t dp8393x_receive(NetClientState *nc, const uint8_t * buf,
     address_space_rw(&s->as, dp8393x_crda(s),
         MEMTXATTRS_UNSPECIFIED, (uint8_t *)s->data, size, 1);
 
-    /* Move to next descriptor */
+    /* Check link field */
     size = sizeof(uint16_t) * width;
     address_space_rw(&s->as, dp8393x_crda(s) + sizeof(uint16_t) * 5 * width,
         MEMTXATTRS_UNSPECIFIED, (uint8_t *)s->data, size, 0);
@@ -852,6 +853,8 @@ static ssize_t dp8393x_receive(NetClientState *nc, const uint8_t * buf,
         dp8393x_put(s, width, 0, 0);
         address_space_rw(&s->as, address, MEMTXATTRS_UNSPECIFIED,
                          (uint8_t *)s->data, size, true);
+
+        /* Move to next descriptor */
         s->regs[SONIC_CRDA] = s->regs[SONIC_LLFA];
         s->regs[SONIC_ISR] |= SONIC_ISR_PKTRX;
         s->regs[SONIC_RSC] = (s->regs[SONIC_RSC] & 0xff00) | (((s->regs[SONIC_RSC] & 0x00ff) + 1) & 0x00ff);