found it. DMA can obviously not handle fragmented pbufs to receive packets. -> debug cleanup needed

bsp_z7/lwip/include/lwipopts.h

@@ -166,7 +166,7 @@ a lot of data that needs to be copied, this should be set high. */
 #define PBUF_POOL_SIZE          120
 
 /* PBUF_POOL_BUFSIZE: the size of each pbuf in the pbuf pool. */
-#define PBUF_POOL_BUFSIZE       256
+#define PBUF_POOL_BUFSIZE       2048
 
 /** SYS_LIGHTWEIGHT_PROT
  * define SYS_LIGHTWEIGHT_PROT in lwipopts.h if you want inter-task protection

bsp_z7/lwip/netif/xemacpsif.c

@@ -273,11 +273,20 @@ s32_t xemacpsif_input(struct netif *netif)
 
 		/* no packet could be read, silently ignore this */
 		if (p == NULL) {
+			outbyte('x');
 			return 0;
 		}
 
 		/* points to packet payload, which starts with an Ethernet header */
 		ethhdr = p->payload;
+		xil_printf(" %p",p->payload);
+
+		for (int j = 0; j < 6; j++) {
+			for (int i = 0; i < 8; i++) {
+				xil_printf(" %02x", *((uint8_t *)(p->payload) + i  + j * 8 ) );
+			}
+			xil_printf("\n");
+		}
 
 	#if LINK_STATS
 		lwip_stats.link.recv++;
@@ -298,13 +307,19 @@ s32_t xemacpsif_input(struct netif *netif)
 	#endif /* PPPOE_SUPPORT */
 				/* full packet send to tcpip_thread to process */
 				if (netif->input(p, netif) != ERR_OK) {
+					outbyte('y');
 					LWIP_DEBUGF(NETIF_DEBUG, ("xemacpsif_input: IP input error\r\n"));
 					pbuf_free(p);
 					p = NULL;
 				}
+				outbyte('o');
 				break;
 
 			default:
+				outbyte('z');
+				for (int i = 0; i < 14; i++) {
+					xil_printf(" %02x", *((uint8_t *)(p->payload) + i ) );
+				}
 				pbuf_free(p);
 				p = NULL;
 				break;

bsp_z7/lwip/netif/xemacpsif_dma.c

@@ -115,6 +115,17 @@ volatile u32_t notifyinfo[4*XLWIP_CONFIG_N_TX_DESC];
  * for BDs. The rest 768 KB of memory is just unused.
  *********************************************************************************/
 
+void printInt(int i) {
+	outbyte(i / 100 + 0x30);
+	i = i % 100;
+	outbyte(i / 10 + 0x30);
+	i = i %10;
+	outbyte(i + 0x30);
+	outbyte(10);
+
+}
+
+
 #if defined __aarch64__
 u8_t emac_bd_space[0x200000] __attribute__ ((aligned (0x200000)));
 #else
@@ -291,6 +302,7 @@ void xemacps_process_sent_bds(xemacpsif_s *xemacpsif, XEmacPs_BdRing *txring)
 
 void emacps_send_handler(void *arg)
 {
+	outbyte('t');
 	struct xemac_s *xemac;
 	xemacpsif_s   *xemacpsif;
 	XEmacPs_BdRing *txringptr;
@@ -498,6 +510,7 @@ void setup_rx_bds(xemacpsif_s *xemacpsif, XEmacPs_BdRing *rxring)
 
 void emacps_recv_handler(void *arg)
 {
+	outbyte('r');
 	struct pbuf *p;
 	XEmacPs_Bd *rxbdset, *curbdptr;
 	struct xemac_s *xemac;
@@ -538,10 +551,13 @@ void emacps_recv_handler(void *arg)
 		}
 
 		for (k = 0, curbdptr=rxbdset; k < bd_processed; k++) {
+			outbyte('p');
 
 			bdindex = XEMACPS_BD_TO_INDEX(rxring, curbdptr);
 			p = (struct pbuf *)rx_pbufs_storage[index + bdindex];
 
+			xil_printf("reading bd index %i bdindex %i p %p payload %p\n", index, bdindex, p, p->payload);
+
 			/*
 			 * Adjust the buffer size to the actual number of bytes received.
 			 */
@@ -550,6 +566,7 @@ void emacps_recv_handler(void *arg)
 #else
 			rx_bytes = XEmacPs_BdGetLength(curbdptr);
 #endif
+			printInt(rx_bytes);
 			pbuf_realloc(p, rx_bytes);
 
 			/* Invalidate RX frame before queuing to handle
@@ -775,6 +792,8 @@ XStatus init_dma(struct xemac_s *xemac)
 #endif
 		XEmacPs_BdSetAddressRx(rxbd, (UINTPTR)p->payload);
 
+		xil_printf("setting bd index %i bdindex %i p %p payload %p\n", index, bdindex, p, p->payload);
+
 		rx_pbufs_storage[index + bdindex] = (UINTPTR)p;
 	}
 	XEmacPs_SetQueuePtr(&(xemacpsif->emacps), xemacpsif->emacps.RxBdRing.BaseBdAddr, 0, XEMACPS_RECV);

bsp_z7/main.c

@@ -67,7 +67,7 @@ int main(void) {
 
   testEth();
 
-  mission();
+  //mission();
 }
 
 static void prvSetupHardware(void) {

bsp_z7/testEth.c

@@ -142,7 +142,7 @@ void udp_receiver(void *arg, struct udp_pcb *pcb, struct pbuf *p,
       xil_printf("%i received len: %i tot_len: %i data: ", now, p->len, p->tot_len);
       char * pointer = p->payload;
       for(uint8_t i = 0; i < p->len; i++) {
-        xil_printf(" %c", pointer + i);
+        xil_printf(" %02x", *(pointer + i));
       }
       xil_printf("\n");
 }